PL/pgSQL — SQL Procedural LanguagePL/pgSQLOverviewPL/pgSQL is a loadable procedural
language for the PostgreSQL database
system. The design goals of PL/pgSQL were to create
a loadable procedural language that
can be used to create functions, procedures, and triggers,
adds control structures to the SQL language,
can perform complex computations,
inherits all user-defined types, functions, procedures, and operators,
can be defined to be trusted by the server,
is easy to use.
Functions created with PL/pgSQL can be
used anywhere that built-in functions could be used.
For example, it is possible to
create complex conditional computation functions and later use
them to define operators or use them in index expressions.
In PostgreSQL 9.0 and later,
PL/pgSQL is installed by default.
However it is still a loadable module, so especially security-conscious
administrators could choose to remove it.
Advantages of Using PL/pgSQL
SQL is the language PostgreSQL
and most other relational databases use as query language. It's
portable and easy to learn. But every SQL
statement must be executed individually by the database server.
That means that your client application must send each query to
the database server, wait for it to be processed, receive and
process the results, do some computation, then send further
queries to the server. All this incurs interprocess
communication and will also incur network overhead if your client
is on a different machine than the database server.
With PL/pgSQL you can group a block of
computation and a series of queries inside
the database server, thus having the power of a procedural
language and the ease of use of SQL, but with considerable
savings of client/server communication overhead.
Extra round trips between
client and server are eliminated Intermediate results that the client does not
need do not have to be marshaled or transferred between server
and client Multiple rounds of query
parsing can be avoided This can result in a considerable performance increase as
compared to an application that does not use stored functions.
Also, with PL/pgSQL you can use all
the data types, operators and functions of SQL.
Supported Argument and Result Data Types
Functions written in PL/pgSQL can accept
as arguments any scalar or array data type supported by the server,
and they can return a result of any of these types. They can also
accept or return any composite type (row type) specified by name.
It is also possible to declare a PL/pgSQL
function as accepting record, which means that any
composite type will do as input, or
as returning record, which means that the result
is a row type whose columns are determined by specification in the
calling query, as discussed in .
PL/pgSQL functions can be declared to accept a variable
number of arguments by using the VARIADIC marker. This
works exactly the same way as for SQL functions, as discussed in
.
PL/pgSQL functions can also be declared to
accept and return the polymorphic types described in
, thus allowing the actual data
types handled by the function to vary from call to call.
Examples appear in .
PL/pgSQL functions can also be declared to return
a set (or table) of any data type that can be returned as
a single instance. Such a function generates its output by executing
RETURN NEXT for each desired element of the result
set, or by using RETURN QUERY to output the result of
evaluating a query.
Finally, a PL/pgSQL function can be declared to return
void if it has no useful return value. (Alternatively, it
could be written as a procedure in that case.)
PL/pgSQL functions can also be declared with output
parameters in place of an explicit specification of the return type.
This does not add any fundamental capability to the language, but
it is often convenient, especially for returning multiple values.
The RETURNS TABLE notation can also be used in place
of RETURNS SETOF.
Specific examples appear in
and
.
Structure of PL/pgSQL
Functions written in PL/pgSQL are defined
to the server by executing commands.
Such a command would normally look like, say,
CREATE FUNCTION somefunc(integer, text) RETURNS integer
AS 'function body text'
LANGUAGE plpgsql;
The function body is simply a string literal so far as CREATE
FUNCTION is concerned. It is often helpful to use dollar quoting
(see ) to write the function
body, rather than the normal single quote syntax. Without dollar quoting,
any single quotes or backslashes in the function body must be escaped by
doubling them. Almost all the examples in this chapter use dollar-quoted
literals for their function bodies.
PL/pgSQL is a block-structured language.
The complete text of a function body must be a
block. A block is defined as:
<<label>> DECLARE
declarations
BEGIN
statements
END label;
Each declaration and each statement within a block is terminated
by a semicolon. A block that appears within another block must
have a semicolon after END, as shown above;
however the final END that
concludes a function body does not require a semicolon.
A common mistake is to write a semicolon immediately after
BEGIN. This is incorrect and will result in a syntax error.
A label is only needed if you want to
identify the block for use
in an EXIT statement, or to qualify the names of the
variables declared in the block. If a label is given after
END, it must match the label at the block's beginning.
All key words are case-insensitive.
Identifiers are implicitly converted to lower case
unless double-quoted, just as they are in ordinary SQL commands.
Comments work the same way in PL/pgSQL code as in
ordinary SQL. A double dash (--) starts a comment
that extends to the end of the line. A /* starts a
block comment that extends to the matching occurrence of
*/. Block comments nest.
Any statement in the statement section of a block
can be a subblock. Subblocks can be used for
logical grouping or to localize variables to a small group
of statements. Variables declared in a subblock mask any
similarly-named variables of outer blocks for the duration
of the subblock; but you can access the outer variables anyway
if you qualify their names with their block's label. For example:
CREATE FUNCTION somefunc() RETURNS integer AS $$
<< outerblock >>
DECLARE
quantity integer := 30;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 30
quantity := 50;
--
-- Create a subblock
--
DECLARE
quantity integer := 80;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 80
RAISE NOTICE 'Outer quantity here is %', outerblock.quantity; -- Prints 50
END;
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 50
RETURN quantity;
END;
$$ LANGUAGE plpgsql;
There is actually a hidden outer block surrounding the body
of any PL/pgSQL function. This block provides the
declarations of the function's parameters (if any), as well as some
special variables such as FOUND (see
). The outer block is
labeled with the function's name, meaning that parameters and special
variables can be qualified with the function's name.
It is important not to confuse the use of
BEGIN/END for grouping statements in
PL/pgSQL with the similarly-named SQL commands
for transaction
control. PL/pgSQL's BEGIN/END
are only for grouping; they do not start or end a transaction.
See for information on managing
transactions in PL/pgSQL.
Also, a block containing an EXCEPTION clause effectively
forms a subtransaction that can be rolled back without affecting the
outer transaction. For more about that see .
Declarations
All variables used in a block must be declared in the
declarations section of the block.
(The only exceptions are that the loop variable of a FOR loop
iterating over a range of integer values is automatically declared as an
integer variable, and likewise the loop variable of a FOR loop
iterating over a cursor's result is automatically declared as a
record variable.)
PL/pgSQL variables can have any SQL data type, such as
integer, varchar, and
char.
Here are some examples of variable declarations:
user_id integer;
quantity numeric(5);
url varchar;
myrow tablename%ROWTYPE;
myfield tablename.columnname%TYPE;
arow RECORD;
The general syntax of a variable declaration is:
name CONSTANT type COLLATE collation_name NOT NULL { DEFAULT | := | = } expression;
The DEFAULT clause, if given, specifies the initial value assigned
to the variable when the block is entered. If the DEFAULT clause
is not given then the variable is initialized to the
SQL null value.
The CONSTANT option prevents the variable from being
assigned to after initialization, so that its value will remain constant
for the duration of the block.
The COLLATE option specifies a collation to use for the
variable (see ).
If NOT NULL
is specified, an assignment of a null value results in a run-time
error. All variables declared as NOT NULL
must have a nonnull default value specified.
Equal (=) can be used instead of PL/SQL-compliant
:=.
A variable's default value is evaluated and assigned to the variable
each time the block is entered (not just once per function call).
So, for example, assigning now() to a variable of type
timestamp causes the variable to have the
time of the current function call, not the time when the function was
precompiled.
Examples:
quantity integer DEFAULT 32;
url varchar := 'http://mysite.com';
transaction_time CONSTANT timestamp with time zone := now();
Once declared, a variable's value can be used in later initialization
expressions in the same block, for example:
DECLARE
x integer := 1;
y integer := x + 1;
Declaring Function Parameters
Parameters passed to functions are named with the identifiers
$1, $2,
etc. Optionally, aliases can be declared for
$n
parameter names for increased readability. Either the alias or the
numeric identifier can then be used to refer to the parameter value.
There are two ways to create an alias. The preferred way is to give a
name to the parameter in the CREATE FUNCTION command,
for example:
CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
The other way is to explicitly declare an alias, using the
declaration syntax
name ALIAS FOR $n;
The same example in this style looks like:
CREATE FUNCTION sales_tax(real) RETURNS real AS $$
DECLARE
subtotal ALIAS FOR $1;
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
These two examples are not perfectly equivalent. In the first case,
subtotal could be referenced as
sales_tax.subtotal, but in the second case it could not.
(Had we attached a label to the inner block, subtotal could
be qualified with that label, instead.)
Some more examples:
CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
DECLARE
v_string ALIAS FOR $1;
index ALIAS FOR $2;
BEGIN
-- some computations using v_string and index here
END;
$$ LANGUAGE plpgsql;
CREATE FUNCTION concat_selected_fields(in_t sometablename) RETURNS text AS $$
BEGIN
RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
END;
$$ LANGUAGE plpgsql;
When a PL/pgSQL function is declared
with output parameters, the output parameters are given
$n names and optional
aliases in just the same way as the normal input parameters. An
output parameter is effectively a variable that starts out NULL;
it should be assigned to during the execution of the function.
The final value of the parameter is what is returned. For instance,
the sales-tax example could also be done this way:
CREATE FUNCTION sales_tax(subtotal real, OUT tax real) AS $$
BEGIN
tax := subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
Notice that we omitted RETURNS real — we could have
included it, but it would be redundant.
To call a function with OUT parameters, omit the
output parameter(s) in the function call:
SELECT sales_tax(100.00);
Output parameters are most useful when returning multiple values.
A trivial example is:
CREATE FUNCTION sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
BEGIN
sum := x + y;
prod := x * y;
END;
$$ LANGUAGE plpgsql;
SELECT * FROM sum_n_product(2, 4);
sum | prod
-----+------
6 | 8
As discussed in , this
effectively creates an anonymous record type for the function's
results. If a RETURNS clause is given, it must say
RETURNS record.
This also works with procedures, for example:
CREATE PROCEDURE sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
BEGIN
sum := x + y;
prod := x * y;
END;
$$ LANGUAGE plpgsql;
In a call to a procedure, all the parameters must be specified. For
output parameters, NULL may be specified when
calling the procedure from plain SQL:
CALL sum_n_product(2, 4, NULL, NULL);
sum | prod
-----+------
6 | 8
However, when calling a procedure
from PL/pgSQL, you should instead write a
variable for any output parameter; the variable will receive the result
of the call. See
for details.
Another way to declare a PL/pgSQL function
is with RETURNS TABLE, for example:
CREATE FUNCTION extended_sales(p_itemno int)
RETURNS TABLE(quantity int, total numeric) AS $$
BEGIN
RETURN QUERY SELECT s.quantity, s.quantity * s.price FROM sales AS s
WHERE s.itemno = p_itemno;
END;
$$ LANGUAGE plpgsql;
This is exactly equivalent to declaring one or more OUT
parameters and specifying RETURNS SETOF
sometype.
When the return type of a PL/pgSQL function
is declared as a polymorphic type (see
), a special
parameter $0 is created. Its data type is the actual
return type of the function, as deduced from the actual input types.
This allows the function to access its actual return type
as shown in .
$0 is initialized to null and can be modified by
the function, so it can be used to hold the return value if desired,
though that is not required. $0 can also be
given an alias. For example, this function works on any data type
that has a + operator:
CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement)
RETURNS anyelement AS $$
DECLARE
result ALIAS FOR $0;
BEGIN
result := v1 + v2 + v3;
RETURN result;
END;
$$ LANGUAGE plpgsql;
The same effect can be obtained by declaring one or more output parameters as
polymorphic types. In this case the
special $0 parameter is not used; the output
parameters themselves serve the same purpose. For example:
CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement,
OUT sum anyelement)
AS $$
BEGIN
sum := v1 + v2 + v3;
END;
$$ LANGUAGE plpgsql;
In practice it might be more useful to declare a polymorphic function
using the anycompatible family of types, so that automatic
promotion of the input arguments to a common type will occur.
For example:
CREATE FUNCTION add_three_values(v1 anycompatible, v2 anycompatible, v3 anycompatible)
RETURNS anycompatible AS $$
BEGIN
RETURN v1 + v2 + v3;
END;
$$ LANGUAGE plpgsql;
With this example, a call such as
SELECT add_three_values(1, 2, 4.7);
will work, automatically promoting the integer inputs to numeric.
The function using anyelement would require you to
cast the three inputs to the same type manually.
ALIASnewname ALIAS FOR oldname;
The ALIAS syntax is more general than is suggested in the
previous section: you can declare an alias for any variable, not just
function parameters. The main practical use for this is to assign
a different name for variables with predetermined names, such as
NEW or OLD within
a trigger function.
Examples:
DECLARE
prior ALIAS FOR old;
updated ALIAS FOR new;
Since ALIAS creates two different ways to name the same
object, unrestricted use can be confusing. It's best to use it only
for the purpose of overriding predetermined names.
Copying Typesvariable%TYPE
%TYPE provides the data type of a variable or
table column. You can use this to declare variables that will hold
database values. For example, let's say you have a column named
user_id in your users
table. To declare a variable with the same data type as
users.user_id you write:
user_id users.user_id%TYPE;
By using %TYPE you don't need to know the data
type of the structure you are referencing, and most importantly,
if the data type of the referenced item changes in the future (for
instance: you change the type of user_id
from integer to real), you might not need
to change your function definition.
%TYPE is particularly valuable in polymorphic
functions, since the data types needed for internal variables can
change from one call to the next. Appropriate variables can be
created by applying %TYPE to the function's
arguments or result placeholders.
Row Typesnametable_name%ROWTYPE;
namecomposite_type_name;
A variable of a composite type is called a row
variable (or row-type variable). Such a variable
can hold a whole row of a SELECT or FOR
query result, so long as that query's column set matches the
declared type of the variable.
The individual fields of the row value
are accessed using the usual dot notation, for example
rowvar.field.
A row variable can be declared to have the same type as the rows of
an existing table or view, by using the
table_name%ROWTYPE
notation; or it can be declared by giving a composite type's name.
(Since every table has an associated composite type of the same name,
it actually does not matter in PostgreSQL whether you
write %ROWTYPE or not. But the form with
%ROWTYPE is more portable.)
Parameters to a function can be
composite types (complete table rows). In that case, the
corresponding identifier $n will be a row variable, and fields can
be selected from it, for example $1.user_id.
Here is an example of using composite types. table1
and table2 are existing tables having at least the
mentioned fields:
CREATE FUNCTION merge_fields(t_row table1) RETURNS text AS $$
DECLARE
t2_row table2%ROWTYPE;
BEGIN
SELECT * INTO t2_row FROM table2 WHERE ... ;
RETURN t_row.f1 || t2_row.f3 || t_row.f5 || t2_row.f7;
END;
$$ LANGUAGE plpgsql;
SELECT merge_fields(t.*) FROM table1 t WHERE ... ;
Record Typesname RECORD;
Record variables are similar to row-type variables, but they have no
predefined structure. They take on the actual row structure of the
row they are assigned during a SELECT or FOR command. The substructure
of a record variable can change each time it is assigned to.
A consequence of this is that until a record variable is first assigned
to, it has no substructure, and any attempt to access a
field in it will draw a run-time error.
Note that RECORD is not a true data type, only a placeholder.
One should also realize that when a PL/pgSQL
function is declared to return type record, this is not quite the
same concept as a record variable, even though such a function might
use a record variable to hold its result. In both cases the actual row
structure is unknown when the function is written, but for a function
returning record the actual structure is determined when the
calling query is parsed, whereas a record variable can change its row
structure on-the-fly.
Collation of PL/pgSQL Variablescollationin PL/pgSQL
When a PL/pgSQL function has one or more
parameters of collatable data types, a collation is identified for each
function call depending on the collations assigned to the actual
arguments, as described in . If a collation is
successfully identified (i.e., there are no conflicts of implicit
collations among the arguments) then all the collatable parameters are
treated as having that collation implicitly. This will affect the
behavior of collation-sensitive operations within the function.
For example, consider
CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
BEGIN
RETURN a < b;
END;
$$ LANGUAGE plpgsql;
SELECT less_than(text_field_1, text_field_2) FROM table1;
SELECT less_than(text_field_1, text_field_2 COLLATE "C") FROM table1;
The first use of less_than will use the common collation
of text_field_1 and text_field_2 for
the comparison, while the second use will use C collation.
Furthermore, the identified collation is also assumed as the collation of
any local variables that are of collatable types. Thus this function
would not work any differently if it were written as
CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
DECLARE
local_a text := a;
local_b text := b;
BEGIN
RETURN local_a < local_b;
END;
$$ LANGUAGE plpgsql;
If there are no parameters of collatable data types, or no common
collation can be identified for them, then parameters and local variables
use the default collation of their data type (which is usually the
database's default collation, but could be different for variables of
domain types).
A local variable of a collatable data type can have a different collation
associated with it by including the COLLATE option in its
declaration, for example
DECLARE
local_a text COLLATE "en_US";
This option overrides the collation that would otherwise be
given to the variable according to the rules above.
Also, of course explicit COLLATE clauses can be written inside
a function if it is desired to force a particular collation to be used in
a particular operation. For example,
CREATE FUNCTION less_than_c(a text, b text) RETURNS boolean AS $$
BEGIN
RETURN a < b COLLATE "C";
END;
$$ LANGUAGE plpgsql;
This overrides the collations associated with the table columns,
parameters, or local variables used in the expression, just as would
happen in a plain SQL command.
Expressions
All expressions used in PL/pgSQL
statements are processed using the server's main
SQL executor. For example, when you write
a PL/pgSQL statement like
IF expression THEN ...
PL/pgSQL will evaluate the expression by
feeding a query like
SELECT expression
to the main SQL engine. While forming the SELECT command,
any occurrences of PL/pgSQL variable names
are replaced by query parameters, as discussed in detail in
.
This allows the query plan for the SELECT to
be prepared just once and then reused for subsequent
evaluations with different values of the variables. Thus, what
really happens on first use of an expression is essentially a
PREPARE command. For example, if we have declared
two integer variables x and y, and we write
IF x < y THEN ...
what happens behind the scenes is equivalent to
PREPARE statement_name(integer, integer) AS SELECT $1 < $2;
and then this prepared statement is EXECUTEd for each
execution of the IF statement, with the current values
of the PL/pgSQL variables supplied as
parameter values. Normally these details are
not important to a PL/pgSQL user, but
they are useful to know when trying to diagnose a problem.
More information appears in .
Since an expression is converted to a
SELECT command, it can contain the same clauses
that an ordinary SELECT would, except that it
cannot include a top-level UNION,
INTERSECT, or EXCEPT clause.
Thus for example one could test whether a table is non-empty with
IF count(*) > 0 FROM my_table THEN ...
since the expression
between IF and THEN is parsed as
though it were SELECT count(*) > 0 FROM my_table.
The SELECT must produce a single column, and not
more than one row. (If it produces no rows, the result is taken as
NULL.)
Basic Statements
In this section and the following ones, we describe all the statement
types that are explicitly understood by
PL/pgSQL.
Anything not recognized as one of these statement types is presumed
to be an SQL command and is sent to the main database engine to execute,
as described in .
Assignment
An assignment of a value to a PL/pgSQL
variable is written as:
variable { := | = } expression;
As explained previously, the expression in such a statement is evaluated
by means of an SQL SELECT command sent to the main
database engine. The expression must yield a single value (possibly
a row value, if the variable is a row or record variable). The target
variable can be a simple variable (optionally qualified with a block
name), a field of a row or record target, or an element or slice of
an array target. Equal (=) can be
used instead of PL/SQL-compliant :=.
If the expression's result data type doesn't match the variable's
data type, the value will be coerced as though by an assignment cast
(see ). If no assignment cast is known
for the pair of data types involved, the PL/pgSQL
interpreter will attempt to convert the result value textually, that is
by applying the result type's output function followed by the variable
type's input function. Note that this could result in run-time errors
generated by the input function, if the string form of the result value
is not acceptable to the input function.
Examples:
tax := subtotal * 0.06;
my_record.user_id := 20;
my_array[j] := 20;
my_array[1:3] := array[1,2,3];
complex_array[n].realpart = 12.3;
Executing SQL Commands
In general, any SQL command that does not return rows can be executed
within a PL/pgSQL function just by writing
the command. For example, you could create and fill a table by writing
CREATE TABLE mytable (id int primary key, data text);
INSERT INTO mytable VALUES (1,'one'), (2,'two');
If the command does return rows (for example SELECT,
or INSERT/UPDATE/DELETE
with RETURNING), there are two ways to proceed.
When the command will return at most one row, or you only care about
the first row of output, write the command as usual but add
an INTO clause to capture the output, as described
in .
To process all of the output rows, write the command as the data
source for a FOR loop, as described in
.
Usually it is not sufficient just to execute statically-defined SQL
commands. Typically you'll want a command to use varying data values,
or even to vary in more fundamental ways such as by using different
table names at different times. Again, there are two ways to proceed
depending on the situation.
PL/pgSQL variable values can be
automatically inserted into optimizable SQL commands, which
are SELECT, INSERT,
UPDATE, DELETE,
MERGE, and certain
utility commands that incorporate one of these, such
as EXPLAIN and CREATE TABLE ... AS
SELECT. In these commands,
any PL/pgSQL variable name appearing
in the command text is replaced by a query parameter, and then the
current value of the variable is provided as the parameter value
at run time. This is exactly like the processing described earlier
for expressions; for details see .
When executing an optimizable SQL command in this way,
PL/pgSQL may cache and re-use the execution
plan for the command, as discussed in
.
Non-optimizable SQL commands (also called utility commands) are not
capable of accepting query parameters. So automatic substitution
of PL/pgSQL variables does not work in such
commands. To include non-constant text in a utility command executed
from PL/pgSQL, you must build the utility
command as a string and then EXECUTE it, as
discussed in .
EXECUTE must also be used if you want to modify
the command in some other way than supplying a data value, for example
by changing a table name.
Sometimes it is useful to evaluate an expression or SELECT
query but discard the result, for example when calling a function
that has side-effects but no useful result value. To do
this in PL/pgSQL, use the
PERFORM statement:
PERFORM query;
This executes query and discards the
result. Write the query the same
way you would write an SQL SELECT command, but replace the
initial keyword SELECT with PERFORM.
For WITH queries, use PERFORM and then
place the query in parentheses. (In this case, the query can only
return one row.)
PL/pgSQL variables will be
substituted into the query just as described above,
and the plan is cached in the same way. Also, the special variable
FOUND is set to true if the query produced at
least one row, or false if it produced no rows (see
).
One might expect that writing SELECT directly
would accomplish this result, but at
present the only accepted way to do it is
PERFORM. An SQL command that can return rows,
such as SELECT, will be rejected as an error
unless it has an INTO clause as discussed in the
next section.
An example:
PERFORM create_mv('cs_session_page_requests_mv', my_query);
Executing a Command with a Single-Row ResultSELECT INTOin PL/pgSQLRETURNING INTOin PL/pgSQL
The result of an SQL command yielding a single row (possibly of multiple
columns) can be assigned to a record variable, row-type variable, or list
of scalar variables. This is done by writing the base SQL command and
adding an INTO clause. For example,
SELECT select_expressions INTO STRICTtarget FROM ...;
INSERT ... RETURNING expressions INTO STRICTtarget;
UPDATE ... RETURNING expressions INTO STRICTtarget;
DELETE ... RETURNING expressions INTO STRICTtarget;
where target can be a record variable, a row
variable, or a comma-separated list of simple variables and
record/row fields.
PL/pgSQL variables will be
substituted into the rest of the command (that is, everything but the
INTO clause) just as described above,
and the plan is cached in the same way.
This works for SELECT,
INSERT/UPDATE/DELETE with
RETURNING, and certain utility commands
that return row sets, such as EXPLAIN.
Except for the INTO clause, the SQL command is the same
as it would be written outside PL/pgSQL.
Note that this interpretation of SELECT with INTO
is quite different from PostgreSQL's regular
SELECT INTO command, wherein the INTO
target is a newly created table. If you want to create a table from a
SELECT result inside a
PL/pgSQL function, use the syntax
CREATE TABLE ... AS SELECT.
If a row variable or a variable list is used as target,
the command's result columns
must exactly match the structure of the target as to number and data
types, or else a run-time error
occurs. When a record variable is the target, it automatically
configures itself to the row type of the command's result columns.
The INTO clause can appear almost anywhere in the SQL
command. Customarily it is written either just before or just after
the list of select_expressions in a
SELECT command, or at the end of the command for other
command types. It is recommended that you follow this convention
in case the PL/pgSQL parser becomes
stricter in future versions.
If STRICT is not specified in the INTO
clause, then target will be set to the first
row returned by the command, or to nulls if the command returned no rows.
(Note that the first row is not
well-defined unless you've used ORDER BY.) Any result rows
after the first row are discarded.
You can check the special FOUND variable (see
) to
determine whether a row was returned:
SELECT * INTO myrec FROM emp WHERE empname = myname;
IF NOT FOUND THEN
RAISE EXCEPTION 'employee % not found', myname;
END IF;
If the STRICT option is specified, the command must
return exactly one row or a run-time error will be reported, either
NO_DATA_FOUND (no rows) or TOO_MANY_ROWS
(more than one row). You can use an exception block if you wish
to catch the error, for example:
BEGIN
SELECT * INTO STRICT myrec FROM emp WHERE empname = myname;
EXCEPTION
WHEN NO_DATA_FOUND THEN
RAISE EXCEPTION 'employee % not found', myname;
WHEN TOO_MANY_ROWS THEN
RAISE EXCEPTION 'employee % not unique', myname;
END;
Successful execution of a command with STRICT
always sets FOUND to true.
For INSERT/UPDATE/DELETE with
RETURNING, PL/pgSQL reports
an error for more than one returned row, even when
STRICT is not specified. This is because there
is no option such as ORDER BY with which to determine
which affected row should be returned.
If print_strict_params is enabled for the function,
then when an error is thrown because the requirements
of STRICT are not met, the DETAIL part of
the error message will include information about the parameters
passed to the command.
You can change the print_strict_params
setting for all functions by setting
plpgsql.print_strict_params, though only subsequent
function compilations will be affected. You can also enable it
on a per-function basis by using a compiler option, for example:
CREATE FUNCTION get_userid(username text) RETURNS int
AS $$
#print_strict_params on
DECLARE
userid int;
BEGIN
SELECT users.userid INTO STRICT userid
FROM users WHERE users.username = get_userid.username;
RETURN userid;
END;
$$ LANGUAGE plpgsql;
On failure, this function might produce an error message such as
ERROR: query returned no rows
DETAIL: parameters: $1 = 'nosuchuser'
CONTEXT: PL/pgSQL function get_userid(text) line 6 at SQL statement
The STRICT option matches the behavior of
Oracle PL/SQL's SELECT INTO and related statements.
Executing Dynamic Commands
Oftentimes you will want to generate dynamic commands inside your
PL/pgSQL functions, that is, commands
that will involve different tables or different data types each
time they are executed. PL/pgSQL's
normal attempts to cache plans for commands (as discussed in
) will not work in such
scenarios. To handle this sort of problem, the
EXECUTE statement is provided:
EXECUTE command-string INTO STRICTtarget USING expression, ... ;
where command-string is an expression
yielding a string (of type text) containing the
command to be executed. The optional target
is a record variable, a row variable, or a comma-separated list of
simple variables and record/row fields, into which the results of
the command will be stored. The optional USING expressions
supply values to be inserted into the command.
No substitution of PL/pgSQL variables is done on the
computed command string. Any required variable values must be inserted
in the command string as it is constructed; or you can use parameters
as described below.
Also, there is no plan caching for commands executed via
EXECUTE. Instead, the command is always planned
each time the statement is run. Thus the command
string can be dynamically created within the function to perform
actions on different tables and columns.
The INTO clause specifies where the results of
an SQL command returning rows should be assigned. If a row variable
or variable list is provided, it must exactly match the structure
of the command's results; if a
record variable is provided, it will configure itself to match the
result structure automatically. If multiple rows are returned,
only the first will be assigned to the INTO
variable(s). If no rows are returned, NULL is assigned to the
INTO variable(s). If no INTO
clause is specified, the command results are discarded.
If the STRICT option is given, an error is reported
unless the command produces exactly one row.
The command string can use parameter values, which are referenced
in the command as $1, $2, etc.
These symbols refer to values supplied in the USING
clause. This method is often preferable to inserting data values
into the command string as text: it avoids run-time overhead of
converting the values to text and back, and it is much less prone
to SQL-injection attacks since there is no need for quoting or escaping.
An example is:
EXECUTE 'SELECT count(*) FROM mytable WHERE inserted_by = $1 AND inserted <= $2'
INTO c
USING checked_user, checked_date;
Note that parameter symbols can only be used for data values
— if you want to use dynamically determined table or column
names, you must insert them into the command string textually.
For example, if the preceding query needed to be done against a
dynamically selected table, you could do this:
EXECUTE 'SELECT count(*) FROM '
|| quote_ident(tabname)
|| ' WHERE inserted_by = $1 AND inserted <= $2'
INTO c
USING checked_user, checked_date;
A cleaner approach is to use format()'s %I
specification to insert table or column names with automatic quoting:
EXECUTE format('SELECT count(*) FROM %I '
'WHERE inserted_by = $1 AND inserted <= $2', tabname)
INTO c
USING checked_user, checked_date;
(This example relies on the SQL rule that string literals separated by a
newline are implicitly concatenated.)
Another restriction on parameter symbols is that they only work in
optimizable SQL commands
(SELECT, INSERT, UPDATE,
DELETE, MERGE, and certain commands containing one of these).
In other statement
types (generically called utility statements), you must insert
values textually even if they are just data values.
An EXECUTE with a simple constant command string and some
USING parameters, as in the first example above, is
functionally equivalent to just writing the command directly in
PL/pgSQL and allowing replacement of
PL/pgSQL variables to happen automatically.
The important difference is that EXECUTE will re-plan
the command on each execution, generating a plan that is specific
to the current parameter values; whereas
PL/pgSQL may otherwise create a generic plan
and cache it for re-use. In situations where the best plan depends
strongly on the parameter values, it can be helpful to use
EXECUTE to positively ensure that a generic plan is not
selected.
SELECT INTO is not currently supported within
EXECUTE; instead, execute a plain SELECT
command and specify INTO as part of the EXECUTE
itself.
The PL/pgSQLEXECUTE statement is not related to the
EXECUTE SQL
statement supported by the
PostgreSQL server. The server's
EXECUTE statement cannot be used directly within
PL/pgSQL functions (and is not needed).
Quoting Values in Dynamic Queriesquote_identuse in PL/pgSQLquote_literaluse in PL/pgSQLquote_nullableuse in PL/pgSQLformatuse in PL/pgSQL
When working with dynamic commands you will often have to handle escaping
of single quotes. The recommended method for quoting fixed text in your
function body is dollar quoting. (If you have legacy code that does
not use dollar quoting, please refer to the
overview in , which can save you
some effort when translating said code to a more reasonable scheme.)
Dynamic values require careful handling since they might contain
quote characters.
An example using format() (this assumes that you are
dollar quoting the function body so quote marks need not be doubled):
EXECUTE format('UPDATE tbl SET %I = $1 '
'WHERE key = $2', colname) USING newvalue, keyvalue;
It is also possible to call the quoting functions directly:
EXECUTE 'UPDATE tbl SET '
|| quote_ident(colname)
|| ' = '
|| quote_literal(newvalue)
|| ' WHERE key = '
|| quote_literal(keyvalue);
This example demonstrates the use of the
quote_ident and
quote_literal functions (see ). For safety, expressions containing column
or table identifiers should be passed through
quote_ident before insertion in a dynamic query.
Expressions containing values that should be literal strings in the
constructed command should be passed through quote_literal.
These functions take the appropriate steps to return the input text
enclosed in double or single quotes respectively, with any embedded
special characters properly escaped.
Because quote_literal is labeled
STRICT, it will always return null when called with a
null argument. In the above example, if newvalue or
keyvalue were null, the entire dynamic query string would
become null, leading to an error from EXECUTE.
You can avoid this problem by using the quote_nullable
function, which works the same as quote_literal except that
when called with a null argument it returns the string NULL.
For example,
EXECUTE 'UPDATE tbl SET '
|| quote_ident(colname)
|| ' = '
|| quote_nullable(newvalue)
|| ' WHERE key = '
|| quote_nullable(keyvalue);
If you are dealing with values that might be null, you should usually
use quote_nullable in place of quote_literal.
As always, care must be taken to ensure that null values in a query do
not deliver unintended results. For example the WHERE clause
'WHERE key = ' || quote_nullable(keyvalue)
will never succeed if keyvalue is null, because the
result of using the equality operator = with a null operand
is always null. If you wish null to work like an ordinary key value,
you would need to rewrite the above as
'WHERE key IS NOT DISTINCT FROM ' || quote_nullable(keyvalue)
(At present, IS NOT DISTINCT FROM is handled much less
efficiently than =, so don't do this unless you must.
See for
more information on nulls and IS DISTINCT.)
Note that dollar quoting is only useful for quoting fixed text.
It would be a very bad idea to try to write this example as:
EXECUTE 'UPDATE tbl SET '
|| quote_ident(colname)
|| ' = $$'
|| newvalue
|| '$$ WHERE key = '
|| quote_literal(keyvalue);
because it would break if the contents of newvalue
happened to contain $$. The same objection would
apply to any other dollar-quoting delimiter you might pick.
So, to safely quote text that is not known in advance, you
must use quote_literal,
quote_nullable, or quote_ident, as appropriate.
Dynamic SQL statements can also be safely constructed using the
format function (see ). For example:
EXECUTE format('UPDATE tbl SET %I = %L '
'WHERE key = %L', colname, newvalue, keyvalue);
%I is equivalent to quote_ident, and
%L is equivalent to quote_nullable.
The format function can be used in conjunction with
the USING clause:
EXECUTE format('UPDATE tbl SET %I = $1 WHERE key = $2', colname)
USING newvalue, keyvalue;
This form is better because the variables are handled in their native
data type format, rather than unconditionally converting them to
text and quoting them via %L. It is also more efficient.
A much larger example of a dynamic command and
EXECUTE can be seen in , which builds and executes a
CREATE FUNCTION command to define a new function.
Obtaining the Result Status
There are several ways to determine the effect of a command. The
first method is to use the GET DIAGNOSTICS
command, which has the form:
GET CURRENT DIAGNOSTICS variable { = | := } item , ... ;
This command allows retrieval of system status indicators.
CURRENT is a noise word (but see also GET STACKED
DIAGNOSTICS in ).
Each item is a key word identifying a status
value to be assigned to the specified variable
(which should be of the right data type to receive it). The currently
available status items are shown
in . Colon-equal
(:=) can be used instead of the SQL-standard =
token. An example:
GET DIAGNOSTICS integer_var = ROW_COUNT;
Available Diagnostics ItemsNameTypeDescriptionROW_COUNTbigintthe number of rows processed by the most
recent SQL commandPG_CONTEXTtextline(s) of text describing the current call stack
(see )PG_ROUTINE_OIDoidOID of the current function
The second method to determine the effects of a command is to check the
special variable named FOUND, which is of
type boolean. FOUND starts out
false within each PL/pgSQL function call.
It is set by each of the following types of statements:
A SELECT INTO statement sets
FOUND true if a row is assigned, false if no
row is returned.
A PERFORM statement sets FOUND
true if it produces (and discards) one or more rows, false if
no row is produced.
UPDATE, INSERT, DELETE,
and MERGE
statements set FOUND true if at least one
row is affected, false if no row is affected.
A FETCH statement sets FOUND
true if it returns a row, false if no row is returned.
A MOVE statement sets FOUND
true if it successfully repositions the cursor, false otherwise.
A FOR or FOREACH statement sets
FOUND true
if it iterates one or more times, else false.
FOUND is set this way when the
loop exits; inside the execution of the loop,
FOUND is not modified by the
loop statement, although it might be changed by the
execution of other statements within the loop body.
RETURN QUERY and RETURN QUERY
EXECUTE statements set FOUND
true if the query returns at least one row, false if no row
is returned.
Other PL/pgSQL statements do not change
the state of FOUND.
Note in particular that EXECUTE
changes the output of GET DIAGNOSTICS, but
does not change FOUND.
FOUND is a local variable within each
PL/pgSQL function; any changes to it
affect only the current function.
Doing Nothing At All
Sometimes a placeholder statement that does nothing is useful.
For example, it can indicate that one arm of an if/then/else
chain is deliberately empty. For this purpose, use the
NULL statement:
NULL;
For example, the following two fragments of code are equivalent:
BEGIN
y := x / 0;
EXCEPTION
WHEN division_by_zero THEN
NULL; -- ignore the error
END;
BEGIN
y := x / 0;
EXCEPTION
WHEN division_by_zero THEN -- ignore the error
END;
Which is preferable is a matter of taste.
In Oracle's PL/SQL, empty statement lists are not allowed, and so
NULL statements are required for situations
such as this. PL/pgSQL allows you to
just write nothing, instead.
Control Structures
Control structures are probably the most useful (and
important) part of PL/pgSQL. With
PL/pgSQL's control structures,
you can manipulate PostgreSQL data in a very
flexible and powerful way.
Returning from a Function
There are two commands available that allow you to return data
from a function: RETURN and RETURN
NEXT.
RETURN
RETURN expression;
RETURN with an expression terminates the
function and returns the value of
expression to the caller. This form
is used for PL/pgSQL functions that do
not return a set.
In a function that returns a scalar type, the expression's result will
automatically be cast into the function's return type as described for
assignments. But to return a composite (row) value, you must write an
expression delivering exactly the requested column set. This may
require use of explicit casting.
If you declared the function with output parameters, write just
RETURN with no expression. The current values
of the output parameter variables will be returned.
If you declared the function to return void, a
RETURN statement can be used to exit the function
early; but do not write an expression following
RETURN.
The return value of a function cannot be left undefined. If
control reaches the end of the top-level block of the function
without hitting a RETURN statement, a run-time
error will occur. This restriction does not apply to functions
with output parameters and functions returning void,
however. In those cases a RETURN statement is
automatically executed if the top-level block finishes.
Some examples:
-- functions returning a scalar type
RETURN 1 + 2;
RETURN scalar_var;
-- functions returning a composite type
RETURN composite_type_var;
RETURN (1, 2, 'three'::text); -- must cast columns to correct types
RETURN NEXT and RETURN QUERYRETURN NEXTin PL/pgSQLRETURN QUERYin PL/pgSQL
RETURN NEXT expression;
RETURN QUERY query;
RETURN QUERY EXECUTE command-string USING expression, ... ;
When a PL/pgSQL function is declared to return
SETOF sometype, the procedure
to follow is slightly different. In that case, the individual
items to return are specified by a sequence of RETURN
NEXT or RETURN QUERY commands, and
then a final RETURN command with no argument
is used to indicate that the function has finished executing.
RETURN NEXT can be used with both scalar and
composite data types; with a composite result type, an entire
table of results will be returned.
RETURN QUERY appends the results of executing
a query to the function's result set. RETURN
NEXT and RETURN QUERY can be freely
intermixed in a single set-returning function, in which case
their results will be concatenated.
RETURN NEXT and RETURN
QUERY do not actually return from the function —
they simply append zero or more rows to the function's result
set. Execution then continues with the next statement in the
PL/pgSQL function. As successive
RETURN NEXT or RETURN
QUERY commands are executed, the result set is built
up. A final RETURN, which should have no
argument, causes control to exit the function (or you can just
let control reach the end of the function).
RETURN QUERY has a variant
RETURN QUERY EXECUTE, which specifies the
query to be executed dynamically. Parameter expressions can
be inserted into the computed query string via USING,
in just the same way as in the EXECUTE command.
If you declared the function with output parameters, write just
RETURN NEXT with no expression. On each
execution, the current values of the output parameter
variable(s) will be saved for eventual return as a row of the
result. Note that you must declare the function as returning
SETOF record when there are multiple output
parameters, or SETOF sometype
when there is just one output parameter of type
sometype, in order to create a set-returning
function with output parameters.
Here is an example of a function using RETURN
NEXT:
CREATE TABLE foo (fooid INT, foosubid INT, fooname TEXT);
INSERT INTO foo VALUES (1, 2, 'three');
INSERT INTO foo VALUES (4, 5, 'six');
CREATE OR REPLACE FUNCTION get_all_foo() RETURNS SETOF foo AS
$BODY$
DECLARE
r foo%rowtype;
BEGIN
FOR r IN
SELECT * FROM foo WHERE fooid > 0
LOOP
-- can do some processing here
RETURN NEXT r; -- return current row of SELECT
END LOOP;
RETURN;
END;
$BODY$
LANGUAGE plpgsql;
SELECT * FROM get_all_foo();
Here is an example of a function using RETURN
QUERY:
CREATE FUNCTION get_available_flightid(date) RETURNS SETOF integer AS
$BODY$
BEGIN
RETURN QUERY SELECT flightid
FROM flight
WHERE flightdate >= $1
AND flightdate < ($1 + 1);
-- Since execution is not finished, we can check whether rows were returned
-- and raise exception if not.
IF NOT FOUND THEN
RAISE EXCEPTION 'No flight at %.', $1;
END IF;
RETURN;
END;
$BODY$
LANGUAGE plpgsql;
-- Returns available flights or raises exception if there are no
-- available flights.
SELECT * FROM get_available_flightid(CURRENT_DATE);
The current implementation of RETURN NEXT
and RETURN QUERY stores the entire result set
before returning from the function, as discussed above. That
means that if a PL/pgSQL function produces a
very large result set, performance might be poor: data will be
written to disk to avoid memory exhaustion, but the function
itself will not return until the entire result set has been
generated. A future version of PL/pgSQL might
allow users to define set-returning functions
that do not have this limitation. Currently, the point at
which data begins being written to disk is controlled by the
configuration variable. Administrators who have sufficient
memory to store larger result sets in memory should consider
increasing this parameter.
Returning from a Procedure
A procedure does not have a return value. A procedure can therefore end
without a RETURN statement. If you wish to use
a RETURN statement to exit the code early, write
just RETURN with no expression.
If the procedure has output parameters, the final values of the output
parameter variables will be returned to the caller.
Calling a Procedure
A PL/pgSQL function, procedure,
or DO block can call a procedure
using CALL. Output parameters are handled
differently from the way that CALL works in plain
SQL. Each OUT or INOUT
parameter of the procedure must
correspond to a variable in the CALL statement, and
whatever the procedure returns is assigned back to that variable after
it returns. For example:
CREATE PROCEDURE triple(INOUT x int)
LANGUAGE plpgsql
AS $$
BEGIN
x := x * 3;
END;
$$;
DO $$
DECLARE myvar int := 5;
BEGIN
CALL triple(myvar);
RAISE NOTICE 'myvar = %', myvar; -- prints 15
END;
$$;
The variable corresponding to an output parameter can be a simple
variable or a field of a composite-type variable. Currently,
it cannot be an element of an array.
ConditionalsIF and CASE statements let you execute
alternative commands based on certain conditions.
PL/pgSQL has three forms of IF:
IF ... THEN ... END IFIF ... THEN ... ELSE ... END IFIF ... THEN ... ELSIF ... THEN ... ELSE ... END IF
and two forms of CASE:
CASE ... WHEN ... THEN ... ELSE ... END CASECASE WHEN ... THEN ... ELSE ... END CASEIF-THEN
IF boolean-expression THEN
statements
END IF;
IF-THEN statements are the simplest form of
IF. The statements between
THEN and END IF will be
executed if the condition is true. Otherwise, they are
skipped.
Example:
IF v_user_id <> 0 THEN
UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;
IF-THEN-ELSE
IF boolean-expression THEN
statements
ELSE
statements
END IF;
IF-THEN-ELSE statements add to
IF-THEN by letting you specify an
alternative set of statements that should be executed if the
condition is not true. (Note this includes the case where the
condition evaluates to NULL.)
Examples:
IF parentid IS NULL OR parentid = ''
THEN
RETURN fullname;
ELSE
RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;
IF v_count > 0 THEN
INSERT INTO users_count (count) VALUES (v_count);
RETURN 't';
ELSE
RETURN 'f';
END IF;
IF-THEN-ELSIF
IF boolean-expression THEN
statements ELSIF boolean-expression THEN
statements ELSIF boolean-expression THEN
statements
...
ELSE
statements
END IF;
Sometimes there are more than just two alternatives.
IF-THEN-ELSIF provides a convenient
method of checking several alternatives in turn.
The IF conditions are tested successively
until the first one that is true is found. Then the
associated statement(s) are executed, after which control
passes to the next statement after END IF.
(Any subsequent IF conditions are not
tested.) If none of the IF conditions is true,
then the ELSE block (if any) is executed.
Here is an example:
IF number = 0 THEN
result := 'zero';
ELSIF number > 0 THEN
result := 'positive';
ELSIF number < 0 THEN
result := 'negative';
ELSE
-- hmm, the only other possibility is that number is null
result := 'NULL';
END IF;
The key word ELSIF can also be spelled
ELSEIF.
An alternative way of accomplishing the same task is to nest
IF-THEN-ELSE statements, as in the
following example:
IF demo_row.sex = 'm' THEN
pretty_sex := 'man';
ELSE
IF demo_row.sex = 'f' THEN
pretty_sex := 'woman';
END IF;
END IF;
However, this method requires writing a matching END IF
for each IF, so it is much more cumbersome than
using ELSIF when there are many alternatives.
Simple CASE
CASE search-expression
WHEN expression, expression ... THEN
statements WHEN expression, expression ... THEN
statements
... ELSE
statements
END CASE;
The simple form of CASE provides conditional execution
based on equality of operands. The search-expression
is evaluated (once) and successively compared to each
expression in the WHEN clauses.
If a match is found, then the corresponding
statements are executed, and then control
passes to the next statement after END CASE. (Subsequent
WHEN expressions are not evaluated.) If no match is
found, the ELSEstatements are
executed; but if ELSE is not present, then a
CASE_NOT_FOUND exception is raised.
Here is a simple example:
CASE x
WHEN 1, 2 THEN
msg := 'one or two';
ELSE
msg := 'other value than one or two';
END CASE;
Searched CASE
CASE
WHEN boolean-expression THEN
statements WHEN boolean-expression THEN
statements
... ELSE
statements
END CASE;
The searched form of CASE provides conditional execution
based on truth of Boolean expressions. Each WHEN clause's
boolean-expression is evaluated in turn,
until one is found that yields true. Then the
corresponding statements are executed, and
then control passes to the next statement after END CASE.
(Subsequent WHEN expressions are not evaluated.)
If no true result is found, the ELSEstatements are executed;
but if ELSE is not present, then a
CASE_NOT_FOUND exception is raised.
Here is an example:
CASE
WHEN x BETWEEN 0 AND 10 THEN
msg := 'value is between zero and ten';
WHEN x BETWEEN 11 AND 20 THEN
msg := 'value is between eleven and twenty';
END CASE;
This form of CASE is entirely equivalent to
IF-THEN-ELSIF, except for the rule that reaching
an omitted ELSE clause results in an error rather
than doing nothing.
Simple Loopsloopin PL/pgSQL
With the LOOP, EXIT,
CONTINUE, WHILE, FOR,
and FOREACH statements, you can arrange for your
PL/pgSQL function to repeat a series of commands.
LOOP <<label>>
LOOP
statements
END LOOP label;
LOOP defines an unconditional loop that is repeated
indefinitely until terminated by an EXIT or
RETURN statement. The optional
label can be used by EXIT
and CONTINUE statements within nested loops to
specify which loop those statements refer to.
EXITEXITin PL/pgSQL
EXIT label WHEN boolean-expression;
If no label is given, the innermost
loop is terminated and the statement following END
LOOP is executed next. If label
is given, it must be the label of the current or some outer
level of nested loop or block. Then the named loop or block is
terminated and control continues with the statement after the
loop's/block's corresponding END.
If WHEN is specified, the loop exit occurs only if
boolean-expression is true. Otherwise, control passes
to the statement after EXIT.
EXIT can be used with all types of loops; it is
not limited to use with unconditional loops.
When used with a
BEGIN block, EXIT passes
control to the next statement after the end of the block.
Note that a label must be used for this purpose; an unlabeled
EXIT is never considered to match a
BEGIN block. (This is a change from
pre-8.4 releases of PostgreSQL, which
would allow an unlabeled EXIT to match
a BEGIN block.)
Examples:
LOOP
-- some computations
IF count > 0 THEN
EXIT; -- exit loop
END IF;
END LOOP;
LOOP
-- some computations
EXIT WHEN count > 0; -- same result as previous example
END LOOP;
<<ablock>>
BEGIN
-- some computations
IF stocks > 100000 THEN
EXIT ablock; -- causes exit from the BEGIN block
END IF;
-- computations here will be skipped when stocks > 100000
END;
CONTINUECONTINUEin PL/pgSQL
CONTINUE label WHEN boolean-expression;
If no label is given, the next iteration of
the innermost loop is begun. That is, all statements remaining
in the loop body are skipped, and control returns
to the loop control expression (if any) to determine whether
another loop iteration is needed.
If label is present, it
specifies the label of the loop whose execution will be
continued.
If WHEN is specified, the next iteration of the
loop is begun only if boolean-expression is
true. Otherwise, control passes to the statement after
CONTINUE.
CONTINUE can be used with all types of loops; it
is not limited to use with unconditional loops.
Examples:
LOOP
-- some computations
EXIT WHEN count > 100;
CONTINUE WHEN count < 50;
-- some computations for count IN [50 .. 100]
END LOOP;
WHILEWHILEin PL/pgSQL <<label>>
WHILE boolean-expression LOOP
statements
END LOOP label;
The WHILE statement repeats a
sequence of statements so long as the
boolean-expression
evaluates to true. The expression is checked just before
each entry to the loop body.
For example:
WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
-- some computations here
END LOOP;
WHILE NOT done LOOP
-- some computations here
END LOOP;
FOR (Integer Variant) <<label>>
FOR name IN REVERSE expression .. expression BY expression LOOP
statements
END LOOP label;
This form of FOR creates a loop that iterates over a range
of integer values. The variable
name is automatically defined as type
integer and exists only inside the loop (any existing
definition of the variable name is ignored within the loop).
The two expressions giving
the lower and upper bound of the range are evaluated once when entering
the loop. If the BY clause isn't specified the iteration
step is 1, otherwise it's the value specified in the BY
clause, which again is evaluated once on loop entry.
If REVERSE is specified then the step value is
subtracted, rather than added, after each iteration.
Some examples of integer FOR loops:
FOR i IN 1..10 LOOP
-- i will take on the values 1,2,3,4,5,6,7,8,9,10 within the loop
END LOOP;
FOR i IN REVERSE 10..1 LOOP
-- i will take on the values 10,9,8,7,6,5,4,3,2,1 within the loop
END LOOP;
FOR i IN REVERSE 10..1 BY 2 LOOP
-- i will take on the values 10,8,6,4,2 within the loop
END LOOP;
If the lower bound is greater than the upper bound (or less than,
in the REVERSE case), the loop body is not
executed at all. No error is raised.
If a label is attached to the
FOR loop then the integer loop variable can be
referenced with a qualified name, using that
label.
Looping through Query Results
Using a different type of FOR loop, you can iterate through
the results of a query and manipulate that data
accordingly. The syntax is:
<<label>>
FOR target IN query LOOP
statements
END LOOP label;
The target is a record variable, row variable,
or comma-separated list of scalar variables.
The target is successively assigned each row
resulting from the query and the loop body is
executed for each row. Here is an example:
CREATE FUNCTION refresh_mviews() RETURNS integer AS $$
DECLARE
mviews RECORD;
BEGIN
RAISE NOTICE 'Refreshing all materialized views...';
FOR mviews IN
SELECT n.nspname AS mv_schema,
c.relname AS mv_name,
pg_catalog.pg_get_userbyid(c.relowner) AS owner
FROM pg_catalog.pg_class c
LEFT JOIN pg_catalog.pg_namespace n ON (n.oid = c.relnamespace)
WHERE c.relkind = 'm'
ORDER BY 1
LOOP
-- Now "mviews" has one record with information about the materialized view
RAISE NOTICE 'Refreshing materialized view %.% (owner: %)...',
quote_ident(mviews.mv_schema),
quote_ident(mviews.mv_name),
quote_ident(mviews.owner);
EXECUTE format('REFRESH MATERIALIZED VIEW %I.%I', mviews.mv_schema, mviews.mv_name);
END LOOP;
RAISE NOTICE 'Done refreshing materialized views.';
RETURN 1;
END;
$$ LANGUAGE plpgsql;
If the loop is terminated by an EXIT statement, the last
assigned row value is still accessible after the loop.
The query used in this type of FOR
statement can be any SQL command that returns rows to the caller:
SELECT is the most common case,
but you can also use INSERT, UPDATE, or
DELETE with a RETURNING clause. Some utility
commands such as EXPLAIN will work too.
PL/pgSQL variables are replaced by query parameters,
and the query plan is cached for possible re-use, as discussed in
detail in and
.
The FOR-IN-EXECUTE statement is another way to iterate over
rows:
<<label>>
FOR target IN EXECUTE text_expression USING expression, ... LOOP
statements
END LOOP label;
This is like the previous form, except that the source query
is specified as a string expression, which is evaluated and replanned
on each entry to the FOR loop. This allows the programmer to
choose the speed of a preplanned query or the flexibility of a dynamic
query, just as with a plain EXECUTE statement.
As with EXECUTE, parameter values can be inserted
into the dynamic command via USING.
Another way to specify the query whose results should be iterated
through is to declare it as a cursor. This is described in
.
Looping through Arrays
The FOREACH loop is much like a FOR loop,
but instead of iterating through the rows returned by an SQL query,
it iterates through the elements of an array value.
(In general, FOREACH is meant for looping through
components of a composite-valued expression; variants for looping
through composites besides arrays may be added in future.)
The FOREACH statement to loop over an array is:
<<label>>
FOREACH target SLICE number IN ARRAY expression LOOP
statements
END LOOP label;
Without SLICE, or if SLICE 0 is specified,
the loop iterates through individual elements of the array produced
by evaluating the expression.
The target variable is assigned each
element value in sequence, and the loop body is executed for each element.
Here is an example of looping through the elements of an integer
array:
CREATE FUNCTION sum(int[]) RETURNS int8 AS $$
DECLARE
s int8 := 0;
x int;
BEGIN
FOREACH x IN ARRAY $1
LOOP
s := s + x;
END LOOP;
RETURN s;
END;
$$ LANGUAGE plpgsql;
The elements are visited in storage order, regardless of the number of
array dimensions. Although the target is
usually just a single variable, it can be a list of variables when
looping through an array of composite values (records). In that case,
for each array element, the variables are assigned from successive
columns of the composite value.
With a positive SLICE value, FOREACH
iterates through slices of the array rather than single elements.
The SLICE value must be an integer constant not larger
than the number of dimensions of the array. The
target variable must be an array,
and it receives successive slices of the array value, where each slice
is of the number of dimensions specified by SLICE.
Here is an example of iterating through one-dimensional slices:
CREATE FUNCTION scan_rows(int[]) RETURNS void AS $$
DECLARE
x int[];
BEGIN
FOREACH x SLICE 1 IN ARRAY $1
LOOP
RAISE NOTICE 'row = %', x;
END LOOP;
END;
$$ LANGUAGE plpgsql;
SELECT scan_rows(ARRAY[[1,2,3],[4,5,6],[7,8,9],[10,11,12]]);
NOTICE: row = {1,2,3}
NOTICE: row = {4,5,6}
NOTICE: row = {7,8,9}
NOTICE: row = {10,11,12}
Trapping Errorsexceptionsin PL/pgSQL
By default, any error occurring in a PL/pgSQL
function aborts execution of the function and the
surrounding transaction. You can trap errors and recover
from them by using a BEGIN block with an
EXCEPTION clause. The syntax is an extension of the
normal syntax for a BEGIN block:
<<label>> DECLARE
declarations
BEGIN
statements
EXCEPTION
WHEN condition OR condition ... THEN
handler_statements WHEN condition OR condition ... THEN
handler_statements
...
END;
If no error occurs, this form of block simply executes all the
statements, and then control passes
to the next statement after END. But if an error
occurs within the statements, further
processing of the statements is
abandoned, and control passes to the EXCEPTION list.
The list is searched for the first condition
matching the error that occurred. If a match is found, the
corresponding handler_statements are
executed, and then control passes to the next statement after
END. If no match is found, the error propagates out
as though the EXCEPTION clause were not there at all:
the error can be caught by an enclosing block with
EXCEPTION, or if there is none it aborts processing
of the function.
The condition names can be any of
those shown in . A category
name matches any error within its category. The special
condition name OTHERS matches every error type except
QUERY_CANCELED and ASSERT_FAILURE.
(It is possible, but often unwise, to trap those two error types
by name.) Condition names are
not case-sensitive. Also, an error condition can be specified
by SQLSTATE code; for example these are equivalent:
WHEN division_by_zero THEN ...
WHEN SQLSTATE '22012' THEN ...
If a new error occurs within the selected
handler_statements, it cannot be caught
by this EXCEPTION clause, but is propagated out.
A surrounding EXCEPTION clause could catch it.
When an error is caught by an EXCEPTION clause,
the local variables of the PL/pgSQL function
remain as they were when the error occurred, but all changes
to persistent database state within the block are rolled back.
As an example, consider this fragment:
INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
BEGIN
UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
x := x + 1;
y := x / 0;
EXCEPTION
WHEN division_by_zero THEN
RAISE NOTICE 'caught division_by_zero';
RETURN x;
END;
When control reaches the assignment to y, it will
fail with a division_by_zero error. This will be caught by
the EXCEPTION clause. The value returned in the
RETURN statement will be the incremented value of
x, but the effects of the UPDATE command will
have been rolled back. The INSERT command preceding the
block is not rolled back, however, so the end result is that the database
contains Tom Jones not Joe Jones.
A block containing an EXCEPTION clause is significantly
more expensive to enter and exit than a block without one. Therefore,
don't use EXCEPTION without need.
Exceptions with UPDATE/INSERT
This example uses exception handling to perform either
UPDATE or INSERT, as appropriate. It is
recommended that applications use INSERT with
ON CONFLICT DO UPDATE rather than actually using
this pattern. This example serves primarily to illustrate use of
PL/pgSQL control flow structures:
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
-- first try to update the key
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
-- not there, so try to insert the key
-- if someone else inserts the same key concurrently,
-- we could get a unique-key failure
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- Do nothing, and loop to try the UPDATE again.
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
This coding assumes the unique_violation error is caused by
the INSERT, and not by, say, an INSERT in a
trigger function on the table. It might also misbehave if there is
more than one unique index on the table, since it will retry the
operation regardless of which index caused the error.
More safety could be had by using the
features discussed next to check that the trapped error was the one
expected.
Obtaining Information about an Error
Exception handlers frequently need to identify the specific error that
occurred. There are two ways to get information about the current
exception in PL/pgSQL: special variables and the
GET STACKED DIAGNOSTICS command.
Within an exception handler, the special variable
SQLSTATE contains the error code that corresponds to
the exception that was raised (refer to
for a list of possible error codes). The special variable
SQLERRM contains the error message associated with the
exception. These variables are undefined outside exception handlers.
Within an exception handler, one may also retrieve
information about the current exception by using the
GET STACKED DIAGNOSTICS command, which has the form:
GET STACKED DIAGNOSTICS variable { = | := } item , ... ;
Each item is a key word identifying a status
value to be assigned to the specified variable
(which should be of the right data type to receive it). The currently
available status items are shown
in .
Error Diagnostics ItemsNameTypeDescriptionRETURNED_SQLSTATEtextthe SQLSTATE error code of the exceptionCOLUMN_NAMEtextthe name of the column related to exceptionCONSTRAINT_NAMEtextthe name of the constraint related to exceptionPG_DATATYPE_NAMEtextthe name of the data type related to exceptionMESSAGE_TEXTtextthe text of the exception's primary messageTABLE_NAMEtextthe name of the table related to exceptionSCHEMA_NAMEtextthe name of the schema related to exceptionPG_EXCEPTION_DETAILtextthe text of the exception's detail message, if anyPG_EXCEPTION_HINTtextthe text of the exception's hint message, if anyPG_EXCEPTION_CONTEXTtextline(s) of text describing the call stack at the time of the
exception (see )
If the exception did not set a value for an item, an empty string
will be returned.
Here is an example:
DECLARE
text_var1 text;
text_var2 text;
text_var3 text;
BEGIN
-- some processing which might cause an exception
...
EXCEPTION WHEN OTHERS THEN
GET STACKED DIAGNOSTICS text_var1 = MESSAGE_TEXT,
text_var2 = PG_EXCEPTION_DETAIL,
text_var3 = PG_EXCEPTION_HINT;
END;
Obtaining Execution Location Information
The GET DIAGNOSTICS command, previously described
in , retrieves information
about current execution state (whereas the GET STACKED
DIAGNOSTICS command discussed above reports information about
the execution state as of a previous error). Its PG_CONTEXT
status item is useful for identifying the current execution
location. PG_CONTEXT returns a text string with line(s)
of text describing the call stack. The first line refers to the current
function and currently executing GET DIAGNOSTICS
command. The second and any subsequent lines refer to calling functions
further up the call stack. For example:
CREATE OR REPLACE FUNCTION outer_func() RETURNS integer AS $$
BEGIN
RETURN inner_func();
END;
$$ LANGUAGE plpgsql;
CREATE OR REPLACE FUNCTION inner_func() RETURNS integer AS $$
DECLARE
stack text;
BEGIN
GET DIAGNOSTICS stack = PG_CONTEXT;
RAISE NOTICE E'--- Call Stack ---\n%', stack;
RETURN 1;
END;
$$ LANGUAGE plpgsql;
SELECT outer_func();
NOTICE: --- Call Stack ---
PL/pgSQL function inner_func() line 5 at GET DIAGNOSTICS
PL/pgSQL function outer_func() line 3 at RETURN
CONTEXT: PL/pgSQL function outer_func() line 3 at RETURN
outer_func
------------
1
(1 row)
GET STACKED DIAGNOSTICS ... PG_EXCEPTION_CONTEXT
returns the same sort of stack trace, but describing the location
at which an error was detected, rather than the current location.
Cursorscursorin PL/pgSQL
Rather than executing a whole query at once, it is possible to set
up a cursor that encapsulates the query, and then read
the query result a few rows at a time. One reason for doing this is
to avoid memory overrun when the result contains a large number of
rows. (However, PL/pgSQL users do not normally need
to worry about that, since FOR loops automatically use a cursor
internally to avoid memory problems.) A more interesting usage is to
return a reference to a cursor that a function has created, allowing the
caller to read the rows. This provides an efficient way to return
large row sets from functions.
Declaring Cursor Variables
All access to cursors in PL/pgSQL goes through
cursor variables, which are always of the special data type
refcursor. One way to create a cursor variable
is just to declare it as a variable of type refcursor.
Another way is to use the cursor declaration syntax,
which in general is:
name NO SCROLL CURSOR ( arguments ) FOR query;
(FOR can be replaced by IS for
Oracle compatibility.)
If SCROLL is specified, the cursor will be capable of
scrolling backward; if NO SCROLL is specified, backward
fetches will be rejected; if neither specification appears, it is
query-dependent whether backward fetches will be allowed.
arguments, if specified, is a
comma-separated list of pairs namedatatype that define names to be
replaced by parameter values in the given query. The actual
values to substitute for these names will be specified later,
when the cursor is opened.
Some examples:
DECLARE
curs1 refcursor;
curs2 CURSOR FOR SELECT * FROM tenk1;
curs3 CURSOR (key integer) FOR SELECT * FROM tenk1 WHERE unique1 = key;
All three of these variables have the data type refcursor,
but the first can be used with any query, while the second has
a fully specified query already bound to it, and the last
has a parameterized query bound to it. (key will be
replaced by an integer parameter value when the cursor is opened.)
The variable curs1
is said to be unbound since it is not bound to
any particular query.
The SCROLL option cannot be used when the cursor's
query uses FOR UPDATE/SHARE. Also, it is
best to use NO SCROLL with a query that involves
volatile functions. The implementation of SCROLL
assumes that re-reading the query's output will give consistent
results, which a volatile function might not do.
Opening Cursors
Before a cursor can be used to retrieve rows, it must be
opened. (This is the equivalent action to the SQL
command DECLARE
CURSOR.)
PL/pgSQL has
three forms of the OPEN statement, two of which use unbound
cursor variables while the third uses a bound cursor variable.
Bound cursor variables can also be used without explicitly opening the cursor,
via the FOR statement described in
.
A FOR loop will open the cursor and then
close it again when the loop completes.
portalin PL/pgSQL
Opening a cursor involves creating a server-internal data structure
called a portal, which holds the execution
state for the cursor's query. A portal has a name, which must be
unique within the session for the duration of the portal's existence.
By default, PL/pgSQL will assign a unique
name to each portal it creates. However, if you assign a non-null
string value to a cursor variable, that string will be used as its
portal name. This feature can be used as described in
.
OPEN FORquery
OPEN unbound_cursorvar NO SCROLL FOR query;
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor variable (that is, as a simple
refcursor variable). The query must be a
SELECT, or something else that returns rows
(such as EXPLAIN). The query
is treated in the same way as other SQL commands in
PL/pgSQL: PL/pgSQL
variable names are substituted, and the query plan is cached for
possible reuse. When a PL/pgSQL
variable is substituted into the cursor query, the value that is
substituted is the one it has at the time of the OPEN;
subsequent changes to the variable will not affect the cursor's
behavior.
The SCROLL and NO SCROLL
options have the same meanings as for a bound cursor.
An example:
OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;
OPEN FOR EXECUTE
OPEN unbound_cursorvar NO SCROLL FOR EXECUTE query_string USING expression, ... ;
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor variable (that is, as a simple
refcursor variable). The query is specified as a string
expression, in the same way as in the EXECUTE
command. As usual, this gives flexibility so the query plan can vary
from one run to the next (see ),
and it also means that variable substitution is not done on the
command string. As with EXECUTE, parameter values
can be inserted into the dynamic command via
format() and USING.
The SCROLL and
NO SCROLL options have the same meanings as for a bound
cursor.
An example:
OPEN curs1 FOR EXECUTE format('SELECT * FROM %I WHERE col1 = $1',tabname) USING keyvalue;
In this example, the table name is inserted into the query via
format(). The comparison value for col1
is inserted via a USING parameter, so it needs
no quoting.
Opening a Bound Cursor
OPEN bound_cursorvar ( argument_name := argument_value, ... ) ;
This form of OPEN is used to open a cursor
variable whose query was bound to it when it was declared. The
cursor cannot be open already. A list of actual argument value
expressions must appear if and only if the cursor was declared to
take arguments. These values will be substituted in the query.
The query plan for a bound cursor is always considered cacheable;
there is no equivalent of EXECUTE in this case.
Notice that SCROLL and NO SCROLL cannot be
specified in OPEN, as the cursor's scrolling
behavior was already determined.
Argument values can be passed using either positional
or named notation. In positional
notation, all arguments are specified in order. In named notation,
each argument's name is specified using := to
separate it from the argument expression. Similar to calling
functions, described in , it
is also allowed to mix positional and named notation.
Examples (these use the cursor declaration examples above):
OPEN curs2;
OPEN curs3(42);
OPEN curs3(key := 42);
Because variable substitution is done on a bound cursor's query,
there are really two ways to pass values into the cursor: either
with an explicit argument to OPEN, or implicitly by
referencing a PL/pgSQL variable in the query.
However, only variables declared before the bound cursor was
declared will be substituted into it. In either case the value to
be passed is determined at the time of the OPEN.
For example, another way to get the same effect as the
curs3 example above is
DECLARE
key integer;
curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key;
BEGIN
key := 42;
OPEN curs4;
Using Cursors
Once a cursor has been opened, it can be manipulated with the
statements described here.
These manipulations need not occur in the same function that
opened the cursor to begin with. You can return a refcursor
value out of a function and let the caller operate on the cursor.
(Internally, a refcursor value is simply the string name
of the portal containing the active query for the cursor. This name
can be passed around, assigned to other refcursor variables,
and so on, without disturbing the portal.)
All portals are implicitly closed at transaction end. Therefore
a refcursor value is usable to reference an open cursor
only until the end of the transaction.
FETCH
FETCH direction { FROM | IN } cursor INTO target;
FETCH retrieves the next row from the
cursor into a target, which might be a row variable, a record
variable, or a comma-separated list of simple variables, just like
SELECT INTO. If there is no next row, the
target is set to NULL(s). As with SELECT
INTO, the special variable FOUND can
be checked to see whether a row was obtained or not.
The direction clause can be any of the
variants allowed in the SQL
command except the ones that can fetch
more than one row; namely, it can be
NEXT,
PRIOR,
FIRST,
LAST,
ABSOLUTEcount,
RELATIVEcount,
FORWARD, or
BACKWARD.
Omitting direction is the same
as specifying NEXT.
In the forms using a count,
the count can be any integer-valued
expression (unlike the SQL FETCH command,
which only allows an integer constant).
direction values that require moving
backward are likely to fail unless the cursor was declared or opened
with the SCROLL option.
cursor must be the name of a refcursor
variable that references an open cursor portal.
Examples:
FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo, bar, baz;
FETCH LAST FROM curs3 INTO x, y;
FETCH RELATIVE -2 FROM curs4 INTO x;
MOVE
MOVE direction { FROM | IN } cursor;
MOVE repositions a cursor without retrieving
any data. MOVE works exactly like the
FETCH command, except it only repositions the
cursor and does not return the row moved to. As with SELECT
INTO, the special variable FOUND can
be checked to see whether there was a next row to move to.
Examples:
MOVE curs1;
MOVE LAST FROM curs3;
MOVE RELATIVE -2 FROM curs4;
MOVE FORWARD 2 FROM curs4;
UPDATE/DELETE WHERE CURRENT OF
UPDATE table SET ... WHERE CURRENT OF cursor;
DELETE FROM table WHERE CURRENT OF cursor;
When a cursor is positioned on a table row, that row can be updated
or deleted using the cursor to identify the row. There are
restrictions on what the cursor's query can be (in particular,
no grouping) and it's best to use FOR UPDATE in the
cursor. For more information see the
reference page.
An example:
UPDATE foo SET dataval = myval WHERE CURRENT OF curs1;
CLOSE
CLOSE cursor;
CLOSE closes the portal underlying an open
cursor. This can be used to release resources earlier than end of
transaction, or to free up the cursor variable to be opened again.
An example:
CLOSE curs1;
Returning CursorsPL/pgSQL functions can return cursors to the
caller. This is useful to return multiple rows or columns,
especially with very large result sets. To do this, the function
opens the cursor and returns the cursor name to the caller (or simply
opens the cursor using a portal name specified by or otherwise known
to the caller). The caller can then fetch rows from the cursor. The
cursor can be closed by the caller, or it will be closed automatically
when the transaction closes.
The portal name used for a cursor can be specified by the
programmer or automatically generated. To specify a portal name,
simply assign a string to the refcursor variable before
opening it. The string value of the refcursor variable
will be used by OPEN as the name of the underlying portal.
However, if the refcursor variable's value is null
(as it will be by default), then
OPEN automatically generates a name that does not
conflict with any existing portal, and assigns it to the
refcursor variable.
Prior to PostgreSQL 16, bound cursor
variables were initialized to contain their own names, rather
than being left as null, so that the underlying portal name would
be the same as the cursor variable's name by default. This was
changed because it created too much risk of conflicts between
similarly-named cursors in different functions.
The following example shows one way a cursor name can be supplied by
the caller:
CREATE TABLE test (col text);
INSERT INTO test VALUES ('123');
CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS '
BEGIN
OPEN $1 FOR SELECT col FROM test;
RETURN $1;
END;
' LANGUAGE plpgsql;
BEGIN;
SELECT reffunc('funccursor');
FETCH ALL IN funccursor;
COMMIT;
The following example uses automatic cursor name generation:
CREATE FUNCTION reffunc2() RETURNS refcursor AS '
DECLARE
ref refcursor;
BEGIN
OPEN ref FOR SELECT col FROM test;
RETURN ref;
END;
' LANGUAGE plpgsql;
-- need to be in a transaction to use cursors.
BEGIN;
SELECT reffunc2();
reffunc2
--------------------
<unnamed cursor 1>
(1 row)
FETCH ALL IN "<unnamed cursor 1>";
COMMIT;
The following example shows one way to return multiple cursors
from a single function:
CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$
BEGIN
OPEN $1 FOR SELECT * FROM table_1;
RETURN NEXT $1;
OPEN $2 FOR SELECT * FROM table_2;
RETURN NEXT $2;
END;
$$ LANGUAGE plpgsql;
-- need to be in a transaction to use cursors.
BEGIN;
SELECT * FROM myfunc('a', 'b');
FETCH ALL FROM a;
FETCH ALL FROM b;
COMMIT;
Looping through a Cursor's Result
There is a variant of the FOR statement that allows
iterating through the rows returned by a cursor. The syntax is:
<<label>>
FOR recordvar IN bound_cursorvar ( argument_name := argument_value, ... ) LOOP
statements
END LOOP label;
The cursor variable must have been bound to some query when it was
declared, and it cannot be open already. The
FOR statement automatically opens the cursor, and it closes
the cursor again when the loop exits. A list of actual argument value
expressions must appear if and only if the cursor was declared to take
arguments. These values will be substituted in the query, in just
the same way as during an OPEN (see ).
The variable recordvar is automatically
defined as type record and exists only inside the loop (any
existing definition of the variable name is ignored within the loop).
Each row returned by the cursor is successively assigned to this
record variable and the loop body is executed.
Transaction Management
In procedures invoked by the CALL command
as well as in anonymous code blocks (DO command),
it is possible to end transactions using the
commands COMMIT and ROLLBACK. A new
transaction is started automatically after a transaction is ended using
these commands, so there is no separate START
TRANSACTION command. (Note that BEGIN and
END have different meanings in PL/pgSQL.)
Here is a simple example:
CREATE PROCEDURE transaction_test1()
LANGUAGE plpgsql
AS $$
BEGIN
FOR i IN 0..9 LOOP
INSERT INTO test1 (a) VALUES (i);
IF i % 2 = 0 THEN
COMMIT;
ELSE
ROLLBACK;
END IF;
END LOOP;
END;
$$;
CALL transaction_test1();
chained transactionsin PL/pgSQL
A new transaction starts out with default transaction characteristics such
as transaction isolation level. In cases where transactions are committed
in a loop, it might be desirable to start new transactions automatically
with the same characteristics as the previous one. The commands
COMMIT AND CHAIN and ROLLBACK AND
CHAIN accomplish this.
Transaction control is only possible in CALL or
DO invocations from the top level or nested
CALL or DO invocations without any
other intervening command. For example, if the call stack is
CALL proc1() → CALL proc2()
→ CALL proc3(), then the second and third
procedures can perform transaction control actions. But if the call stack
is CALL proc1() → SELECT
func2() → CALL proc3(), then the last
procedure cannot do transaction control, because of the
SELECT in between.
Special considerations apply to cursor loops. Consider this example:
CREATE PROCEDURE transaction_test2()
LANGUAGE plpgsql
AS $$
DECLARE
r RECORD;
BEGIN
FOR r IN SELECT * FROM test2 ORDER BY x LOOP
INSERT INTO test1 (a) VALUES (r.x);
COMMIT;
END LOOP;
END;
$$;
CALL transaction_test2();
Normally, cursors are automatically closed at transaction commit.
However, a cursor created as part of a loop like this is automatically
converted to a holdable cursor by the first COMMIT or
ROLLBACK. That means that the cursor is fully
evaluated at the first COMMIT or
ROLLBACK rather than row by row. The cursor is still
removed automatically after the loop, so this is mostly invisible to the
user.
Transaction commands are not allowed in cursor loops driven by commands
that are not read-only (for example UPDATE
... RETURNING).
A transaction cannot be ended inside a block with exception handlers.
Errors and MessagesReporting Errors and MessagesRAISEin PL/pgSQLreporting errorsin PL/pgSQL
Use the RAISE statement to report messages and
raise errors.
RAISE level 'format' , expression, ... USING option = expression, ... ;
RAISE levelcondition_name USING option = expression, ... ;
RAISE level SQLSTATE 'sqlstate' USING option = expression, ... ;
RAISE level USING option = expression, ... ;
RAISE ;
The level option specifies
the error severity. Allowed levels are DEBUG,
LOG, INFO,
NOTICE, WARNING,
and EXCEPTION, with EXCEPTION
being the default.
EXCEPTION raises an error (which normally aborts the
current transaction); the other levels only generate messages of different
priority levels.
Whether messages of a particular priority are reported to the client,
written to the server log, or both is controlled by the
and
configuration
variables. See for more
information.
After level if any,
you can specify a format string
(which must be a simple string literal, not an expression). The
format string specifies the error message text to be reported.
The format string can be followed
by optional argument expressions to be inserted into the message.
Inside the format string, % is replaced by the
string representation of the next optional argument's value. Write
%% to emit a literal %.
The number of arguments must match the number of %
placeholders in the format string, or an error is raised during
the compilation of the function.
In this example, the value of v_job_id will replace the
% in the string:
RAISE NOTICE 'Calling cs_create_job(%)', v_job_id;
You can attach additional information to the error report by writing
USING followed by option = expression items. Each
expression can be any
string-valued expression. The allowed option key words are:
MESSAGESets the error message text. This option can't be used in the
form of RAISE that includes a format string
before USING.DETAILSupplies an error detail message.HINTSupplies a hint message.ERRCODESpecifies the error code (SQLSTATE) to report, either by condition
name, as shown in , or directly as a
five-character SQLSTATE code.COLUMNCONSTRAINTDATATYPETABLESCHEMASupplies the name of a related object.
This example will abort the transaction with the given error message
and hint:
RAISE EXCEPTION 'Nonexistent ID --> %', user_id
USING HINT = 'Please check your user ID';
These two examples show equivalent ways of setting the SQLSTATE:
RAISE 'Duplicate user ID: %', user_id USING ERRCODE = 'unique_violation';
RAISE 'Duplicate user ID: %', user_id USING ERRCODE = '23505';
There is a second RAISE syntax in which the main argument
is the condition name or SQLSTATE to be reported, for example:
RAISE division_by_zero;
RAISE SQLSTATE '22012';
In this syntax, USING can be used to supply a custom
error message, detail, or hint. Another way to do the earlier
example is
RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;
Still another variant is to write RAISE USING or RAISE
level USING and put
everything else into the USING list.
The last variant of RAISE has no parameters at all.
This form can only be used inside a BEGIN block's
EXCEPTION clause;
it causes the error currently being handled to be re-thrown.
Before PostgreSQL 9.1, RAISE without
parameters was interpreted as re-throwing the error from the block
containing the active exception handler. Thus an EXCEPTION
clause nested within that handler could not catch it, even if the
RAISE was within the nested EXCEPTION clause's
block. This was deemed surprising as well as being incompatible with
Oracle's PL/SQL.
If no condition name nor SQLSTATE is specified in a
RAISE EXCEPTION command, the default is to use
raise_exception (P0001).
If no message text is specified, the default is to use the condition
name or SQLSTATE as message text.
When specifying an error code by SQLSTATE code, you are not
limited to the predefined error codes, but can select any
error code consisting of five digits and/or upper-case ASCII
letters, other than 00000. It is recommended that
you avoid throwing error codes that end in three zeroes, because
these are category codes and can only be trapped by trapping
the whole category.
Checking AssertionsASSERTin PL/pgSQLassertionsin PL/pgSQLplpgsql.check_asserts configuration parameter
The ASSERT statement is a convenient shorthand for
inserting debugging checks into PL/pgSQL
functions.
ASSERT condition , message;
The condition is a Boolean
expression that is expected to always evaluate to true; if it does,
the ASSERT statement does nothing further. If the
result is false or null, then an ASSERT_FAILURE exception
is raised. (If an error occurs while evaluating
the condition, it is
reported as a normal error.)
If the optional message is
provided, it is an expression whose result (if not null) replaces the
default error message text assertion failed, should
the condition fail.
The message expression is
not evaluated in the normal case where the assertion succeeds.
Testing of assertions can be enabled or disabled via the configuration
parameter plpgsql.check_asserts, which takes a Boolean
value; the default is on. If this parameter
is off then ASSERT statements do nothing.
Note that ASSERT is meant for detecting program
bugs, not for reporting ordinary error conditions. Use
the RAISE statement, described above, for that.
Trigger Functionstriggerin PL/pgSQLPL/pgSQL can be used to define trigger
functions on data changes or database events.
A trigger function is created with the CREATE FUNCTION
command, declaring it as a function with no arguments and a return type of
trigger (for data change triggers) or
event_trigger (for database event triggers).
Special local variables named TG_something are
automatically defined to describe the condition that triggered the call.
Triggers on Data Changes
A data change trigger is declared as a
function with no arguments and a return type of trigger.
Note that the function must be declared with no arguments even if it
expects to receive some arguments specified in CREATE TRIGGER
— such arguments are passed via TG_ARGV, as described
below.
When a PL/pgSQL function is called as a
trigger, several special variables are created automatically in the
top-level block. They are:
NEWrecord
new database row for INSERT/UPDATE operations in row-level
triggers. This variable is null in statement-level triggers
and for DELETE operations.
OLDrecord
old database row for UPDATE/DELETE operations in row-level
triggers. This variable is null in statement-level triggers
and for INSERT operations.
TG_NAMEname
name of the trigger which fired.
TG_WHENtextBEFORE, AFTER, or
INSTEAD OF, depending on the trigger's definition.
TG_LEVELtextROW or STATEMENT,
depending on the trigger's definition.
TG_OPtext
operation for which the trigger was fired:
INSERT, UPDATE,
DELETE, or TRUNCATE.
TG_RELIDoid (references pg_class.oid)
object ID of the table that caused the trigger invocation.
TG_RELNAMEname
table that caused the trigger
invocation. This is now deprecated, and could disappear in a future
release. Use TG_TABLE_NAME instead.
TG_TABLE_NAMEname
table that caused the trigger invocation.
TG_TABLE_SCHEMAname
schema of the table that caused the trigger invocation.
TG_NARGSinteger
number of arguments given to the trigger
function in the CREATE TRIGGER statement.
TG_ARGVtext[]
arguments from
the CREATE TRIGGER statement.
The index counts from 0. Invalid
indexes (less than 0 or greater than or equal to tg_nargs)
result in a null value.
A trigger function must return either NULL or a
record/row value having exactly the structure of the table the
trigger was fired for.
Row-level triggers fired BEFORE can return null to signal the
trigger manager to skip the rest of the operation for this row
(i.e., subsequent triggers are not fired, and the
INSERT/UPDATE/DELETE does not occur
for this row). If a nonnull
value is returned then the operation proceeds with that row value.
Returning a row value different from the original value
of NEW alters the row that will be inserted or
updated. Thus, if the trigger function wants the triggering
action to succeed normally without altering the row
value, NEW (or a value equal thereto) has to be
returned. To alter the row to be stored, it is possible to
replace single values directly in NEW and return the
modified NEW, or to build a complete new record/row to
return. In the case of a before-trigger
on DELETE, the returned value has no direct
effect, but it has to be nonnull to allow the trigger action to
proceed. Note that NEW is null
in DELETE triggers, so returning that is
usually not sensible. The usual idiom in DELETE
triggers is to return OLD.
INSTEAD OF triggers (which are always row-level triggers,
and may only be used on views) can return null to signal that they did
not perform any updates, and that the rest of the operation for this
row should be skipped (i.e., subsequent triggers are not fired, and the
row is not counted in the rows-affected status for the surrounding
INSERT/UPDATE/DELETE).
Otherwise a nonnull value should be returned, to signal
that the trigger performed the requested operation. For
INSERT and UPDATE operations, the return value
should be NEW, which the trigger function may modify to
support INSERT RETURNING and UPDATE RETURNING
(this will also affect the row value passed to any subsequent triggers,
or passed to a special EXCLUDED alias reference within
an INSERT statement with an ON CONFLICT DO
UPDATE clause). For DELETE operations, the return
value should be OLD.
The return value of a row-level trigger
fired AFTER or a statement-level trigger
fired BEFORE or AFTER is
always ignored; it might as well be null. However, any of these types of
triggers might still abort the entire operation by raising an error.
shows an example of a
trigger function in PL/pgSQL.
A PL/pgSQL Trigger Function
This example trigger ensures that any time a row is inserted or updated
in the table, the current user name and time are stamped into the
row. And it checks that an employee's name is given and that the
salary is a positive value.
CREATE TABLE emp (
empname text,
salary integer,
last_date timestamp,
last_user text
);
CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
BEGIN
-- Check that empname and salary are given
IF NEW.empname IS NULL THEN
RAISE EXCEPTION 'empname cannot be null';
END IF;
IF NEW.salary IS NULL THEN
RAISE EXCEPTION '% cannot have null salary', NEW.empname;
END IF;
-- Who works for us when they must pay for it?
IF NEW.salary < 0 THEN
RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
END IF;
-- Remember who changed the payroll when
NEW.last_date := current_timestamp;
NEW.last_user := current_user;
RETURN NEW;
END;
$emp_stamp$ LANGUAGE plpgsql;
CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE FUNCTION emp_stamp();
Another way to log changes to a table involves creating a new table that
holds a row for each insert, update, or delete that occurs. This approach
can be thought of as auditing changes to a table.
shows an example of an
audit trigger function in PL/pgSQL.
A PL/pgSQL Trigger Function for Auditing
This example trigger ensures that any insert, update or delete of a row
in the emp table is recorded (i.e., audited) in the emp_audit table.
The current time and user name are stamped into the row, together with
the type of operation performed on it.
CREATE TABLE emp (
empname text NOT NULL,
salary integer
);
CREATE TABLE emp_audit(
operation char(1) NOT NULL,
stamp timestamp NOT NULL,
userid text NOT NULL,
empname text NOT NULL,
salary integer
);
CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
BEGIN
--
-- Create a row in emp_audit to reflect the operation performed on emp,
-- making use of the special variable TG_OP to work out the operation.
--
IF (TG_OP = 'DELETE') THEN
INSERT INTO emp_audit SELECT 'D', now(), current_user, OLD.*;
ELSIF (TG_OP = 'UPDATE') THEN
INSERT INTO emp_audit SELECT 'U', now(), current_user, NEW.*;
ELSIF (TG_OP = 'INSERT') THEN
INSERT INTO emp_audit SELECT 'I', now(), current_user, NEW.*;
END IF;
RETURN NULL; -- result is ignored since this is an AFTER trigger
END;
$emp_audit$ LANGUAGE plpgsql;
CREATE TRIGGER emp_audit
AFTER INSERT OR UPDATE OR DELETE ON emp
FOR EACH ROW EXECUTE FUNCTION process_emp_audit();
A variation of the previous example uses a view joining the main table
to the audit table, to show when each entry was last modified. This
approach still records the full audit trail of changes to the table,
but also presents a simplified view of the audit trail, showing just
the last modified timestamp derived from the audit trail for each entry.
shows an example
of an audit trigger on a view in PL/pgSQL.
A PL/pgSQL View Trigger Function for Auditing
This example uses a trigger on the view to make it updatable, and
ensure that any insert, update or delete of a row in the view is
recorded (i.e., audited) in the emp_audit table. The current time
and user name are recorded, together with the type of operation
performed, and the view displays the last modified time of each row.
CREATE TABLE emp (
empname text PRIMARY KEY,
salary integer
);
CREATE TABLE emp_audit(
operation char(1) NOT NULL,
userid text NOT NULL,
empname text NOT NULL,
salary integer,
stamp timestamp NOT NULL
);
CREATE VIEW emp_view AS
SELECT e.empname,
e.salary,
max(ea.stamp) AS last_updated
FROM emp e
LEFT JOIN emp_audit ea ON ea.empname = e.empname
GROUP BY 1, 2;
CREATE OR REPLACE FUNCTION update_emp_view() RETURNS TRIGGER AS $$
BEGIN
--
-- Perform the required operation on emp, and create a row in emp_audit
-- to reflect the change made to emp.
--
IF (TG_OP = 'DELETE') THEN
DELETE FROM emp WHERE empname = OLD.empname;
IF NOT FOUND THEN RETURN NULL; END IF;
OLD.last_updated = now();
INSERT INTO emp_audit VALUES('D', current_user, OLD.*);
RETURN OLD;
ELSIF (TG_OP = 'UPDATE') THEN
UPDATE emp SET salary = NEW.salary WHERE empname = OLD.empname;
IF NOT FOUND THEN RETURN NULL; END IF;
NEW.last_updated = now();
INSERT INTO emp_audit VALUES('U', current_user, NEW.*);
RETURN NEW;
ELSIF (TG_OP = 'INSERT') THEN
INSERT INTO emp VALUES(NEW.empname, NEW.salary);
NEW.last_updated = now();
INSERT INTO emp_audit VALUES('I', current_user, NEW.*);
RETURN NEW;
END IF;
END;
$$ LANGUAGE plpgsql;
CREATE TRIGGER emp_audit
INSTEAD OF INSERT OR UPDATE OR DELETE ON emp_view
FOR EACH ROW EXECUTE FUNCTION update_emp_view();
One use of triggers is to maintain a summary table
of another table. The resulting summary can be used in place of the
original table for certain queries — often with vastly reduced run
times.
This technique is commonly used in Data Warehousing, where the tables
of measured or observed data (called fact tables) might be extremely large.
shows an example of a
trigger function in PL/pgSQL that maintains
a summary table for a fact table in a data warehouse.
A PL/pgSQL Trigger Function for Maintaining a Summary Table
The schema detailed here is partly based on the Grocery Store
example from The Data Warehouse Toolkit
by Ralph Kimball.
--
-- Main tables - time dimension and sales fact.
--
CREATE TABLE time_dimension (
time_key integer NOT NULL,
day_of_week integer NOT NULL,
day_of_month integer NOT NULL,
month integer NOT NULL,
quarter integer NOT NULL,
year integer NOT NULL
);
CREATE UNIQUE INDEX time_dimension_key ON time_dimension(time_key);
CREATE TABLE sales_fact (
time_key integer NOT NULL,
product_key integer NOT NULL,
store_key integer NOT NULL,
amount_sold numeric(12,2) NOT NULL,
units_sold integer NOT NULL,
amount_cost numeric(12,2) NOT NULL
);
CREATE INDEX sales_fact_time ON sales_fact(time_key);
--
-- Summary table - sales by time.
--
CREATE TABLE sales_summary_bytime (
time_key integer NOT NULL,
amount_sold numeric(15,2) NOT NULL,
units_sold numeric(12) NOT NULL,
amount_cost numeric(15,2) NOT NULL
);
CREATE UNIQUE INDEX sales_summary_bytime_key ON sales_summary_bytime(time_key);
--
-- Function and trigger to amend summarized column(s) on UPDATE, INSERT, DELETE.
--
CREATE OR REPLACE FUNCTION maint_sales_summary_bytime() RETURNS TRIGGER
AS $maint_sales_summary_bytime$
DECLARE
delta_time_key integer;
delta_amount_sold numeric(15,2);
delta_units_sold numeric(12);
delta_amount_cost numeric(15,2);
BEGIN
-- Work out the increment/decrement amount(s).
IF (TG_OP = 'DELETE') THEN
delta_time_key = OLD.time_key;
delta_amount_sold = -1 * OLD.amount_sold;
delta_units_sold = -1 * OLD.units_sold;
delta_amount_cost = -1 * OLD.amount_cost;
ELSIF (TG_OP = 'UPDATE') THEN
-- forbid updates that change the time_key -
-- (probably not too onerous, as DELETE + INSERT is how most
-- changes will be made).
IF ( OLD.time_key != NEW.time_key) THEN
RAISE EXCEPTION 'Update of time_key : % -> % not allowed',
OLD.time_key, NEW.time_key;
END IF;
delta_time_key = OLD.time_key;
delta_amount_sold = NEW.amount_sold - OLD.amount_sold;
delta_units_sold = NEW.units_sold - OLD.units_sold;
delta_amount_cost = NEW.amount_cost - OLD.amount_cost;
ELSIF (TG_OP = 'INSERT') THEN
delta_time_key = NEW.time_key;
delta_amount_sold = NEW.amount_sold;
delta_units_sold = NEW.units_sold;
delta_amount_cost = NEW.amount_cost;
END IF;
-- Insert or update the summary row with the new values.
<<insert_update>>
LOOP
UPDATE sales_summary_bytime
SET amount_sold = amount_sold + delta_amount_sold,
units_sold = units_sold + delta_units_sold,
amount_cost = amount_cost + delta_amount_cost
WHERE time_key = delta_time_key;
EXIT insert_update WHEN found;
BEGIN
INSERT INTO sales_summary_bytime (
time_key,
amount_sold,
units_sold,
amount_cost)
VALUES (
delta_time_key,
delta_amount_sold,
delta_units_sold,
delta_amount_cost
);
EXIT insert_update;
EXCEPTION
WHEN UNIQUE_VIOLATION THEN
-- do nothing
END;
END LOOP insert_update;
RETURN NULL;
END;
$maint_sales_summary_bytime$ LANGUAGE plpgsql;
CREATE TRIGGER maint_sales_summary_bytime
AFTER INSERT OR UPDATE OR DELETE ON sales_fact
FOR EACH ROW EXECUTE FUNCTION maint_sales_summary_bytime();
INSERT INTO sales_fact VALUES(1,1,1,10,3,15);
INSERT INTO sales_fact VALUES(1,2,1,20,5,35);
INSERT INTO sales_fact VALUES(2,2,1,40,15,135);
INSERT INTO sales_fact VALUES(2,3,1,10,1,13);
SELECT * FROM sales_summary_bytime;
DELETE FROM sales_fact WHERE product_key = 1;
SELECT * FROM sales_summary_bytime;
UPDATE sales_fact SET units_sold = units_sold * 2;
SELECT * FROM sales_summary_bytime;
AFTER triggers can also make use of transition
tables to inspect the entire set of rows changed by the triggering
statement. The CREATE TRIGGER command assigns names to one
or both transition tables, and then the function can refer to those names
as though they were read-only temporary tables.
shows an example.
Auditing with Transition Tables
This example produces the same results as
, but instead of using a
trigger that fires for every row, it uses a trigger that fires once
per statement, after collecting the relevant information in a transition
table. This can be significantly faster than the row-trigger approach
when the invoking statement has modified many rows. Notice that we must
make a separate trigger declaration for each kind of event, since the
REFERENCING clauses must be different for each case. But
this does not stop us from using a single trigger function if we choose.
(In practice, it might be better to use three separate functions and
avoid the run-time tests on TG_OP.)
CREATE TABLE emp (
empname text NOT NULL,
salary integer
);
CREATE TABLE emp_audit(
operation char(1) NOT NULL,
stamp timestamp NOT NULL,
userid text NOT NULL,
empname text NOT NULL,
salary integer
);
CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
BEGIN
--
-- Create rows in emp_audit to reflect the operations performed on emp,
-- making use of the special variable TG_OP to work out the operation.
--
IF (TG_OP = 'DELETE') THEN
INSERT INTO emp_audit
SELECT 'D', now(), current_user, o.* FROM old_table o;
ELSIF (TG_OP = 'UPDATE') THEN
INSERT INTO emp_audit
SELECT 'U', now(), current_user, n.* FROM new_table n;
ELSIF (TG_OP = 'INSERT') THEN
INSERT INTO emp_audit
SELECT 'I', now(), current_user, n.* FROM new_table n;
END IF;
RETURN NULL; -- result is ignored since this is an AFTER trigger
END;
$emp_audit$ LANGUAGE plpgsql;
CREATE TRIGGER emp_audit_ins
AFTER INSERT ON emp
REFERENCING NEW TABLE AS new_table
FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
CREATE TRIGGER emp_audit_upd
AFTER UPDATE ON emp
REFERENCING OLD TABLE AS old_table NEW TABLE AS new_table
FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
CREATE TRIGGER emp_audit_del
AFTER DELETE ON emp
REFERENCING OLD TABLE AS old_table
FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
Triggers on EventsPL/pgSQL can be used to define
event triggers.
PostgreSQL requires that a function that
is to be called as an event trigger must be declared as a function with
no arguments and a return type of event_trigger.
When a PL/pgSQL function is called as an
event trigger, several special variables are created automatically
in the top-level block. They are:
TG_EVENTtext
event the trigger is fired for.
TG_TAGtext
command tag for which the trigger is fired.
shows an example of an
event trigger function in PL/pgSQL.
A PL/pgSQL Event Trigger Function
This example trigger simply raises a NOTICE message
each time a supported command is executed.
CREATE OR REPLACE FUNCTION snitch() RETURNS event_trigger AS $$
BEGIN
RAISE NOTICE 'snitch: % %', tg_event, tg_tag;
END;
$$ LANGUAGE plpgsql;
CREATE EVENT TRIGGER snitch ON ddl_command_start EXECUTE FUNCTION snitch();
PL/pgSQL under the Hood
This section discusses some implementation details that are
frequently important for PL/pgSQL users to know.
Variable Substitution
SQL statements and expressions within a PL/pgSQL function
can refer to variables and parameters of the function. Behind the scenes,
PL/pgSQL substitutes query parameters for such references.
Query parameters will only be substituted in places where they are
syntactically permissible. As an extreme case, consider
this example of poor programming style:
INSERT INTO foo (foo) VALUES (foo(foo));
The first occurrence of foo must syntactically be a table
name, so it will not be substituted, even if the function has a variable
named foo. The second occurrence must be the name of a
column of that table, so it will not be substituted either. Likewise
the third occurrence must be a function name, so it also will not be
substituted for. Only the last occurrence is a candidate to be a
reference to a variable of the PL/pgSQL
function.
Another way to understand this is that variable substitution can only
insert data values into an SQL command; it cannot dynamically change which
database objects are referenced by the command. (If you want to do
that, you must build a command string dynamically, as explained in
.)
Since the names of variables are syntactically no different from the names
of table columns, there can be ambiguity in statements that also refer to
tables: is a given name meant to refer to a table column, or a variable?
Let's change the previous example to
INSERT INTO dest (col) SELECT foo + bar FROM src;
Here, dest and src must be table names, and
col must be a column of dest, but foo
and bar might reasonably be either variables of the function
or columns of src.
By default, PL/pgSQL will report an error if a name
in an SQL statement could refer to either a variable or a table column.
You can fix such a problem by renaming the variable or column,
or by qualifying the ambiguous reference, or by telling
PL/pgSQL which interpretation to prefer.
The simplest solution is to rename the variable or column.
A common coding rule is to use a
different naming convention for PL/pgSQL
variables than you use for column names. For example,
if you consistently name function variables
v_something while none of your
column names start with v_, no conflicts will occur.
Alternatively you can qualify ambiguous references to make them clear.
In the above example, src.foo would be an unambiguous reference
to the table column. To create an unambiguous reference to a variable,
declare it in a labeled block and use the block's label
(see ). For example,
<<block>>
DECLARE
foo int;
BEGIN
foo := ...;
INSERT INTO dest (col) SELECT block.foo + bar FROM src;
Here block.foo means the variable even if there is a column
foo in src. Function parameters, as well as
special variables such as FOUND, can be qualified by the
function's name, because they are implicitly declared in an outer block
labeled with the function's name.
Sometimes it is impractical to fix all the ambiguous references in a
large body of PL/pgSQL code. In such cases you can
specify that PL/pgSQL should resolve ambiguous references
as the variable (which is compatible with PL/pgSQL's
behavior before PostgreSQL 9.0), or as the
table column (which is compatible with some other systems such as
Oracle).
plpgsql.variable_conflict configuration parameter
To change this behavior on a system-wide basis, set the configuration
parameter plpgsql.variable_conflict to one of
error, use_variable, or
use_column (where error is the factory default).
This parameter affects subsequent compilations
of statements in PL/pgSQL functions, but not statements
already compiled in the current session.
Because changing this setting
can cause unexpected changes in the behavior of PL/pgSQL
functions, it can only be changed by a superuser.
You can also set the behavior on a function-by-function basis, by
inserting one of these special commands at the start of the function
text:
#variable_conflict error
#variable_conflict use_variable
#variable_conflict use_column
These commands affect only the function they are written in, and override
the setting of plpgsql.variable_conflict. An example is
CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
#variable_conflict use_variable
DECLARE
curtime timestamp := now();
BEGIN
UPDATE users SET last_modified = curtime, comment = comment
WHERE users.id = id;
END;
$$ LANGUAGE plpgsql;
In the UPDATE command, curtime, comment,
and id will refer to the function's variable and parameters
whether or not users has columns of those names. Notice
that we had to qualify the reference to users.id in the
WHERE clause to make it refer to the table column.
But we did not have to qualify the reference to comment
as a target in the UPDATE list, because syntactically
that must be a column of users. We could write the same
function without depending on the variable_conflict setting
in this way:
CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
<<fn>>
DECLARE
curtime timestamp := now();
BEGIN
UPDATE users SET last_modified = fn.curtime, comment = stamp_user.comment
WHERE users.id = stamp_user.id;
END;
$$ LANGUAGE plpgsql;
Variable substitution does not happen in a command string given
to EXECUTE or one of its variants. If you need to
insert a varying value into such a command, do so as part of
constructing the string value, or use USING, as illustrated in
.
Variable substitution currently works only in SELECT,
INSERT, UPDATE,
DELETE, and commands containing one of
these (such as EXPLAIN and CREATE TABLE
... AS SELECT),
because the main SQL engine allows query parameters only in these
commands. To use a non-constant name or value in other statement
types (generically called utility statements), you must construct
the utility statement as a string and EXECUTE it.
Plan Caching
The PL/pgSQL interpreter parses the function's source
text and produces an internal binary instruction tree the first time the
function is called (within each session). The instruction tree
fully translates the
PL/pgSQL statement structure, but individual
SQL expressions and SQL commands
used in the function are not translated immediately.
preparing a queryin PL/pgSQL
As each expression and SQL command is first
executed in the function, the PL/pgSQL interpreter
parses and analyzes the command to create a prepared statement,
using the SPI manager's
SPI_prepare function.
Subsequent visits to that expression or command
reuse the prepared statement. Thus, a function with conditional code
paths that are seldom visited will never incur the overhead of
analyzing those commands that are never executed within the current
session. A disadvantage is that errors
in a specific expression or command cannot be detected until that
part of the function is reached in execution. (Trivial syntax
errors will be detected during the initial parsing pass, but
anything deeper will not be detected until execution.)
PL/pgSQL (or more precisely, the SPI manager) can
furthermore attempt to cache the execution plan associated with any
particular prepared statement. If a cached plan is not used, then
a fresh execution plan is generated on each visit to the statement,
and the current parameter values (that is, PL/pgSQL
variable values) can be used to optimize the selected plan. If the
statement has no parameters, or is executed many times, the SPI manager
will consider creating a generic plan that is not dependent
on specific parameter values, and caching that for re-use. Typically
this will happen only if the execution plan is not very sensitive to
the values of the PL/pgSQL variables referenced in it.
If it is, generating a plan each time is a net win. See for more information about the behavior of
prepared statements.
Because PL/pgSQL saves prepared statements
and sometimes execution plans in this way,
SQL commands that appear directly in a
PL/pgSQL function must refer to the
same tables and columns on every execution; that is, you cannot use
a parameter as the name of a table or column in an SQL command. To get
around this restriction, you can construct dynamic commands using
the PL/pgSQLEXECUTE
statement — at the price of performing new parse analysis and
constructing a new execution plan on every execution.
The mutable nature of record variables presents another problem in this
connection. When fields of a record variable are used in
expressions or statements, the data types of the fields must not
change from one call of the function to the next, since each
expression will be analyzed using the data type that is present
when the expression is first reached. EXECUTE can be
used to get around this problem when necessary.
If the same function is used as a trigger for more than one table,
PL/pgSQL prepares and caches statements
independently for each such table — that is, there is a cache
for each trigger function and table combination, not just for each
function. This alleviates some of the problems with varying
data types; for instance, a trigger function will be able to work
successfully with a column named key even if it happens
to have different types in different tables.
Likewise, functions having polymorphic argument types have a separate
statement cache for each combination of actual argument types they have
been invoked for, so that data type differences do not cause unexpected
failures.
Statement caching can sometimes have surprising effects on the
interpretation of time-sensitive values. For example there
is a difference between what these two functions do:
CREATE FUNCTION logfunc1(logtxt text) RETURNS void AS $$
BEGIN
INSERT INTO logtable VALUES (logtxt, 'now');
END;
$$ LANGUAGE plpgsql;
and:
CREATE FUNCTION logfunc2(logtxt text) RETURNS void AS $$
DECLARE
curtime timestamp;
BEGIN
curtime := 'now';
INSERT INTO logtable VALUES (logtxt, curtime);
END;
$$ LANGUAGE plpgsql;
In the case of logfunc1, the
PostgreSQL main parser knows when
analyzing the INSERT that the
string 'now' should be interpreted as
timestamp, because the target column of
logtable is of that type. Thus,
'now' will be converted to a timestamp
constant when the
INSERT is analyzed, and then used in all
invocations of logfunc1 during the lifetime
of the session. Needless to say, this isn't what the programmer
wanted. A better idea is to use the now() or
current_timestamp function.
In the case of logfunc2, the
PostgreSQL main parser does not know
what type 'now' should become and therefore
it returns a data value of type text containing the string
now. During the ensuing assignment
to the local variable curtime, the
PL/pgSQL interpreter casts this
string to the timestamp type by calling the
textout and timestamp_in
functions for the conversion. So, the computed time stamp is updated
on each execution as the programmer expects. Even though this
happens to work as expected, it's not terribly efficient, so
use of the now() function would still be a better idea.
Tips for Developing in PL/pgSQL
One good way to develop in
PL/pgSQL is to use the text editor of your
choice to create your functions, and in another window, use
psql to load and test those functions.
If you are doing it this way, it
is a good idea to write the function using CREATE OR
REPLACE FUNCTION. That way you can just reload the file to update
the function definition. For example:
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $$
....
$$ LANGUAGE plpgsql;
While running psql, you can load or reload such
a function definition file with:
\i filename.sql
and then immediately issue SQL commands to test the function.
Another good way to develop in PL/pgSQL is with a
GUI database access tool that facilitates development in a
procedural language. One example of such a tool is
pgAdmin, although others exist. These tools often
provide convenient features such as escaping single quotes and
making it easier to recreate and debug functions.
Handling of Quotation Marks
The code of a PL/pgSQL function is specified in
CREATE FUNCTION as a string literal. If you
write the string literal in the ordinary way with surrounding
single quotes, then any single quotes inside the function body
must be doubled; likewise any backslashes must be doubled (assuming
escape string syntax is used).
Doubling quotes is at best tedious, and in more complicated cases
the code can become downright incomprehensible, because you can
easily find yourself needing half a dozen or more adjacent quote marks.
It's recommended that you instead write the function body as a
dollar-quoted string literal (see ). In the dollar-quoting
approach, you never double any quote marks, but instead take care to
choose a different dollar-quoting delimiter for each level of
nesting you need. For example, you might write the CREATE
FUNCTION command as:
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $PROC$
....
$PROC$ LANGUAGE plpgsql;
Within this, you might use quote marks for simple literal strings in
SQL commands and $$ to delimit fragments of SQL commands
that you are assembling as strings. If you need to quote text that
includes $$, you could use $Q$, and so on.
The following chart shows what you have to do when writing quote
marks without dollar quoting. It might be useful when translating
pre-dollar quoting code into something more comprehensible.
1 quotation mark
To begin and end the function body, for example:
CREATE FUNCTION foo() RETURNS integer AS '
....
' LANGUAGE plpgsql;
Anywhere within a single-quoted function body, quote marks
must appear in pairs.
2 quotation marks
For string literals inside the function body, for example:
a_output := ''Blah'';
SELECT * FROM users WHERE f_name=''foobar'';
In the dollar-quoting approach, you'd just write:
a_output := 'Blah';
SELECT * FROM users WHERE f_name='foobar';
which is exactly what the PL/pgSQL parser would see
in either case.
4 quotation marks
When you need a single quotation mark in a string constant inside the
function body, for example:
a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''
The value actually appended to a_output would be:
AND name LIKE 'foobar' AND xyz.
In the dollar-quoting approach, you'd write:
a_output := a_output || $$ AND name LIKE 'foobar' AND xyz$$
being careful that any dollar-quote delimiters around this are not
just $$.
6 quotation marks
When a single quotation mark in a string inside the function body is
adjacent to the end of that string constant, for example:
a_output := a_output || '' AND name LIKE ''''foobar''''''
The value appended to a_output would then be:
AND name LIKE 'foobar'.
In the dollar-quoting approach, this becomes:
a_output := a_output || $$ AND name LIKE 'foobar'$$
10 quotation marks
When you want two single quotation marks in a string constant (which
accounts for 8 quotation marks) and this is adjacent to the end of that
string constant (2 more). You will probably only need that if
you are writing a function that generates other functions, as in
.
For example:
a_output := a_output || '' if v_'' ||
referrer_keys.kind || '' like ''''''''''
|| referrer_keys.key_string || ''''''''''
then return '''''' || referrer_keys.referrer_type
|| ''''''; end if;'';
The value of a_output would then be:
if v_... like ''...'' then return ''...''; end if;
In the dollar-quoting approach, this becomes:
a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
|| referrer_keys.key_string || $$'
then return '$$ || referrer_keys.referrer_type
|| $$'; end if;$$;
where we assume we only need to put single quote marks into
a_output, because it will be re-quoted before use.
Additional Compile-Time and Run-Time Checks
To aid the user in finding instances of simple but common problems before
they cause harm, PL/pgSQL provides additional
checks. When enabled, depending on the configuration, they
can be used to emit either a WARNING or an ERROR
during the compilation of a function. A function which has received
a WARNING can be executed without producing further messages,
so you are advised to test in a separate development environment.
Setting plpgsql.extra_warnings, or
plpgsql.extra_errors, as appropriate, to "all"
is encouraged in development and/or testing environments.
These additional checks are enabled through the configuration variables
plpgsql.extra_warnings for warnings and
plpgsql.extra_errors for errors. Both can be set either to
a comma-separated list of checks, "none" or
"all". The default is "none". Currently
the list of available checks includes:
shadowed_variables
Checks if a declaration shadows a previously defined variable.
strict_multi_assignment
Some PL/pgSQL commands allow assigning
values to more than one variable at a time, such as
SELECT INTO. Typically, the number of target
variables and the number of source variables should match, though
PL/pgSQL will use NULL
for missing values and extra variables are ignored. Enabling this
check will cause PL/pgSQL to throw a
WARNING or ERROR whenever the
number of target variables and the number of source variables are
different.
too_many_rows
Enabling this check will cause PL/pgSQL to
check if a given query returns more than one row when an
INTO clause is used. As an INTO
statement will only ever use one row, having a query return multiple
rows is generally either inefficient and/or nondeterministic and
therefore is likely an error.
The following example shows the effect of plpgsql.extra_warnings
set to shadowed_variables:
SET plpgsql.extra_warnings TO 'shadowed_variables';
CREATE FUNCTION foo(f1 int) RETURNS int AS $$
DECLARE
f1 int;
BEGIN
RETURN f1;
END;
$$ LANGUAGE plpgsql;
WARNING: variable "f1" shadows a previously defined variable
LINE 3: f1 int;
^
CREATE FUNCTION
The below example shows the effects of setting
plpgsql.extra_warnings to
strict_multi_assignment:
SET plpgsql.extra_warnings TO 'strict_multi_assignment';
CREATE OR REPLACE FUNCTION public.foo()
RETURNS void
LANGUAGE plpgsql
AS $$
DECLARE
x int;
y int;
BEGIN
SELECT 1 INTO x, y;
SELECT 1, 2 INTO x, y;
SELECT 1, 2, 3 INTO x, y;
END;
$$;
SELECT foo();
WARNING: number of source and target fields in assignment does not match
DETAIL: strict_multi_assignment check of extra_warnings is active.
HINT: Make sure the query returns the exact list of columns.
WARNING: number of source and target fields in assignment does not match
DETAIL: strict_multi_assignment check of extra_warnings is active.
HINT: Make sure the query returns the exact list of columns.
foo
-----
(1 row)
Porting from Oracle PL/SQLOracleporting from PL/SQL to PL/pgSQLPL/SQL (Oracle)porting to PL/pgSQL
This section explains differences between
PostgreSQL's PL/pgSQL
language and Oracle's PL/SQL language,
to help developers who port applications from
Oracle to PostgreSQL.
PL/pgSQL is similar to PL/SQL in many
aspects. It is a block-structured, imperative language, and all
variables have to be declared. Assignments, loops, and conditionals
are similar. The main differences you should keep in mind when
porting from PL/SQL to
PL/pgSQL are:
If a name used in an SQL command could be either a column name of a
table used in the command or a reference to a variable of the function,
PL/SQL treats it as a column name.
By default, PL/pgSQL will throw an error
complaining that the name is ambiguous. You can specify
plpgsql.variable_conflict = use_column
to change this behavior to match PL/SQL,
as explained in .
It's often best to avoid such ambiguities in the first place,
but if you have to port a large amount of code that depends on
this behavior, setting variable_conflict may be the
best solution.
In PostgreSQL the function body must be written as
a string literal. Therefore you need to use dollar quoting or escape
single quotes in the function body. (See .)
Data type names often need translation. For example, in Oracle string
values are commonly declared as being of type varchar2, which
is a non-SQL-standard type. In PostgreSQL,
use type varchar or text instead. Similarly, replace
type number with numeric, or use some other numeric
data type if there's a more appropriate one.
Instead of packages, use schemas to organize your functions
into groups.
Since there are no packages, there are no package-level variables
either. This is somewhat annoying. You can keep per-session state
in temporary tables instead.
Integer FOR loops with REVERSE work
differently: PL/SQL counts down from the second
number to the first, while PL/pgSQL counts down
from the first number to the second, requiring the loop bounds
to be swapped when porting. This incompatibility is unfortunate
but is unlikely to be changed. (See .)
FOR loops over queries (other than cursors) also work
differently: the target variable(s) must have been declared,
whereas PL/SQL always declares them implicitly.
An advantage of this is that the variable values are still accessible
after the loop exits.
There are various notational differences for the use of cursor
variables.
Porting Examples shows how to port a simple
function from PL/SQL to PL/pgSQL.
Porting a Simple Function from PL/SQL to PL/pgSQL
Here is an OraclePL/SQL function:
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar2,
v_version varchar2)
RETURN varchar2 IS
BEGIN
IF v_version IS NULL THEN
RETURN v_name;
END IF;
RETURN v_name || '/' || v_version;
END;
/
show errors;
Let's go through this function and see the differences compared to
PL/pgSQL:
The type name varchar2 has to be changed to varchar
or text. In the examples in this section, we'll
use varchar, but text is often a better choice if
you do not need specific string length limits.
The RETURN key word in the function
prototype (not the function body) becomes
RETURNS in
PostgreSQL.
Also, IS becomes AS, and you need to
add a LANGUAGE clause because PL/pgSQL
is not the only possible function language.
In PostgreSQL, the function body is considered
to be a string literal, so you need to use quote marks or dollar
quotes around it. This substitutes for the terminating /
in the Oracle approach.
The show errors command does not exist in
PostgreSQL, and is not needed since errors are
reported automatically.
This is how this function would look when ported to
PostgreSQL:
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar,
v_version varchar)
RETURNS varchar AS $$
BEGIN
IF v_version IS NULL THEN
RETURN v_name;
END IF;
RETURN v_name || '/' || v_version;
END;
$$ LANGUAGE plpgsql;
shows how to port a
function that creates another function and how to handle the
ensuing quoting problems.
Porting a Function that Creates Another Function from PL/SQL to PL/pgSQL
The following procedure grabs rows from a
SELECT statement and builds a large function
with the results in IF statements, for the
sake of efficiency.
This is the Oracle version:
CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
CURSOR referrer_keys IS
SELECT * FROM cs_referrer_keys
ORDER BY try_order;
func_cmd VARCHAR(4000);
BEGIN
func_cmd := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR2,
v_domain IN VARCHAR2, v_url IN VARCHAR2) RETURN VARCHAR2 IS BEGIN';
FOR referrer_key IN referrer_keys LOOP
func_cmd := func_cmd ||
' IF v_' || referrer_key.kind
|| ' LIKE ''' || referrer_key.key_string
|| ''' THEN RETURN ''' || referrer_key.referrer_type
|| '''; END IF;';
END LOOP;
func_cmd := func_cmd || ' RETURN NULL; END;';
EXECUTE IMMEDIATE func_cmd;
END;
/
show errors;
Here is how this function would end up in PostgreSQL:
CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc() AS $func$
DECLARE
referrer_keys CURSOR IS
SELECT * FROM cs_referrer_keys
ORDER BY try_order;
func_body text;
func_cmd text;
BEGIN
func_body := 'BEGIN';
FOR referrer_key IN referrer_keys LOOP
func_body := func_body ||
' IF v_' || referrer_key.kind
|| ' LIKE ' || quote_literal(referrer_key.key_string)
|| ' THEN RETURN ' || quote_literal(referrer_key.referrer_type)
|| '; END IF;' ;
END LOOP;
func_body := func_body || ' RETURN NULL; END;';
func_cmd :=
'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host varchar,
v_domain varchar,
v_url varchar)
RETURNS varchar AS '
|| quote_literal(func_body)
|| ' LANGUAGE plpgsql;' ;
EXECUTE func_cmd;
END;
$func$ LANGUAGE plpgsql;
Notice how the body of the function is built separately and passed
through quote_literal to double any quote marks in it. This
technique is needed because we cannot safely use dollar quoting for
defining the new function: we do not know for sure what strings will
be interpolated from the referrer_key.key_string field.
(We are assuming here that referrer_key.kind can be
trusted to always be host, domain, or
url, but referrer_key.key_string might be
anything, in particular it might contain dollar signs.) This function
is actually an improvement on the Oracle original, because it will
not generate broken code when referrer_key.key_string or
referrer_key.referrer_type contain quote marks.
shows how to port a function
with OUT parameters and string manipulation.
PostgreSQL does not have a built-in
instr function, but you can create one
using a combination of other
functions. In there is a
PL/pgSQL implementation of
instr that you can use to make your porting
easier.
Porting a Procedure With String Manipulation and
OUT Parameters from PL/SQL to
PL/pgSQL
The following Oracle PL/SQL procedure is used
to parse a URL and return several elements (host, path, and query).
This is the Oracle version:
CREATE OR REPLACE PROCEDURE cs_parse_url(
v_url IN VARCHAR2,
v_host OUT VARCHAR2, -- This will be passed back
v_path OUT VARCHAR2, -- This one too
v_query OUT VARCHAR2) -- And this one
IS
a_pos1 INTEGER;
a_pos2 INTEGER;
BEGIN
v_host := NULL;
v_path := NULL;
v_query := NULL;
a_pos1 := instr(v_url, '//');
IF a_pos1 = 0 THEN
RETURN;
END IF;
a_pos2 := instr(v_url, '/', a_pos1 + 2);
IF a_pos2 = 0 THEN
v_host := substr(v_url, a_pos1 + 2);
v_path := '/';
RETURN;
END IF;
v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
a_pos1 := instr(v_url, '?', a_pos2 + 1);
IF a_pos1 = 0 THEN
v_path := substr(v_url, a_pos2);
RETURN;
END IF;
v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
v_query := substr(v_url, a_pos1 + 1);
END;
/
show errors;
Here is a possible translation into PL/pgSQL:
CREATE OR REPLACE FUNCTION cs_parse_url(
v_url IN VARCHAR,
v_host OUT VARCHAR, -- This will be passed back
v_path OUT VARCHAR, -- This one too
v_query OUT VARCHAR) -- And this one
AS $$
DECLARE
a_pos1 INTEGER;
a_pos2 INTEGER;
BEGIN
v_host := NULL;
v_path := NULL;
v_query := NULL;
a_pos1 := instr(v_url, '//');
IF a_pos1 = 0 THEN
RETURN;
END IF;
a_pos2 := instr(v_url, '/', a_pos1 + 2);
IF a_pos2 = 0 THEN
v_host := substr(v_url, a_pos1 + 2);
v_path := '/';
RETURN;
END IF;
v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
a_pos1 := instr(v_url, '?', a_pos2 + 1);
IF a_pos1 = 0 THEN
v_path := substr(v_url, a_pos2);
RETURN;
END IF;
v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
v_query := substr(v_url, a_pos1 + 1);
END;
$$ LANGUAGE plpgsql;
This function could be used like this:
SELECT * FROM cs_parse_url('http://foobar.com/query.cgi?baz');
shows how to port a procedure
that uses numerous features that are specific to Oracle.
Porting a Procedure from PL/SQL to PL/pgSQL
The Oracle version:
CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
a_running_job_count INTEGER;
BEGIN
LOCK TABLE cs_jobs IN EXCLUSIVE MODE;
SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;
IF a_running_job_count > 0 THEN
COMMIT; -- free lock
raise_application_error(-20000,
'Unable to create a new job: a job is currently running.');
END IF;
DELETE FROM cs_active_job;
INSERT INTO cs_active_job(job_id) VALUES (v_job_id);
BEGIN
INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
EXCEPTION
WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists
END;
COMMIT;
END;
/
show errors
This is how we could port this procedure to PL/pgSQL:
CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id integer) AS $$
DECLARE
a_running_job_count integer;
BEGIN
LOCK TABLE cs_jobs IN EXCLUSIVE MODE;
SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;
IF a_running_job_count > 0 THEN
COMMIT; -- free lock
RAISE EXCEPTION 'Unable to create a new job: a job is currently running'; --
END IF;
DELETE FROM cs_active_job;
INSERT INTO cs_active_job(job_id) VALUES (v_job_id);
BEGIN
INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
EXCEPTION
WHEN unique_violation THEN --
-- don't worry if it already exists
END;
COMMIT;
END;
$$ LANGUAGE plpgsql;
The syntax of RAISE is considerably different from
Oracle's statement, although the basic case RAISEexception_name works
similarly.
The exception names supported by PL/pgSQL are
different from Oracle's. The set of built-in exception names
is much larger (see ). There
is not currently a way to declare user-defined exception names,
although you can throw user-chosen SQLSTATE values instead.
Other Things to Watch For
This section explains a few other things to watch for when porting
Oracle PL/SQL functions to
PostgreSQL.
Implicit Rollback after Exceptions
In PL/pgSQL, when an exception is caught by an
EXCEPTION clause, all database changes since the block's
BEGIN are automatically rolled back. That is, the behavior
is equivalent to what you'd get in Oracle with:
BEGIN
SAVEPOINT s1;
... code here ...
EXCEPTION
WHEN ... THEN
ROLLBACK TO s1;
... code here ...
WHEN ... THEN
ROLLBACK TO s1;
... code here ...
END;
If you are translating an Oracle procedure that uses
SAVEPOINT and ROLLBACK TO in this style,
your task is easy: just omit the SAVEPOINT and
ROLLBACK TO. If you have a procedure that uses
SAVEPOINT and ROLLBACK TO in a different way
then some actual thought will be required.
EXECUTE
The PL/pgSQL version of
EXECUTE works similarly to the
PL/SQL version, but you have to remember to use
quote_literal and
quote_ident as described in . Constructs of the
type EXECUTE 'SELECT * FROM $1'; will not work
reliably unless you use these functions.
Optimizing PL/pgSQL FunctionsPostgreSQL gives you two function creation
modifiers to optimize execution: volatility (whether
the function always returns the same result when given the same
arguments) and strictness (whether the function
returns null if any argument is null). Consult the
reference page for details.
When making use of these optimization attributes, your
CREATE FUNCTION statement might look something
like this:
CREATE FUNCTION foo(...) RETURNS integer AS $$
...
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
Appendix
This section contains the code for a set of Oracle-compatible
instr functions that you can use to simplify
your porting efforts.
instr function 0 THEN
temp_str := substring(string FROM beg_index);
pos := position(string_to_search_for IN temp_str);
IF pos = 0 THEN
RETURN 0;
ELSE
RETURN pos + beg_index - 1;
END IF;
ELSIF beg_index < 0 THEN
ss_length := char_length(string_to_search_for);
length := char_length(string);
beg := length + 1 + beg_index;
WHILE beg > 0 LOOP
temp_str := substring(string FROM beg FOR ss_length);
IF string_to_search_for = temp_str THEN
RETURN beg;
END IF;
beg := beg - 1;
END LOOP;
RETURN 0;
ELSE
RETURN 0;
END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
CREATE FUNCTION instr(string varchar, string_to_search_for varchar,
beg_index integer, occur_index integer)
RETURNS integer AS $$
DECLARE
pos integer NOT NULL DEFAULT 0;
occur_number integer NOT NULL DEFAULT 0;
temp_str varchar;
beg integer;
i integer;
length integer;
ss_length integer;
BEGIN
IF occur_index <= 0 THEN
RAISE 'argument ''%'' is out of range', occur_index
USING ERRCODE = '22003';
END IF;
IF beg_index > 0 THEN
beg := beg_index - 1;
FOR i IN 1..occur_index LOOP
temp_str := substring(string FROM beg + 1);
pos := position(string_to_search_for IN temp_str);
IF pos = 0 THEN
RETURN 0;
END IF;
beg := beg + pos;
END LOOP;
RETURN beg;
ELSIF beg_index < 0 THEN
ss_length := char_length(string_to_search_for);
length := char_length(string);
beg := length + 1 + beg_index;
WHILE beg > 0 LOOP
temp_str := substring(string FROM beg FOR ss_length);
IF string_to_search_for = temp_str THEN
occur_number := occur_number + 1;
IF occur_number = occur_index THEN
RETURN beg;
END IF;
END IF;
beg := beg - 1;
END LOOP;
RETURN 0;
ELSE
RETURN 0;
END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
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