PL/pgSQL PL/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. 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. 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 . 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 . 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'; user_id CONSTANT integer := 10; 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. 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; 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. 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. newname 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. variable%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. name table_name%ROWTYPE; name composite_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 ... ; name 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 in 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. 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 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 . 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 and . 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 variable, or an element of an array that is a simple variable or field. 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; For any SQL command that does not return rows, for example INSERT without a RETURNING clause, you can execute the command within a PL/pgSQL function just by writing the command. Any PL/pgSQL variable name appearing in the command text is treated as a 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 a SQL command in this way, PL/pgSQL may cache and re-use the execution plan for the command, as discussed in . 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 for commands that return no result, 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. A 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); SELECT INTO in PL/pgSQL RETURNING INTO in PL/pgSQL The result of a 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 STRICT target FROM ...; INSERT ... RETURNING expressions INTO STRICT target; UPDATE ... RETURNING expressions INTO STRICT target; DELETE ... RETURNING expressions INTO STRICT target; 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 query, and the plan is cached, just as described above for commands that do not return rows. This works for SELECT, INSERT/UPDATE/DELETE with RETURNING, and utility commands that return row-set results (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 or a variable list is used as target, the query'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 query 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 query, or to nulls if the query 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 query 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 query. 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. To handle cases where you need to process multiple result rows from a SQL query, see . 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 STRICT target 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 a SQL command returning rows should be assigned. If a row or variable list is provided, it must exactly match the structure of the query's results (when a record variable is used, 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. If no rows are returned, NULL is assigned to the INTO variable(s). If no INTO clause is specified, the query results are discarded. If the STRICT option is given, an error is reported unless the query 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 for table or column names (strings separated by a newline are concatenated): EXECUTE format('SELECT count(*) FROM %I ' 'WHERE inserted_by = $1 AND inserted <= $2', tabname) INTO c USING checked_user, checked_date; Another restriction on parameter symbols is that they only work in SELECT, INSERT, UPDATE, and DELETE commands. 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/pgSQL EXECUTE statement is not related to the SQL statement supported by the PostgreSQL server. The server's EXECUTE statement cannot be used directly within PL/pgSQL functions (and is not needed). quote_ident use in PL/pgSQL quote_literal use in PL/pgSQL quote_nullable use in PL/pgSQL format use 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. 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; Name Type Description ROW_COUNT bigint the number of rows processed by the most recent SQL command PG_CONTEXT text line(s) of text describing the current call stack (see ) 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, and DELETE 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. 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 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. There are two commands available that allow you to return data from a function: RETURN and RETURN NEXT. 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 in PL/pgSQL RETURN QUERY in 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. 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. 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 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; $$; IF and CASE statements let you execute alternative commands based on certain conditions. PL/pgSQL has three forms of IF: IF ... THEN ... END IF IF ... THEN ... ELSE ... END IF IF ... THEN ... ELSIF ... THEN ... ELSE ... END IF and two forms of CASE: CASE ... WHEN ... THEN ... ELSE ... END CASE CASE WHEN ... THEN ... ELSE ... END CASE 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 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 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. 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 ELSE statements 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; 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 ELSE statements 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. loop in 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. <<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. EXIT in 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; CONTINUE in 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; WHILE in 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; <<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. 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 substituted into the query text, 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 . The FOREACH loop is much like a FOR loop, but instead of iterating through the rows returned by a 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} exceptions in 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. 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. 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 . Name Type Description RETURNED_SQLSTATE text the SQLSTATE error code of the exception COLUMN_NAME text the name of the column related to exception CONSTRAINT_NAME text the name of the constraint related to exception PG_DATATYPE_NAME text the name of the data type related to exception MESSAGE_TEXT text the text of the exception's primary message TABLE_NAME text the name of the table related to exception SCHEMA_NAME text the name of the schema related to exception PG_EXCEPTION_DETAIL text the text of the exception's detail message, if any PG_EXCEPTION_HINT text the text of the exception's hint message, if any PG_EXCEPTION_CONTEXT text line(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; 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. cursor in 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. 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 name datatype 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. 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 . 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 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. 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; 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 a so-called 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 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, ABSOLUTE count, RELATIVE count, 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 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 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 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; PL/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 is null, OPEN automatically generates a name that does not conflict with any existing portal, and assigns it to the refcursor variable. A bound cursor variable is initialized to the string value representing its name, so that the portal name is the same as the cursor variable name, unless the programmer overrides it by assignment before opening the cursor. But an unbound cursor variable defaults to the null value initially, so it will receive an automatically-generated unique name, unless overridden. 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; $$ LANGUAGE plpgsql; -- need to be in a transaction to use cursors. BEGIN; SELECT * FROM myfunc('a', 'b'); FETCH ALL FROM a; FETCH ALL FROM b; COMMIT; There is a variant of the FOR statement that allows iterating through the rows returned by a cursor. The syntax is: <<label>> FOR recordvar IN bound_cursorvar ( argument_name := argument_value , ... ) LOOP statements END LOOP label ; The cursor variable must have been bound to some query when it was declared, and it cannot be open already. The FOR statement automatically opens the cursor, and it closes the cursor again when the loop exits. A list of actual argument value expressions must appear if and only if the cursor was declared to take arguments. These values will be substituted in the query, in just the same way as during an OPEN (see ). The variable recordvar is automatically defined as type record and exists only inside the loop (any existing definition of the variable name is ignored within the loop). Each row returned by the cursor is successively assigned to this record variable and the loop body is executed. In procedures invoked by the CALL command as well as in anonymous code blocks (DO command), it is possible to end transactions using the commands COMMIT and ROLLBACK. A new transaction is started automatically after a transaction is ended using these commands, so there is no separate START TRANSACTION command. (Note that BEGIN and END have different meanings in PL/pgSQL.) Here is a simple example: CREATE PROCEDURE transaction_test1() LANGUAGE plpgsql AS $$ 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; $$; CALL transaction_test1(); chained transactions in PL/pgSQL A new transaction starts out with default transaction characteristics such as transaction isolation level. In cases where transactions are committed in a loop, it might be desirable to start new transactions automatically with the same characteristics as the previous one. The commands COMMIT AND CHAIN and ROLLBACK AND CHAIN accomplish this. Transaction control is only possible in CALL or DO invocations from the top level or nested CALL or DO invocations without any other intervening command. For example, if the call stack is CALL proc1() → CALL proc2() → CALL proc3(), then the second and third procedures can perform transaction control actions. But if the call stack is CALL proc1() → SELECT func2() → CALL proc3(), then the last procedure cannot do transaction control, because of the SELECT in between. Special considerations apply to cursor loops. Consider this example: CREATE PROCEDURE transaction_test2() LANGUAGE plpgsql AS $$ 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. RAISE in PL/pgSQL reporting errors in PL/pgSQL Use the RAISE statement to report messages and raise errors. RAISE level 'format' , expression , ... USING option = expression , ... ; RAISE level condition_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 write a format (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: MESSAGE Sets the error message text. This option can't be used in the form of RAISE that includes a format string before USING. DETAIL Supplies an error detail message. HINT Supplies a hint message. ERRCODE Specifies the error code (SQLSTATE) to report, either by condition name, as shown in , or directly as a five-character SQLSTATE code. COLUMN CONSTRAINT DATATYPE TABLE SCHEMA Supplies 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 ERRCODE_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. ASSERT in PL/pgSQL assertions in PL/pgSQL plpgsql.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 in PL/pgSQL PL/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. 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: NEW Data type RECORD; variable holding the new database row for INSERT/UPDATE operations in row-level triggers. This variable is null in statement-level triggers and for DELETE operations. OLD Data type RECORD; variable holding the old database row for UPDATE/DELETE operations in row-level triggers. This variable is null in statement-level triggers and for INSERT operations. TG_NAME Data type name; variable that contains the name of the trigger actually fired. TG_WHEN Data type text; a string of BEFORE, AFTER, or INSTEAD OF, depending on the trigger's definition. TG_LEVEL Data type text; a string of either ROW or STATEMENT depending on the trigger's definition. TG_OP Data type text; a string of INSERT, UPDATE, DELETE, or TRUNCATE telling for which operation the trigger was fired. TG_RELID Data type oid; the object ID of the table that caused the trigger invocation. TG_RELNAME Data type name; the name of the table that caused the trigger invocation. This is now deprecated, and could disappear in a future release. Use TG_TABLE_NAME instead. TG_TABLE_NAME Data type name; the name of the table that caused the trigger invocation. TG_TABLE_SCHEMA Data type name; the name of the schema of the table that caused the trigger invocation. TG_NARGS Data type integer; the number of arguments given to the trigger function in the CREATE TRIGGER statement. TG_ARGV[] Data type array of text; the 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. 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. 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(), user, OLD.*; ELSIF (TG_OP = 'UPDATE') THEN INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*; ELSIF (TG_OP = 'INSERT') THEN INSERT INTO emp_audit SELECT 'I', now(), 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. 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', 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', 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', 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. 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. 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(), user, o.* FROM old_table o; ELSIF (TG_OP = 'UPDATE') THEN INSERT INTO emp_audit SELECT 'U', now(), user, n.* FROM new_table n; ELSIF (TG_OP = 'INSERT') THEN INSERT INTO emp_audit SELECT 'I', now(), 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(); PL/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_EVENT Data type text; a string representing the event the trigger is fired for. TG_TAG Data type text; variable that contains the command tag for which the trigger is fired. shows an example of an event trigger function in PL/pgSQL. 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(); This section discusses some implementation details that are frequently important for PL/pgSQL users to know. 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. Parameters will only be substituted in places where a parameter or column reference is syntactically allowed. As an extreme case, consider this example of poor programming style: INSERT INTO foo (foo) VALUES (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 the table, so it will not be substituted either. Only the third occurrence is a candidate to be a reference to the function's variable. PostgreSQL versions before 9.0 would try to substitute the variable in all three cases, leading to syntax errors. 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 a 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 the 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, and DELETE commands, 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. 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 query in 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/pgSQL EXECUTE 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; $$ LANGUAGE plpgsql; In the case of logfunc1, the PostgreSQL main parser knows when analyzing the INSERT that the string 'now' should be interpreted as timestamp, because the target column of logtable is of that type. Thus, 'now' will be converted to a timestamp constant when the INSERT is analyzed, and then used in all invocations of logfunc1 during the lifetime of the session. Needless to say, this isn't what the programmer wanted. A better idea is to use the now() or current_timestamp function. In the case of logfunc2, the PostgreSQL main parser does not know what type 'now' should become and therefore it returns a data value of type text containing the string now. During the ensuing assignment to the local variable curtime, the PL/pgSQL interpreter casts this string to the timestamp type by calling the textout and timestamp_in functions for the conversion. So, the computed time stamp is updated on each execution as the programmer expects. Even though this happens to work as expected, it's not terribly efficient, so use of the now() function would still be a better idea. One good way to develop in PL/pgSQL is to use the text editor of your choice to create your functions, and in another window, use psql to load and test those functions. If you are doing it this way, it is a good idea to write the function using CREATE OR REPLACE FUNCTION. That way you can just reload the file to update the function definition. For example: CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $$ .... $$ LANGUAGE plpgsql; While running psql, you can load or reload such a function definition file with: \i filename.sql and then immediately issue SQL commands to test the function. Another good way to develop in PL/pgSQL is with a GUI database access tool that facilitates development in a procedural language. One example of such a tool is pgAdmin, although others exist. These tools often provide convenient features such as escaping single quotes and making it easier to recreate and debug functions. The code of a PL/pgSQL function is specified in CREATE FUNCTION as a string literal. If you write the string literal in the ordinary way with surrounding single quotes, then any single quotes inside the function body must be doubled; likewise any backslashes must be doubled (assuming escape string syntax is used). Doubling quotes is at best tedious, and in more complicated cases the code can become downright incomprehensible, because you can easily find yourself needing half a dozen or more adjacent quote marks. It's recommended that you instead write the function body as a dollar-quoted string literal (see ). In the dollar-quoting approach, you never double any quote marks, but instead take care to choose a different dollar-quoting delimiter for each level of nesting you need. For example, you might write the CREATE FUNCTION command as: CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $PROC$ .... $PROC$ LANGUAGE plpgsql; Within this, you might use quote marks for simple literal strings in SQL commands and $$ 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. 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) Oracle porting from PL/SQL to PL/pgSQL PL/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 a SQL command could be either a column name of a table or a reference to a variable of the function, PL/SQL treats it as a column name. This corresponds to PL/pgSQL's plpgsql.variable_conflict = use_column behavior, which is not the default, 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. shows how to port a simple function from PL/SQL to PL/pgSQL. Here is an Oracle PL/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. 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. 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. 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 RAISE exception_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. This section explains a few other things to watch for when porting Oracle PL/SQL functions to PostgreSQL. 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. 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. PostgreSQL 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; 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; ]]>