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diff --git a/doc/src/sgml/ref/create_function.sgml b/doc/src/sgml/ref/create_function.sgml new file mode 100644 index 0000000..7e6d52c --- /dev/null +++ b/doc/src/sgml/ref/create_function.sgml @@ -0,0 +1,935 @@ +<!-- +doc/src/sgml/ref/create_function.sgml +PostgreSQL documentation +--> + +<refentry id="sql-createfunction"> + <indexterm zone="sql-createfunction"> + <primary>CREATE FUNCTION</primary> + </indexterm> + + <refmeta> + <refentrytitle>CREATE FUNCTION</refentrytitle> + <manvolnum>7</manvolnum> + <refmiscinfo>SQL - Language Statements</refmiscinfo> + </refmeta> + + <refnamediv> + <refname>CREATE FUNCTION</refname> + <refpurpose>define a new function</refpurpose> + </refnamediv> + + <refsynopsisdiv> +<synopsis> +CREATE [ OR REPLACE ] FUNCTION + <replaceable class="parameter">name</replaceable> ( [ [ <replaceable class="parameter">argmode</replaceable> ] [ <replaceable class="parameter">argname</replaceable> ] <replaceable class="parameter">argtype</replaceable> [ { DEFAULT | = } <replaceable class="parameter">default_expr</replaceable> ] [, ...] ] ) + [ RETURNS <replaceable class="parameter">rettype</replaceable> + | RETURNS TABLE ( <replaceable class="parameter">column_name</replaceable> <replaceable class="parameter">column_type</replaceable> [, ...] ) ] + { LANGUAGE <replaceable class="parameter">lang_name</replaceable> + | TRANSFORM { FOR TYPE <replaceable class="parameter">type_name</replaceable> } [, ... ] + | WINDOW + | { IMMUTABLE | STABLE | VOLATILE } + | [ NOT ] LEAKPROOF + | { CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT } + | { [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER } + | PARALLEL { UNSAFE | RESTRICTED | SAFE } + | COST <replaceable class="parameter">execution_cost</replaceable> + | ROWS <replaceable class="parameter">result_rows</replaceable> + | SUPPORT <replaceable class="parameter">support_function</replaceable> + | SET <replaceable class="parameter">configuration_parameter</replaceable> { TO <replaceable class="parameter">value</replaceable> | = <replaceable class="parameter">value</replaceable> | FROM CURRENT } + | AS '<replaceable class="parameter">definition</replaceable>' + | AS '<replaceable class="parameter">obj_file</replaceable>', '<replaceable class="parameter">link_symbol</replaceable>' + | <replaceable class="parameter">sql_body</replaceable> + } ... +</synopsis> + </refsynopsisdiv> + + <refsect1 id="sql-createfunction-description"> + <title>Description</title> + + <para> + <command>CREATE FUNCTION</command> defines a new function. + <command>CREATE OR REPLACE FUNCTION</command> will either create a + new function, or replace an existing definition. + To be able to define a function, the user must have the + <literal>USAGE</literal> privilege on the language. + </para> + + <para> + If a schema name is included, then the function is created in the + specified schema. Otherwise it is created in the current schema. + The name of the new function must not match any existing function or procedure + with the same input argument types in the same schema. However, + functions and procedures of different argument types can share a name (this is + called <firstterm>overloading</firstterm>). + </para> + + <para> + To replace the current definition of an existing function, use + <command>CREATE OR REPLACE FUNCTION</command>. It is not possible + to change the name or argument types of a function this way (if you + tried, you would actually be creating a new, distinct function). + Also, <command>CREATE OR REPLACE FUNCTION</command> will not let + you change the return type of an existing function. To do that, + you must drop and recreate the function. (When using <literal>OUT</literal> + parameters, that means you cannot change the types of any + <literal>OUT</literal> parameters except by dropping the function.) + </para> + + <para> + When <command>CREATE OR REPLACE FUNCTION</command> is used to replace an + existing function, the ownership and permissions of the function + do not change. All other function properties are assigned the + values specified or implied in the command. You must own the function + to replace it (this includes being a member of the owning role). + </para> + + <para> + If you drop and then recreate a function, the new function is not + the same entity as the old; you will have to drop existing rules, views, + triggers, etc. that refer to the old function. Use + <command>CREATE OR REPLACE FUNCTION</command> to change a function + definition without breaking objects that refer to the function. + Also, <command>ALTER FUNCTION</command> can be used to change most of the + auxiliary properties of an existing function. + </para> + + <para> + The user that creates the function becomes the owner of the function. + </para> + + <para> + To be able to create a function, you must have <literal>USAGE</literal> + privilege on the argument types and the return type. + </para> + + <para> + Refer to <xref linkend="xfunc"/> for further information on writing + functions. + </para> + </refsect1> + + <refsect1> + <title>Parameters</title> + + <variablelist> + + <varlistentry> + <term><replaceable class="parameter">name</replaceable></term> + + <listitem> + <para> + The name (optionally schema-qualified) of the function to create. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">argmode</replaceable></term> + + <listitem> + <para> + The mode of an argument: <literal>IN</literal>, <literal>OUT</literal>, + <literal>INOUT</literal>, or <literal>VARIADIC</literal>. + If omitted, the default is <literal>IN</literal>. + Only <literal>OUT</literal> arguments can follow a <literal>VARIADIC</literal> one. + Also, <literal>OUT</literal> and <literal>INOUT</literal> arguments cannot be used + together with the <literal>RETURNS TABLE</literal> notation. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">argname</replaceable></term> + + <listitem> + <para> + The name of an argument. Some languages (including SQL and PL/pgSQL) + let you use the name in the function body. For other languages the + name of an input argument is just extra documentation, so far as + the function itself is concerned; but you can use input argument names + when calling a function to improve readability (see <xref + linkend="sql-syntax-calling-funcs"/>). In any case, the name + of an output argument is significant, because it defines the column + name in the result row type. (If you omit the name for an output + argument, the system will choose a default column name.) + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">argtype</replaceable></term> + + <listitem> + <para> + The data type(s) of the function's arguments (optionally + schema-qualified), if any. The argument types can be base, composite, + or domain types, or can reference the type of a table column. + </para> + <para> + Depending on the implementation language it might also be allowed + to specify <quote>pseudo-types</quote> such as <type>cstring</type>. + Pseudo-types indicate that the actual argument type is either + incompletely specified, or outside the set of ordinary SQL data types. + </para> + <para> + The type of a column is referenced by writing + <literal><replaceable + class="parameter">table_name</replaceable>.<replaceable + class="parameter">column_name</replaceable>%TYPE</literal>. + Using this feature can sometimes help make a function independent of + changes to the definition of a table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">default_expr</replaceable></term> + + <listitem> + <para> + An expression to be used as default value if the parameter is + not specified. The expression has to be coercible to the + argument type of the parameter. + Only input (including <literal>INOUT</literal>) parameters can have a default + value. All input parameters following a + parameter with a default value must have default values as well. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">rettype</replaceable></term> + + <listitem> + <para> + The return data type (optionally schema-qualified). The return type + can be a base, composite, or domain type, + or can reference the type of a table column. + Depending on the implementation language it might also be allowed + to specify <quote>pseudo-types</quote> such as <type>cstring</type>. + If the function is not supposed to return a value, specify + <type>void</type> as the return type. + </para> + <para> + When there are <literal>OUT</literal> or <literal>INOUT</literal> parameters, + the <literal>RETURNS</literal> clause can be omitted. If present, it + must agree with the result type implied by the output parameters: + <literal>RECORD</literal> if there are multiple output parameters, or + the same type as the single output parameter. + </para> + <para> + The <literal>SETOF</literal> + modifier indicates that the function will return a set of + items, rather than a single item. + </para> + <para> + The type of a column is referenced by writing + <literal><replaceable + class="parameter">table_name</replaceable>.<replaceable + class="parameter">column_name</replaceable>%TYPE</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">column_name</replaceable></term> + + <listitem> + <para> + The name of an output column in the <literal>RETURNS TABLE</literal> + syntax. This is effectively another way of declaring a named + <literal>OUT</literal> parameter, except that <literal>RETURNS TABLE</literal> + also implies <literal>RETURNS SETOF</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">column_type</replaceable></term> + + <listitem> + <para> + The data type of an output column in the <literal>RETURNS TABLE</literal> + syntax. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">lang_name</replaceable></term> + + <listitem> + <para> + The name of the language that the function is implemented in. + It can be <literal>sql</literal>, <literal>c</literal>, + <literal>internal</literal>, or the name of a user-defined + procedural language, e.g., <literal>plpgsql</literal>. The default is + <literal>sql</literal> if <replaceable + class="parameter">sql_body</replaceable> is specified. Enclosing the + name in single quotes is deprecated and requires matching case. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>TRANSFORM { FOR TYPE <replaceable class="parameter">type_name</replaceable> } [, ... ] }</literal></term> + + <listitem> + <para> + Lists which transforms a call to the function should apply. Transforms + convert between SQL types and language-specific data types; + see <xref linkend="sql-createtransform"/>. Procedural language + implementations usually have hardcoded knowledge of the built-in types, + so those don't need to be listed here. If a procedural language + implementation does not know how to handle a type and no transform is + supplied, it will fall back to a default behavior for converting data + types, but this depends on the implementation. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>WINDOW</literal></term> + + <listitem> + <para><literal>WINDOW</literal> indicates that the function is a + <firstterm>window function</firstterm> rather than a plain function. + This is currently only useful for functions written in C. + The <literal>WINDOW</literal> attribute cannot be changed when + replacing an existing function definition. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>IMMUTABLE</literal></term> + <term><literal>STABLE</literal></term> + <term><literal>VOLATILE</literal></term> + + <listitem> + <para> + These attributes inform the query optimizer about the behavior + of the function. At most one choice + can be specified. If none of these appear, + <literal>VOLATILE</literal> is the default assumption. + </para> + + <para><literal>IMMUTABLE</literal> indicates that the function + cannot modify the database and always + returns the same result when given the same argument values; that + is, it does not do database lookups or otherwise use information not + directly present in its argument list. If this option is given, + any call of the function with all-constant arguments can be + immediately replaced with the function value. + </para> + + <para><literal>STABLE</literal> indicates that the function + cannot modify the database, + and that within a single table scan it will consistently + return the same result for the same argument values, but that its + result could change across SQL statements. This is the appropriate + selection for functions whose results depend on database lookups, + parameter variables (such as the current time zone), etc. (It is + inappropriate for <literal>AFTER</literal> triggers that wish to + query rows modified by the current command.) Also note + that the <function>current_timestamp</function> family of functions qualify + as stable, since their values do not change within a transaction. + </para> + + <para><literal>VOLATILE</literal> indicates that the function value can + change even within a single table scan, so no optimizations can be + made. Relatively few database functions are volatile in this sense; + some examples are <literal>random()</literal>, <literal>currval()</literal>, + <literal>timeofday()</literal>. But note that any function that has + side-effects must be classified volatile, even if its result is quite + predictable, to prevent calls from being optimized away; an example is + <literal>setval()</literal>. + </para> + + <para> + For additional details see <xref linkend="xfunc-volatility"/>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>LEAKPROOF</literal></term> + <listitem> + <para> + <literal>LEAKPROOF</literal> indicates that the function has no side + effects. It reveals no information about its arguments other than by + its return value. For example, a function which throws an error message + for some argument values but not others, or which includes the argument + values in any error message, is not leakproof. This affects how the + system executes queries against views created with the + <literal>security_barrier</literal> option or tables with row level + security enabled. The system will enforce conditions from security + policies and security barrier views before any user-supplied conditions + from the query itself that contain non-leakproof functions, in order to + prevent the inadvertent exposure of data. Functions and operators + marked as leakproof are assumed to be trustworthy, and may be executed + before conditions from security policies and security barrier views. + In addition, functions which do not take arguments or which are not + passed any arguments from the security barrier view or table do not have + to be marked as leakproof to be executed before security conditions. See + <xref linkend="sql-createview"/> and <xref linkend="rules-privileges"/>. + This option can only be set by the superuser. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>CALLED ON NULL INPUT</literal></term> + <term><literal>RETURNS NULL ON NULL INPUT</literal></term> + <term><literal>STRICT</literal></term> + + <listitem> + <para><literal>CALLED ON NULL INPUT</literal> (the default) indicates + that the function will be called normally when some of its + arguments are null. It is then the function author's + responsibility to check for null values if necessary and respond + appropriately. + </para> + + <para><literal>RETURNS NULL ON NULL INPUT</literal> or + <literal>STRICT</literal> indicates that the function always + returns null whenever any of its arguments are null. If this + parameter is specified, the function is not executed when there + are null arguments; instead a null result is assumed + automatically. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal><optional>EXTERNAL</optional> SECURITY INVOKER</literal></term> + <term><literal><optional>EXTERNAL</optional> SECURITY DEFINER</literal></term> + + <listitem> + <para><literal>SECURITY INVOKER</literal> indicates that the function + is to be executed with the privileges of the user that calls it. + That is the default. <literal>SECURITY DEFINER</literal> + specifies that the function is to be executed with the + privileges of the user that owns it. + </para> + + <para> + The key word <literal>EXTERNAL</literal> is allowed for SQL + conformance, but it is optional since, unlike in SQL, this feature + applies to all functions not only external ones. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>PARALLEL</literal></term> + + <listitem> + <para><literal>PARALLEL UNSAFE</literal> indicates that the function + can't be executed in parallel mode and the presence of such a + function in an SQL statement forces a serial execution plan. This is + the default. <literal>PARALLEL RESTRICTED</literal> indicates that + the function can be executed in parallel mode, but the execution is + restricted to parallel group leader. <literal>PARALLEL SAFE</literal> + indicates that the function is safe to run in parallel mode without + restriction. + </para> + + <para> + Functions should be labeled parallel unsafe if they modify any database + state, or if they make changes to the transaction such as using + sub-transactions, or if they access sequences or attempt to make + persistent changes to settings (e.g., <literal>setval</literal>). They should + be labeled as parallel restricted if they access temporary tables, + client connection state, cursors, prepared statements, or miscellaneous + backend-local state which the system cannot synchronize in parallel mode + (e.g., <literal>setseed</literal> cannot be executed other than by the group + leader because a change made by another process would not be reflected + in the leader). In general, if a function is labeled as being safe when + it is restricted or unsafe, or if it is labeled as being restricted when + it is in fact unsafe, it may throw errors or produce wrong answers + when used in a parallel query. C-language functions could in theory + exhibit totally undefined behavior if mislabeled, since there is no way + for the system to protect itself against arbitrary C code, but in most + likely cases the result will be no worse than for any other function. + If in doubt, functions should be labeled as <literal>UNSAFE</literal>, which is + the default. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>COST</literal> <replaceable class="parameter">execution_cost</replaceable></term> + + <listitem> + <para> + A positive number giving the estimated execution cost for the function, + in units of <xref linkend="guc-cpu-operator-cost"/>. If the function + returns a set, this is the cost per returned row. If the cost is + not specified, 1 unit is assumed for C-language and internal functions, + and 100 units for functions in all other languages. Larger values + cause the planner to try to avoid evaluating the function more often + than necessary. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>ROWS</literal> <replaceable class="parameter">result_rows</replaceable></term> + + <listitem> + <para> + A positive number giving the estimated number of rows that the planner + should expect the function to return. This is only allowed when the + function is declared to return a set. The default assumption is + 1000 rows. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SUPPORT</literal> <replaceable class="parameter">support_function</replaceable></term> + + <listitem> + <para> + The name (optionally schema-qualified) of a <firstterm>planner support + function</firstterm> to use for this function. See + <xref linkend="xfunc-optimization"/> for details. + You must be superuser to use this option. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable>configuration_parameter</replaceable></term> + <term><replaceable>value</replaceable></term> + <listitem> + <para> + The <literal>SET</literal> clause causes the specified configuration + parameter to be set to the specified value when the function is + entered, and then restored to its prior value when the function exits. + <literal>SET FROM CURRENT</literal> saves the value of the parameter that + is current when <command>CREATE FUNCTION</command> is executed as the value + to be applied when the function is entered. + </para> + + <para> + If a <literal>SET</literal> clause is attached to a function, then + the effects of a <command>SET LOCAL</command> command executed inside the + function for the same variable are restricted to the function: the + configuration parameter's prior value is still restored at function exit. + However, an ordinary + <command>SET</command> command (without <literal>LOCAL</literal>) overrides the + <literal>SET</literal> clause, much as it would do for a previous <command>SET + LOCAL</command> command: the effects of such a command will persist after + function exit, unless the current transaction is rolled back. + </para> + + <para> + See <xref linkend="sql-set"/> and + <xref linkend="runtime-config"/> + for more information about allowed parameter names and values. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">definition</replaceable></term> + + <listitem> + <para> + A string constant defining the function; the meaning depends on the + language. It can be an internal function name, the path to an + object file, an SQL command, or text in a procedural language. + </para> + + <para> + It is often helpful to use dollar quoting (see <xref + linkend="sql-syntax-dollar-quoting"/>) to write the function definition + string, rather than the normal single quote syntax. Without dollar + quoting, any single quotes or backslashes in the function definition must + be escaped by doubling them. + </para> + + </listitem> + </varlistentry> + + <varlistentry> + <term><literal><replaceable class="parameter">obj_file</replaceable>, <replaceable class="parameter">link_symbol</replaceable></literal></term> + + <listitem> + <para> + This form of the <literal>AS</literal> clause is used for + dynamically loadable C language functions when the function name + in the C language source code is not the same as the name of + the SQL function. The string <replaceable + class="parameter">obj_file</replaceable> is the name of the shared + library file containing the compiled C function, and is interpreted + as for the <link linkend="sql-load"><command>LOAD</command></link> command. The string + <replaceable class="parameter">link_symbol</replaceable> is the + function's link symbol, that is, the name of the function in the C + language source code. If the link symbol is omitted, it is assumed to + be the same as the name of the SQL function being defined. The C names + of all functions must be different, so you must give overloaded C + functions different C names (for example, use the argument types as + part of the C names). + </para> + + <para> + When repeated <command>CREATE FUNCTION</command> calls refer to + the same object file, the file is only loaded once per session. + To unload and + reload the file (perhaps during development), start a new session. + </para> + + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">sql_body</replaceable></term> + + <listitem> + <para> + The body of a <literal>LANGUAGE SQL</literal> function. This can + either be a single statement +<programlisting> +RETURN <replaceable>expression</replaceable> +</programlisting> + or a block +<programlisting> +BEGIN ATOMIC + <replaceable>statement</replaceable>; + <replaceable>statement</replaceable>; + ... + <replaceable>statement</replaceable>; +END +</programlisting> + </para> + + <para> + This is similar to writing the text of the function body as a string + constant (see <replaceable>definition</replaceable> above), but there + are some differences: This form only works for <literal>LANGUAGE + SQL</literal>, the string constant form works for all languages. This + form is parsed at function definition time, the string constant form is + parsed at execution time; therefore this form cannot support + polymorphic argument types and other constructs that are not resolvable + at function definition time. This form tracks dependencies between the + function and objects used in the function body, so <literal>DROP + ... CASCADE</literal> will work correctly, whereas the form using + string literals may leave dangling functions. Finally, this form is + more compatible with the SQL standard and other SQL implementations. + </para> + </listitem> + </varlistentry> + + </variablelist> + </refsect1> + + <refsect1 id="sql-createfunction-overloading"> + <title>Overloading</title> + + <para> + <productname>PostgreSQL</productname> allows function + <firstterm>overloading</firstterm>; that is, the same name can be + used for several different functions so long as they have distinct + input argument types. Whether or not you use it, this capability entails + security precautions when calling functions in databases where some users + mistrust other users; see <xref linkend="typeconv-func"/>. + </para> + + <para> + Two functions are considered the same if they have the same names and + <emphasis>input</emphasis> argument types, ignoring any <literal>OUT</literal> + parameters. Thus for example these declarations conflict: +<programlisting> +CREATE FUNCTION foo(int) ... +CREATE FUNCTION foo(int, out text) ... +</programlisting> + </para> + + <para> + Functions that have different argument type lists will not be considered + to conflict at creation time, but if defaults are provided they might + conflict in use. For example, consider +<programlisting> +CREATE FUNCTION foo(int) ... +CREATE FUNCTION foo(int, int default 42) ... +</programlisting> + A call <literal>foo(10)</literal> will fail due to the ambiguity about which + function should be called. + </para> + + </refsect1> + + <refsect1 id="sql-createfunction-notes"> + <title>Notes</title> + + <para> + The full <acronym>SQL</acronym> type syntax is allowed for + declaring a function's arguments and return value. However, + parenthesized type modifiers (e.g., the precision field for + type <type>numeric</type>) are discarded by <command>CREATE FUNCTION</command>. + Thus for example + <literal>CREATE FUNCTION foo (varchar(10)) ...</literal> + is exactly the same as + <literal>CREATE FUNCTION foo (varchar) ...</literal>. + </para> + + <para> + When replacing an existing function with <command>CREATE OR REPLACE + FUNCTION</command>, there are restrictions on changing parameter names. + You cannot change the name already assigned to any input parameter + (although you can add names to parameters that had none before). + If there is more than one output parameter, you cannot change the + names of the output parameters, because that would change the + column names of the anonymous composite type that describes the + function's result. These restrictions are made to ensure that + existing calls of the function do not stop working when it is replaced. + </para> + + <para> + If a function is declared <literal>STRICT</literal> with a <literal>VARIADIC</literal> + argument, the strictness check tests that the variadic array <emphasis>as + a whole</emphasis> is non-null. The function will still be called if the + array has null elements. + </para> + + </refsect1> + + <refsect1 id="sql-createfunction-examples"> + <title>Examples</title> + + <para> + Add two integers using an SQL function: +<programlisting> +CREATE FUNCTION add(integer, integer) RETURNS integer + AS 'select $1 + $2;' + LANGUAGE SQL + IMMUTABLE + RETURNS NULL ON NULL INPUT; +</programlisting> + The same function written in a more SQL-conforming style, using argument + names and an unquoted body: +<programlisting> +CREATE FUNCTION add(a integer, b integer) RETURNS integer + LANGUAGE SQL + IMMUTABLE + RETURNS NULL ON NULL INPUT + RETURN a + b; +</programlisting> + </para> + + <para> + Increment an integer, making use of an argument name, in + <application>PL/pgSQL</application>: +<programlisting> +CREATE OR REPLACE FUNCTION increment(i integer) RETURNS integer AS $$ + BEGIN + RETURN i + 1; + END; +$$ LANGUAGE plpgsql; +</programlisting> + </para> + + <para> + Return a record containing multiple output parameters: +<programlisting> +CREATE FUNCTION dup(in int, out f1 int, out f2 text) + AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$ + LANGUAGE SQL; + +SELECT * FROM dup(42); +</programlisting> + You can do the same thing more verbosely with an explicitly named + composite type: +<programlisting> +CREATE TYPE dup_result AS (f1 int, f2 text); + +CREATE FUNCTION dup(int) RETURNS dup_result + AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$ + LANGUAGE SQL; + +SELECT * FROM dup(42); +</programlisting> + Another way to return multiple columns is to use a <literal>TABLE</literal> + function: +<programlisting> +CREATE FUNCTION dup(int) RETURNS TABLE(f1 int, f2 text) + AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$ + LANGUAGE SQL; + +SELECT * FROM dup(42); +</programlisting> + However, a <literal>TABLE</literal> function is different from the + preceding examples, because it actually returns a <emphasis>set</emphasis> + of records, not just one record. + </para> + </refsect1> + + <refsect1 id="sql-createfunction-security"> + <title>Writing <literal>SECURITY DEFINER</literal> Functions Safely</title> + + <indexterm> + <primary><varname>search_path</varname> configuration parameter</primary> + <secondary>use in securing functions</secondary> + </indexterm> + + <para> + Because a <literal>SECURITY DEFINER</literal> function is executed + with the privileges of the user that owns it, care is needed to + ensure that the function cannot be misused. For security, + <xref linkend="guc-search-path"/> should be set to exclude any schemas + writable by untrusted users. This prevents + malicious users from creating objects (e.g., tables, functions, and + operators) that mask objects intended to be used by the function. + Particularly important in this regard is the + temporary-table schema, which is searched first by default, and + is normally writable by anyone. A secure arrangement can be obtained + by forcing the temporary schema to be searched last. To do this, + write <literal>pg_temp</literal><indexterm><primary>pg_temp</primary><secondary>securing functions</secondary></indexterm> as the last entry in <varname>search_path</varname>. + This function illustrates safe usage: + +<programlisting> +CREATE FUNCTION check_password(uname TEXT, pass TEXT) +RETURNS BOOLEAN AS $$ +DECLARE passed BOOLEAN; +BEGIN + SELECT (pwd = $2) INTO passed + FROM pwds + WHERE username = $1; + + RETURN passed; +END; +$$ LANGUAGE plpgsql + SECURITY DEFINER + -- Set a secure search_path: trusted schema(s), then 'pg_temp'. + SET search_path = admin, pg_temp; +</programlisting> + + This function's intention is to access a table <literal>admin.pwds</literal>. + But without the <literal>SET</literal> clause, or with a <literal>SET</literal> clause + mentioning only <literal>admin</literal>, the function could be subverted by + creating a temporary table named <literal>pwds</literal>. + </para> + + <para> + Before <productname>PostgreSQL</productname> version 8.3, the + <literal>SET</literal> clause was not available, and so older functions may + contain rather complicated logic to save, set, and restore + <varname>search_path</varname>. The <literal>SET</literal> clause is far easier + to use for this purpose. + </para> + + <para> + Another point to keep in mind is that by default, execute privilege + is granted to <literal>PUBLIC</literal> for newly created functions + (see <xref linkend="ddl-priv"/> for more + information). Frequently you will wish to restrict use of a security + definer function to only some users. To do that, you must revoke + the default <literal>PUBLIC</literal> privileges and then grant execute + privilege selectively. To avoid having a window where the new function + is accessible to all, create it and set the privileges within a single + transaction. For example: + </para> + +<programlisting> +BEGIN; +CREATE FUNCTION check_password(uname TEXT, pass TEXT) ... SECURITY DEFINER; +REVOKE ALL ON FUNCTION check_password(uname TEXT, pass TEXT) FROM PUBLIC; +GRANT EXECUTE ON FUNCTION check_password(uname TEXT, pass TEXT) TO admins; +COMMIT; +</programlisting> + + </refsect1> + + <refsect1 id="sql-createfunction-compat"> + <title>Compatibility</title> + + <para> + A <command>CREATE FUNCTION</command> command is defined in the SQL + standard. The <productname>PostgreSQL</productname> implementation can be + used in a compatible way but has many extensions. Conversely, the SQL + standard specifies a number of optional features that are not implemented + in <productname>PostgreSQL</productname>. + </para> + + <para> + The following are important compatibility issues: + + <itemizedlist> + <listitem> + <para> + <literal>OR REPLACE</literal> is a PostgreSQL extension. + </para> + </listitem> + + <listitem> + <para> + For compatibility with some other database systems, <replaceable + class="parameter">argmode</replaceable> can be written either before or + after <replaceable class="parameter">argname</replaceable>. But only + the first way is standard-compliant. + </para> + </listitem> + + <listitem> + <para> + For parameter defaults, the SQL standard specifies only the syntax with + the <literal>DEFAULT</literal> key word. The syntax with + <literal>=</literal> is used in T-SQL and Firebird. + </para> + </listitem> + + <listitem> + <para> + The <literal>SETOF</literal> modifier is a PostgreSQL extension. + </para> + </listitem> + + <listitem> + <para> + Only <literal>SQL</literal> is standardized as a language. + </para> + </listitem> + + <listitem> + <para> + All other attributes except <literal>CALLED ON NULL INPUT</literal> and + <literal>RETURNS NULL ON NULL INPUT</literal> are not standardized. + </para> + </listitem> + + <listitem> + <para> + For the body of <literal>LANGUAGE SQL</literal> functions, the SQL + standard only specifies the <replaceable>sql_body</replaceable> form. + </para> + </listitem> + </itemizedlist> + </para> + + <para> + Simple <literal>LANGUAGE SQL</literal> functions can be written in a way + that is both standard-conforming and portable to other implementations. + More complex functions using advanced features, optimization attributes, or + other languages will necessarily be specific to PostgreSQL in a significant + way. + </para> + </refsect1> + + + <refsect1> + <title>See Also</title> + + <simplelist type="inline"> + <member><xref linkend="sql-alterfunction"/></member> + <member><xref linkend="sql-dropfunction"/></member> + <member><xref linkend="sql-grant"/></member> + <member><xref linkend="sql-load"/></member> + <member><xref linkend="sql-revoke"/></member> + </simplelist> + </refsect1> + +</refentry> |