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[postgresql] / doc / src / sgml / ref / create_function.sgml

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<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>
    | 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>'
  } ...
    [ WITH ( <replaceable class="parameter">attribute</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
   with the same input argument types in the same schema.  However,
   functions of different argument types can share a name (this is
   called <firstterm>overloading</>).
  </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</>
   parameters, that means you cannot change the types of any
   <literal>OUT</> parameters except by dropping the function.)
  </para>

  <para>
   When <command>CREATE OR REPLACE FUNCTION</> 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</> 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>
 </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>OUT</>,
       <literal>INOUT</>, or <literal>VARIADIC</>.
       If omitted, the default is <literal>IN</>.
       Only <literal>OUT</> arguments can follow a <literal>VARIADIC</> one.
       Also, <literal>OUT</> and <literal>INOUT</> arguments cannot be used
       together with the <literal>RETURNS TABLE</> 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</> such as <type>cstring</>.
       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</>) 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</> such as <type>cstring</>.
       If the function is not supposed to return a value, specify
       <type>void</> as the return type.
      </para>
      <para>
       When there are <literal>OUT</> or <literal>INOUT</> parameters,
       the <literal>RETURNS</> clause can be omitted.  If present, it
       must agree with the result type implied by the output parameters:
       <literal>RECORD</> 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</>
       syntax.  This is effectively another way of declaring a named
       <literal>OUT</> parameter, except that <literal>RETURNS TABLE</>
       also implies <literal>RETURNS SETOF</>.
      </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</>
       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>.  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</> rather than a plain function.
       This is currently only useful for functions written in C.
       The <literal>WINDOW</> 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</> triggers that wish to
       query rows modified by the current command.)  Also note
       that the <function>current_timestamp</> 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>currval()</>,
       <literal>timeofday()</>.  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()</>.
      </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</>).  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</> 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</>, which is
      the default.
     </para>
    </listitem>
   </varlistentry>

    <varlistentry>
     <term><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><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><replaceable>configuration_parameter</replaceable></term>
     <term><replaceable>value</replaceable></term>
     <listitem>
      <para>
       The <literal>SET</> 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</> saves the value of the parameter that
       is current when <command>CREATE FUNCTION</> is executed as the value
       to be applied when the function is entered.
      </para>

      <para>
       If a <literal>SET</> clause is attached to a function, then
       the effects of a <command>SET LOCAL</> 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 (without <literal>LOCAL</>) overrides the
       <literal>SET</> clause, much as it would do for a previous <command>SET
       LOCAL</> 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
       file containing the dynamically loadable object, and
       <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.
      </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">attribute</replaceable></term>

     <listitem>
      <para>
       The historical way to specify optional pieces of information
       about the function.  The following attributes can appear here:

      <variablelist>
       <varlistentry>
        <term><literal>isStrict</></term>
        <listitem>
         <para>
          Equivalent to <literal>STRICT</literal> or <literal>RETURNS NULL ON NULL INPUT</literal>.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>isCachable</></term>
        <listitem>
         <para><literal>isCachable</literal> is an obsolete equivalent of
          <literal>IMMUTABLE</literal>; it's still accepted for
          backwards-compatibility reasons.
         </para>
        </listitem>
       </varlistentry>

      </variablelist>

      Attribute names are not case-sensitive.
     </para>
    </listitem>
   </varlistentry>

   </variablelist>

   <para>
    Refer to <xref linkend="xfunc"> for further information on writing
    functions.
   </para>

 </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.  However, 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>
    Two functions are considered the same if they have the same names and
    <emphasis>input</> argument types, ignoring any <literal>OUT</>
    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)</> 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</>.
    Thus for example
    <literal>CREATE FUNCTION foo (varchar(10)) ...</>
    is exactly the same as
    <literal>CREATE FUNCTION foo (varchar) ...</>.
   </para>

   <para>
    When replacing an existing function with <command>CREATE OR REPLACE
    FUNCTION</>, 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</> with a <literal>VARIADIC</>
    argument, the strictness check tests that the variadic array <emphasis>as
    a whole</> 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>
   Here are some trivial examples to help you get started.  For more
   information and examples, see <xref linkend="xfunc">.
<programlisting>
CREATE FUNCTION add(integer, integer) RETURNS integer
    AS 'select $1 + $2;'
    LANGUAGE SQL
    IMMUTABLE
    RETURNS NULL ON NULL INPUT;
</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</>
   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</> function is different from the
   preceding examples, because it actually returns a <emphasis>set</>
   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</>
   <secondary>use in securing functions</>
  </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</><indexterm><primary>pg_temp</><secondary>securing functions</></> as the last entry in <varname>search_path</>.
    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</>.
    But without the <literal>SET</> clause, or with a <literal>SET</> clause
    mentioning only <literal>admin</>, the function could be subverted by
    creating a temporary table named <literal>pwds</>.
   </para>

   <para>
    Before <productname>PostgreSQL</productname> version 8.3, the
    <literal>SET</> clause was not available, and so older functions may
    contain rather complicated logic to save, set, and restore
    <varname>search_path</>.  The <literal>SET</> 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</> for newly created functions
    (see <xref linkend="sql-grant"> 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</> 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 SQL:1999 and later.
   The <productname>PostgreSQL</productname> version is similar but
   not fully compatible.  The attributes are not portable, neither are the
   different available languages.
  </para>

  <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>

  <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>
 </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>