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$PostgreSQL: pgsql/doc/src/sgml/ref/create_function.sgml,v 1.75 2007/04/23 16:52:53 neilc Exp $
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<refentry id="SQL-CREATEFUNCTION">
 <refmeta>
  <refentrytitle id="SQL-CREATEFUNCTION-TITLE">CREATE FUNCTION</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE FUNCTION</refname>
  <refpurpose>define a new function</refpurpose>
 </refnamediv>

 <indexterm zone="sql-createfunction">
  <primary>CREATE FUNCTION</primary>
 </indexterm>

 <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> [, ...] ] )
    [ RETURNS <replaceable class="parameter">rettype</replaceable> ]
  { LANGUAGE <replaceable class="parameter">langname</replaceable>
    | IMMUTABLE | STABLE | VOLATILE
    | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
    | [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
    | COST <replaceable class="parameter">execution_cost</replaceable>
    | ROWS <replaceable class="parameter">result_rows</replaceable>
    | 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.
  </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 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 names or types of any
   <literal>OUT</> parameters except by dropping the function.)
  </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.
  </para>

  <para>
   The user that creates the function becomes the owner of the function.
  </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: either <literal>IN</>, <literal>OUT</>,
       or <literal>INOUT</>.  If omitted, the default is <literal>IN</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">argname</replaceable></term>

     <listitem>
      <para>
       The name of an argument. Some languages (currently only PL/pgSQL) let
       you use the name in the function body.  For other languages the
       name of an input argument is just extra documentation.  But the name
       of an output argument is significant, since 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>pseudotypes</> such as <type>cstring</>.
       Pseudotypes 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">tablename</replaceable>.<replaceable
       class="parameter">columnname</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">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>pseudotypes</> 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">tablename</replaceable>.<replaceable
       class="parameter">columnname</replaceable>%TYPE</literal>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">langname</replaceable></term>

     <listitem>
      <para>
       The name of the language that the function is implemented in.
       Can be <literal>SQL</literal>, <literal>C</literal>,
       <literal>internal</literal>, or the name of a user-defined
       procedural language.  For backward compatibility,
       the name can be enclosed by single quotes.
      </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.  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>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 created 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><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 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>
     </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>
     </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>

 </refsect1>

 <refsect1 id="sql-createfunction-notes">
  <title>Notes</title>

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

   <para>
    The full <acronym>SQL</acronym> type syntax is allowed for
    input arguments and return value. However, some details of the
    type specification (e.g., the precision field for
    type <type>numeric</type>) are the responsibility of the
    underlying function implementation and are silently swallowed
    (i.e., not recognized or
    enforced) by the <command>CREATE FUNCTION</command> command.
   </para>

   <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
    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>
    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), use the <xref
    linkend="sql-load" endterm="sql-load-title"> command.
   </para>

   <para>
    Use <xref linkend="sql-dropfunction"
    endterm="sql-dropfunction-title"> to remove user-defined
    functions.
   </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>

   <para>
    To be able to define a function, the user must have the
    <literal>USAGE</literal> privilege on the language.
   </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>
  </para>
 </refsect1>

 <refsect1 id="sql-createfunction-security">
  <title>Writing <literal>SECURITY DEFINER</literal> Functions Safely</title>

   <para>
    Because a <literal>SECURITY DEFINER</literal> function is executed
    with the privileges of the user that created 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 that mask objects 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 had
    by forcing the temporary schema to be searched last.  To do this,
    write <literal>pg_temp</> as the last entry in <varname>search_path</>.
    This function illustrates safe usage:
   </para>

<programlisting>
CREATE FUNCTION check_password(uname TEXT, pass TEXT)
RETURNS BOOLEAN AS $$
DECLARE passed BOOLEAN;
        old_path TEXT;
BEGIN
        -- Save old search_path; notice we must qualify current_setting
        -- to ensure we invoke the right function
        old_path := pg_catalog.current_setting('search_path');

        -- Set a secure search_path: trusted schemas, then 'pg_temp'.
        -- We set is_local = true so that the old value will be restored
        -- in event of an error before we reach the function end.
        PERFORM pg_catalog.set_config('search_path', 'admin, pg_temp', true);

        -- Do whatever secure work we came for.
        SELECT  (pwd = $2) INTO passed
        FROM    pwds
        WHERE   username = $1;

        -- Restore caller's search_path
        PERFORM pg_catalog.set_config('search_path', old_path, true);

        RETURN passed;
END;
$$ LANGUAGE plpgsql SECURITY DEFINER;
</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>
 </refsect1>


 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterfunction" endterm="sql-alterfunction-title"></member>
   <member><xref linkend="sql-dropfunction" endterm="sql-dropfunction-title"></member>
   <member><xref linkend="sql-grant" endterm="sql-grant-title"></member>
   <member><xref linkend="sql-load" endterm="sql-load-title"></member>
   <member><xref linkend="sql-revoke" endterm="sql-revoke-title"></member>
   <member><xref linkend="app-createlang" endterm="app-createlang-title"></member>
  </simplelist>
 </refsect1>

</refentry>
