create_function.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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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
    | 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 may share a name (this is
   called <firstterm>overloading</>).
  </para>

  <para>
   To update the 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 can't 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 may be base, composite,
       or domain types, or may reference the type of a table column.
      </para>
      <para>
       Depending on the implementation language it may 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 
       may be a base, composite, or domain type,
       or may reference the type of a table column.
       Depending on the implementation language it may also be allowed
       to specify <quote>pseudotypes</> such as <type>cstring</>.
      </para>
      <para>
       When there are <literal>OUT</> or <literal>INOUT</> parameters,
       the <literal>RETURNS</> clause may 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.
       May be <literal>SQL</literal>, <literal>C</literal>,
       <literal>internal</literal>, or the name of a user-defined
       procedural language.  For backward compatibility,
       the name may 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 system whether it is safe to
       replace multiple evaluations of the function with a single
       evaluation, for run-time optimization.  At most one choice
       may be specified.  If none of these appear,
       <literal>VOLATILE</literal> is the default assumption.
      </para>

      <para>
       <literal>IMMUTABLE</literal> indicates that the function 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 within a single table scan
       the function 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>