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Postgres documentation
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<refentry id="SQL-CREATELANGUAGE">
 <refmeta>
  <refentrytitle id="sql-createlanguage-title">
   CREATE LANGUAGE
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   CREATE LANGUAGE
  </refname>
  <refpurpose>
   Defines a new language for functions
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE [ TRUSTED ] [ PROCEDURAL ] LANGUAGE <replaceable class="parameter">langname</replaceable>
    HANDLER <replaceable class="parameter">call_handler</replaceable>
  </synopsis>
  
  <refsect2 id="R2-SQL-CREATELANGUAGE-1">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term>TRUSTED</term>
      <listitem>
       <para>
	<function> TRUSTED</function> specifies that the call handler for
	the language is safe; that is, it offers an unprivileged user
	no functionality to bypass access restrictions. If
	this keyword is omitted when registering the language,
	only users with the <productname>Postgres</productname>
	superuser privilege can use
	this language to create new functions.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">langname</replaceable></term>
      <listitem>
       <para>   
	The name of the new procedural language.
	The language name is case insensitive. A procedural 
	language cannot override one of the built-in languages of
	<productname>Postgres</productname>.
       </para>
       <para>
        For backward compatibility, the name may be enclosed by single
        quotes.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>HANDLER <replaceable class="parameter">call_handler</replaceable></term>
      <listitem>
       <para>
	<replaceable class="parameter">call_handler</replaceable> is the name
	of a previously
	registered function that will be called to execute the PL
	procedures.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>
   
  </refsect2>
  
  <refsect2 id="R2-SQL-CREATELANGUAGE-2">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE
       </computeroutput></term>
      <listitem>
       <para>
	This message is returned if the language is successfully
	created.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><computeroutput>
ERROR:  PL handler function <replaceable class="parameter">funcname</replaceable>() doesn't exist
       </computeroutput></term>
      <listitem>
       <para>
	This error is returned if the function
	<replaceable class="parameter">funcname</replaceable>()
	is not found.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-CREATELANGUAGE-1">
  <refsect1info>
   <date>1998-09-09</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   Using <command>CREATE LANGUAGE</command>, a 
   <productname>Postgres</productname> user can register
   a new language with <productname>Postgres</productname>. 
   Subsequently, functions and
   trigger procedures can be defined in this new language.
   The user must have the <productname>Postgres</productname>
   superuser privilege to
   register a new language.
  </para>

  <refsect2 id="R2-SQL-CREATELANGUAGE-3">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Writing PL handlers
   </title>

    <note>
     <para>
      In <productname>Postgres</productname> 7.1 and later, call handlers
      must adhere to the "version 1" function manager interface, not the
      old-style interface.
     </para>
    </note>

   <para>
    The call handler for a procedural language must be written
    in a compiled language such as C and registered with
    <productname>Postgres</productname> as a function taking
    no arguments and returning the
    <type>opaque</type> type, a placeholder for unspecified or undefined types.
    This prevents the call handler from being
    called directly as a function from queries.
    (However, arguments may be supplied in the actual call when a
    PL function in the language offered by the handler is to be executed.)
   </para>

   <para>
    The call handler is called in the same way as any other
    function: it receives a pointer to a FunctionCallInfoData struct
    containing argument values and information about the called function,
    and it is expected to return a Datum result (and possibly set the
    <literal>isnull</literal> field of the FunctionCallInfoData struct,
    if it wishes to return an SQL NULL result).  The difference between
    a call handler and an ordinary callee function is that the
    <literal>flinfo-&gt;fn_oid</literal> field of the FunctionCallInfoData
    struct will contain the OID of the PL function to be called, not of
    the call handler itself.  The call handler must use this field to
    determine which function to execute.  Also, the passed argument list
    has been set up according to the declaration of the target PL function,
    not of the call handler.
   </para>

   <para>
    It's up to the call handler to fetch the
    <filename>pg_proc</filename> entry and
    to analyze the argument and return types of the called
    procedure. The AS clause from the
    <command>CREATE FUNCTION</command> of
    the procedure will be found in the <literal>prosrc</literal>
    attribute of the
    <filename>pg_proc</filename> table entry. This may be the
    source text in the procedural
    language itself (like for PL/Tcl), a pathname to a
    file, or anything else that tells the call handler what to
    do in detail.
   </para>

   <para>
    Often, the same function is called many times per SQL statement.
    A call handler can avoid repeated lookups of information about the
    called function by using the <literal>flinfo-&gt;fn_extra</literal> field.
    This will initially be NULL, but can be set by the call handler to
    point at information about the PL function.  On subsequent calls,
    if <literal>flinfo-&gt;fn_extra</literal> is already non-NULL then it
    can be used and the information lookup step skipped.  The call handler
    must be careful that <literal>flinfo-&gt;fn_extra</literal> is made to
    point at memory that will live at least until the end of the current
    query, since an FmgrInfo data structure could be kept that long.
    One way to do this is to allocate the extra data in the memory context
    specified by <literal>flinfo-&gt;fn_mcxt</literal>; such data will
    normally have the same lifespan as the FmgrInfo itself.  But the handler
    could also choose to use a longer-lived context so that it can cache
    function definition information across queries.
   </para>

   <para>
    When a PL function is invoked as a trigger, no explicit arguments
    are passed, but the FunctionCallInfoData's
    <literal>context</literal> field points at a TriggerData node,
    rather than being NULL as it is in a plain function call.
    A PL handler should provide mechanisms for PL functions to get
    at the trigger information.
   </para>
  </refsect2>
   
  <refsect2 id="R2-SQL-CREATELANGUAGE-4">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    Use <command>CREATE FUNCTION</command>
    to create a function.
   </para>
   <para>
    Use <command>DROP LANGUAGE</command>  to drop procedural languages.
   </para>
   <para>
    Refer to the table <filename>pg_language</filename>
    for further information:
<programlisting>
<computeroutput>
        Table "pg_language"
   Attribute   |  Type   | Modifier
---------------+---------+----------
 lanname       | name    |
 lanispl       | boolean |
 lanpltrusted  | boolean |
 lanplcallfoid | oid     |
 lancompiler   | text    |

   lanname   | lanispl | lanpltrusted | lanplcallfoid | lancompiler
-------------+---------+--------------+---------------+-------------
 internal    | f       | f            |             0 | n/a
 C           | f       | f            |             0 | /bin/cc
 sql         | f       | f            |             0 | postgres
</computeroutput>
</programlisting>
   </para>

   <para>
    The call handler for a procedural language must normally be written
    in C and registered as 'internal' or 'C' language, depending
    on whether it is linked into the backend or dynamically loaded.
    The call handler cannot use the old-style 'C' function interface.
   </para>

   <para> 
    At present, the definitions for a procedural language cannot be
    changed once they have been created.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-CREATELANGUAGE-6">
  <title>
   Usage
  </title>
  <para>
   This is a template for a PL handler written in C:
   <programlisting>
#include "executor/spi.h"
#include "commands/trigger.h"
#include "utils/elog.h"
#include "fmgr.h"
#include "access/heapam.h"
#include "utils/syscache.h"
#include "catalog/pg_proc.h"
#include "catalog/pg_type.h"

PG_FUNCTION_INFO_V1(plsample_call_handler);

Datum
plsample_call_handler(PG_FUNCTION_ARGS)
{
     Datum          retval;

     if (CALLED_AS_TRIGGER(fcinfo))
     {
          /*
           * Called as a trigger procedure
           */
          TriggerData    *trigdata = (TriggerData *) fcinfo->context;

          retval = ...
     } else {
          /*
           * Called as a function
           */

          retval = ...
     }

     return retval;
}
   </programlisting>
  </para>

  <para>
   Only a few thousand lines of code have to be added instead
   of the dots to complete the PL call handler. 
   See <command>CREATE FUNCTION</command> for information on how to compile
   it into a loadable module.
  </para>
  <para>
   The following commands then register the sample procedural
   language:
   <programlisting>
CREATE FUNCTION plsample_call_handler () RETURNS opaque
    AS '/usr/local/pgsql/lib/plsample.so'
    LANGUAGE C;
CREATE LANGUAGE plsample
    HANDLER plsample_call_handler;
   </programlisting>
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-CREATELANGUAGE-7">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-CREATELANGUAGE-5">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    SQL92
   </title>

   <para>
    <command>CREATE LANGUAGE</command>
    is a <productname>Postgres</productname> extension.
    There is no <command>CREATE LANGUAGE</command> statement in
    <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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