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Move the "how to write a PL call handler" parts from the CREATE LANGUAGE

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man page to the Programmer's Guide.
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Peter Eisentraut
2001-09-06 10:28:39 +00:00
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doc/src/sgml/ref/create_language.sgml
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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_language.sgml,v 1.16 2001/09/03 12:57:49 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_language.sgml,v 1.17 2001/09/06 10:28:39 petere Exp $
Postgres documentation
-->
<refentry id="SQL-CREATELANGUAGE">
<docinfo>
<date>2001-09-05</date>
</docinfo>
<refmeta>
<refentrytitle id="sql-createlanguage-title">
CREATE LANGUAGE
</refentrytitle>
<refentrytitle id="sql-createlanguage-title">CREATE LANGUAGE</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
CREATE LANGUAGE
</refname>
<refpurpose>
define a new procedural language
</refpurpose>
<refname>CREATE LANGUAGE</refname>
<refpurpose>define a new procedural language</refpurpose>
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>1999-07-20</date>
</refsynopsisdivinfo>
<synopsis>
<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>
</synopsis>
</refsynopsisdiv>
<refsect1 id="R1-SQL-CREATELANGUAGE-1">
<refsect1info>
<date>1998-09-09</date>
</refsect1info>
<title>
Description
</title>
<refsect1 id="sql-createlanguage-description">
<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
Using <command>CREATE LANGUAGE</command>, a
<productname>PostgreSQL</productname> user can register a new
procedural language with a <productname>PostgreSQL</productname>
database. Subsequently, functions and trigger procedures can be
defined in this new language. The user must have the
<productname>PostgreSQL</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>
<para>
<command>CREATE LANGUAGE</command> effectively associates the
language name with a call handler that is responsible for executing
functions written in the language. Refer to the
<citetitle>Programmer's Guide</citetitle> for more information
about language call handlers.
</para>
<note>
<para>
Note that procedural languages are local to individual databases.
To make a language available in all databases by default, it should
be installed into the <literal>template1</literal> database.
</para>
</refsect1>
<refsect1 id="sql-createlanguage-parameters">
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>TRUSTED</literal></term>
<listitem>
<para>
<literal>TRUSTED</literal> specifies that the call handler for
the language is safe, that is, it does not offer an
unprivileged user any functionality to bypass access
restrictions. If this keyword is omitted when registering the
language, only users with the
<productname>PostgreSQL</productname> superuser privilege can
use this language to create new functions.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PROCEDURAL</literal></term>
<listitem>
<para>
This is a noise word.
</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>PostgreSQL</productname>.
</para>
<para>
For backward compatibility, the name may be enclosed by single
quotes.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>HANDLER</literal> <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 procedural language functions. The call
handler for a procedural language must be written in a compiled
language such as C with version 1 call convention and
registered with <productname>PostgreSQL</productname> as a
function taking no arguments and returning the
<type>opaque</type> type, a placeholder for unspecified or
undefined types.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id="sql-createlanguage-diagnostics">
<title>Diagnostics</title>
<msgset>
<msgentry>
<msg>
<msgmain>
<msgtext>
<screen>
CREATE
</screen>
</msgtext>
</msgmain>
</msg>
<msgexplan>
<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.
This message is returned if the language is successfully
created.
</para>
</note>
</msgexplan>
</msgentry>
<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>
<msgentry>
<msg>
<msgmain>
<msgtext>
<screen>
ERROR: PL handler function <replaceable class="parameter">funcname</replaceable>() doesn't exist
</screen>
</msgtext>
</msgmain>
</msg>
<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>
<msgexplan>
<para>
This error is returned if the function <replaceable
class="parameter">funcname</replaceable>() is not found.
</para>
</msgexplan>
</msgentry>
</msgset>
</refsect1>
<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>
<refsect1 id="sql-createlanguage-notes">
<title>Notes</title>
<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>
This command normally should not be executed directly by users.
For the procedural languages supplied in the
<productname>PostgreSQL</productname> distribution, the <xref
linkend="app-createlang"> program should be used, which will also
install the correct call handler. (<command>createlang</command>
will call <command>CREATE LANGUAGE</command> internally.)
</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>
<para>
Use the <xref linkend="sql-createfunction"> command to create a new
function.
</para>
<para>
Use <xref linkend="sql-droplanguage">, or better yet the <xref
linkend="app-droplang"> program, to drop procedural languages.
</para>
<para>
The system catalog <classname>pg_language</classname> records
information about the currently installed procedural languages.
<screen>
Table "pg_language"
Attribute | Type | Modifier
---------------+---------+----------
@ -261,107 +205,61 @@ ERROR: PL handler function <replaceable class="parameter">funcname</replaceable
internal | f | f | 0 | n/a
C | f | f | 0 | /bin/cc
sql | f | f | 0 | postgres
</computeroutput>
</programlisting>
</para>
</screen>
</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>
<para>
At present, the definition of a procedural language cannot be
changed once is has been created.
</para>
</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>
<refsect1 id="sql-createlanguage-examples">
<title>Examples</title>
<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>
The following two commands executed in sequence will register a new
procedural language and the associated call handler.
<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>
</programlisting>
</para>
</refsect1>
<refsect1 id="R1-SQL-CREATELANGUAGE-7">
<title>
Compatibility
</title>
<refsect1 id="sql-createlanguage-compat">
<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>PostgreSQL</productname> extension.
</para>
</refsect1>
<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>
<title>History</title>
<para>
The <command>CREATE LANGUAGE</command> command first appeared in
<productname>PostgreSQL</productname> 6.3.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<simplelist type="inline">
<member><xref linkend="app-createlang"></member>
<member><xref linkend="sql-createfunction"></member>
<member><xref linkend="app-droplang"></member>
<member><xref linkend="sql-droplanguage"></member>
<member><citetitle>PostgreSQL Programmer's Guide</citetitle></member>
</simplelist>
</para>
</refsect1>
</refentry>