mirror of
https://github.com/postgres/postgres.git
synced 2025-10-25 13:17:41 +03:00
Move docs about index cost estimation functions and writing a procedural
language handler to the 'Internals' area, per my proposal of yesterday. Clean up the trigger documentation a bit. Push SPI chapter to the end of its part, and reorder the Internals chapters into what seems a more sensible order (at the moment anyway).
This commit is contained in:
Tom Lane
174
doc/src/sgml/plhandler.sgml
Normal file
174
doc/src/sgml/plhandler.sgml
Normal file
@@ -0,0 +1,174 @@
|
||||
<!--
|
||||
$Header: /cvsroot/pgsql/doc/src/sgml/plhandler.sgml,v 1.1 2003/10/22 22:28:10 tgl Exp $
|
||||
-->
|
||||
|
||||
<chapter id="plhandler">
|
||||
<title>Writing A Procedural Language Handler</title>
|
||||
|
||||
<indexterm zone="plhandler">
|
||||
<primary>procedural language</primary>
|
||||
<secondary>handler for</secondary>
|
||||
</indexterm>
|
||||
|
||||
<para>
|
||||
All calls to functions that are written in a language other than
|
||||
the current <quote>version 1</quote> interface for compiled
|
||||
languages (this includes functions in user-defined procedural languages,
|
||||
functions written in SQL, and functions using the version 0 compiled
|
||||
language interface), go through a <firstterm>call handler</firstterm>
|
||||
function for the specific language. It is the responsibility of
|
||||
the call handler to execute the function in a meaningful way, such
|
||||
as by interpreting the supplied source text. This chapter outlines
|
||||
how a new procedural language's call handler can be written.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The call handler for a procedural language is a
|
||||
<quote>normal</quote> function that must be written in a compiled
|
||||
language such as C, using the version-1 interface, and registered
|
||||
with <productname>PostgreSQL</productname> as taking no arguments
|
||||
and returning the type <type>language_handler</type>. This
|
||||
special pseudotype identifies the function as a call handler and
|
||||
prevents it from being called directly in SQL commands.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The call handler is called in the same way as any other function:
|
||||
It receives a pointer to a
|
||||
<structname>FunctionCallInfoData</structname> <type>struct</> containing
|
||||
argument values and information about the called function, and it
|
||||
is expected to return a <type>Datum</type> result (and possibly
|
||||
set the <structfield>isnull</structfield> field of the
|
||||
<structname>FunctionCallInfoData</structname> structure, if it wishes
|
||||
to return an SQL null result). The difference between a call
|
||||
handler and an ordinary callee function is that the
|
||||
<structfield>flinfo->fn_oid</structfield> field of the
|
||||
<structname>FunctionCallInfoData</structname> structure will contain
|
||||
the OID of the actual 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 function,
|
||||
not of the call handler.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
It's up to the call handler to fetch the entry of the function from the
|
||||
system table
|
||||
<classname>pg_proc</classname> and to analyze the argument
|
||||
and return types of the called function. The <literal>AS</> clause from the
|
||||
<command>CREATE FUNCTION</command> of the function will be found
|
||||
in the <literal>prosrc</literal> column of the
|
||||
<classname>pg_proc</classname> row. This may be the source
|
||||
text in the procedural language itself (like for PL/Tcl), a
|
||||
path name 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
|
||||
<structfield>flinfo->fn_extra</structfield> field. This will
|
||||
initially be <symbol>NULL</>, but can be set by the call handler to point at
|
||||
information about the called function. On subsequent calls, if
|
||||
<structfield>flinfo->fn_extra</structfield> is already non-<symbol>NULL</>
|
||||
then it can be used and the information lookup step skipped. The
|
||||
call handler must make sure that
|
||||
<structfield>flinfo->fn_extra</structfield> is made to point at
|
||||
memory that will live at least until the end of the current query,
|
||||
since an <structname>FmgrInfo</structname> data structure could be
|
||||
kept that long. One way to do this is to allocate the extra data
|
||||
in the memory context specified by
|
||||
<structfield>flinfo->fn_mcxt</structfield>; such data will
|
||||
normally have the same lifespan as the
|
||||
<structname>FmgrInfo</structname> itself. But the handler could
|
||||
also choose to use a longer-lived memory context so that it can cache
|
||||
function definition information across queries.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
When a procedural-language function is invoked as a trigger, no arguments
|
||||
are passed in the usual way, but the
|
||||
<structname>FunctionCallInfoData</structname>'s
|
||||
<structfield>context</structfield> field points at a
|
||||
<structname>TriggerData</structname> structure, rather than being <symbol>NULL</>
|
||||
as it is in a plain function call. A language handler should
|
||||
provide mechanisms for procedural-language functions to get at the trigger
|
||||
information.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
This is a template for a procedural-language handler written in C:
|
||||
<programlisting>
|
||||
#include "postgres.h"
|
||||
#include "executor/spi.h"
|
||||
#include "commands/trigger.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>
|
||||
Only a few thousand lines of code have to be added instead of the
|
||||
dots to complete the call handler.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
After having compiled the handler function into a loadable module
|
||||
(see <xref linkend="dfunc">), the following commands then
|
||||
register the sample procedural language:
|
||||
<programlisting>
|
||||
CREATE FUNCTION plsample_call_handler() RETURNS language_handler
|
||||
AS '<replaceable>filename</replaceable>'
|
||||
LANGUAGE C;
|
||||
CREATE LANGUAGE plsample
|
||||
HANDLER plsample_call_handler;
|
||||
</programlisting>
|
||||
</para>
|
||||
|
||||
</chapter>
|
||||
|
||||
<!-- Keep this comment at the end of the file
|
||||
Local variables:
|
||||
mode:sgml
|
||||
sgml-omittag:nil
|
||||
sgml-shorttag:t
|
||||
sgml-minimize-attributes:nil
|
||||
sgml-always-quote-attributes:t
|
||||
sgml-indent-step:1
|
||||
sgml-indent-data:t
|
||||
sgml-parent-document:nil
|
||||
sgml-default-dtd-file:"./reference.ced"
|
||||
sgml-exposed-tags:nil
|
||||
sgml-local-catalogs:("/usr/lib/sgml/catalog")
|
||||
sgml-local-ecat-files:nil
|
||||
End:
|
||||
-->
|
||||
Reference in New Issue
Block a user