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181 lines
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181 lines
6.6 KiB
Plaintext
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$PostgreSQL: pgsql/doc/src/sgml/plhandler.sgml,v 1.5 2005/01/22 22:06:17 momjian Exp $
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<chapter id="plhandler">
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<title>Writing A Procedural Language Handler</title>
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<indexterm zone="plhandler">
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<primary>procedural language</primary>
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<secondary>handler for</secondary>
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</indexterm>
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<para>
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All calls to functions that are written in a language other than
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the current <quote>version 1</quote> interface for compiled
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languages (this includes functions in user-defined procedural languages,
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functions written in SQL, and functions using the version 0 compiled
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language interface), go through a <firstterm>call handler</firstterm>
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function for the specific language. It is the responsibility of
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the call handler to execute the function in a meaningful way, such
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as by interpreting the supplied source text. This chapter outlines
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how a new procedural language's call handler can be written.
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</para>
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<para>
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The call handler for a procedural language is a
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<quote>normal</quote> function that must be written in a compiled
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language such as C, using the version-1 interface, and registered
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with <productname>PostgreSQL</productname> as taking no arguments
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and returning the type <type>language_handler</type>. This
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special pseudotype identifies the function as a call handler and
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prevents it from being called directly in SQL commands.
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</para>
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<para>
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The call handler is called in the same way as any other function:
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It receives a pointer to a
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<structname>FunctionCallInfoData</structname> <type>struct</> containing
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argument values and information about the called function, and it
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is expected to return a <type>Datum</type> result (and possibly
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set the <structfield>isnull</structfield> field of the
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<structname>FunctionCallInfoData</structname> structure, if it wishes
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to return an SQL null result). The difference between a call
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handler and an ordinary callee function is that the
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<structfield>flinfo->fn_oid</structfield> field of the
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<structname>FunctionCallInfoData</structname> structure will contain
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the OID of the actual function to be called, not of the call
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handler itself. The call handler must use this field to determine
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which function to execute. Also, the passed argument list has
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been set up according to the declaration of the target function,
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not of the call handler.
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</para>
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<para>
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It's up to the call handler to fetch the entry of the function from the
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system table
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<classname>pg_proc</classname> and to analyze the argument
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and return types of the called function. The <literal>AS</> clause from the
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<command>CREATE FUNCTION</command> command for the function will be found
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in the <literal>prosrc</literal> column of the
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<classname>pg_proc</classname> row. This is commonly source
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text in the procedural language, but in theory it could be something else,
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such as a path name to a file, or anything else that tells the call handler
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what to do in detail.
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</para>
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<para>
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Often, the same function is called many times per SQL statement.
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A call handler can avoid repeated lookups of information about the
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called function by using the
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<structfield>flinfo->fn_extra</structfield> field. This will
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initially be <symbol>NULL</>, but can be set by the call handler to point at
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information about the called function. On subsequent calls, if
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<structfield>flinfo->fn_extra</structfield> is already non-<symbol>NULL</>
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then it can be used and the information lookup step skipped. The
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call handler must make sure that
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<structfield>flinfo->fn_extra</structfield> is made to point at
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memory that will live at least until the end of the current query,
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since an <structname>FmgrInfo</structname> data structure could be
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kept that long. One way to do this is to allocate the extra data
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in the memory context specified by
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<structfield>flinfo->fn_mcxt</structfield>; such data will
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normally have the same lifespan as the
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<structname>FmgrInfo</structname> itself. But the handler could
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also choose to use a longer-lived memory context so that it can cache
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function definition information across queries.
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</para>
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<para>
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When a procedural-language function is invoked as a trigger, no arguments
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are passed in the usual way, but the
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<structname>FunctionCallInfoData</structname>'s
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<structfield>context</structfield> field points at a
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<structname>TriggerData</structname> structure, rather than being <symbol>NULL</>
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as it is in a plain function call. A language handler should
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provide mechanisms for procedural-language functions to get at the trigger
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information.
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</para>
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<para>
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This is a template for a procedural-language handler written in C:
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<programlisting>
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#include "postgres.h"
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#include "executor/spi.h"
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#include "commands/trigger.h"
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#include "fmgr.h"
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#include "access/heapam.h"
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#include "utils/syscache.h"
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#include "catalog/pg_proc.h"
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#include "catalog/pg_type.h"
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PG_FUNCTION_INFO_V1(plsample_call_handler);
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Datum
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plsample_call_handler(PG_FUNCTION_ARGS)
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{
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Datum retval;
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if (CALLED_AS_TRIGGER(fcinfo))
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{
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/*
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* Called as a trigger procedure
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*/
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TriggerData *trigdata = (TriggerData *) fcinfo->context;
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retval = ...
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}
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else
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{
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/*
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* Called as a function
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*/
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retval = ...
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}
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return retval;
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}
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</programlisting>
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Only a few thousand lines of code have to be added instead of the
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dots to complete the call handler.
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</para>
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<para>
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After having compiled the handler function into a loadable module
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(see <xref linkend="dfunc">), the following commands then
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register the sample procedural language:
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<programlisting>
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CREATE FUNCTION plsample_call_handler() RETURNS language_handler
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AS '<replaceable>filename</replaceable>'
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LANGUAGE C;
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CREATE LANGUAGE plsample
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HANDLER plsample_call_handler;
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</programlisting>
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</para>
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<para>
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The procedural languages included in the standard distribution
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are good references when trying to write your own call handler.
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Look into the <filename>src/pl</> subdirectory of the source tree.
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</para>
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</chapter>
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