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Recommend use of CREATE EXTENSION rather than plain CREATE LANGUAGE where relevant. Encourage PL authors to provide extension wrappers for their PLs.
237 lines
9.9 KiB
Plaintext
237 lines
9.9 KiB
Plaintext
<!-- doc/src/sgml/plhandler.sgml -->
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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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For more details on C language calling conventions and dynamic loading,
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see <xref linkend="xfunc-c">.
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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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<classname>pg_proc</classname> system catalog 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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#ifdef PG_MODULE_MAGIC
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PG_MODULE_MAGIC;
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#endif
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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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Although providing a call handler is sufficient to create a minimal
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procedural language, there are two other functions that can optionally
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be provided to make the language more convenient to use. These
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are a <firstterm>validator</firstterm> and an
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<firstterm>inline handler</firstterm>. A validator can be provided
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to allow language-specific checking to be done during
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<xref linkend="sql-createfunction">.
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An inline handler can be provided to allow the language to support
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anonymous code blocks executed via the <xref linkend="sql-do"
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> command.
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</para>
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<para>
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If a validator is provided by a procedural language, it
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must be declared as a function taking a single parameter of type
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<type>oid</>. The validator's result is ignored, so it is customarily
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declared to return <type>void</>. The validator will be called at
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the end of a <command>CREATE FUNCTION</> command that has created
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or updated a function written in the procedural language.
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The passed-in OID is the OID of the function's <classname>pg_proc</>
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row. The validator must fetch this row in the usual way, and do
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whatever checking is appropriate. Typical checks include verifying
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that the function's argument and result types are supported by the
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language, and that the function's body is syntactically correct
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in the language. If the validator finds the function to be okay,
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it should just return. If it finds an error, it should report that
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via the normal <function>ereport()</> error reporting mechanism.
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Throwing an error will force a transaction rollback and thus prevent
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the incorrect function definition from being committed.
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</para>
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<para>
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Validator functions should typically honor the <xref
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linkend="guc-check-function-bodies"> parameter: if it is turned off then
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any expensive or context-sensitive checking should be skipped.
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In particular, this parameter is turned off by <application>pg_dump</>
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so that it can load procedural language functions without worrying
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about possible dependencies of the function bodies on other database
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objects. (Because of this requirement, the call handler should avoid
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assuming that the validator has fully checked the function. The point
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of having a validator is not to let the call handler omit checks, but
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to notify the user immediately if there are obvious errors in a
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<command>CREATE FUNCTION</> command.)
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</para>
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<para>
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If an inline handler is provided by a procedural language, it
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must be declared as a function taking a single parameter of type
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<type>internal</>. The inline handler's result is ignored, so it is
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customarily declared to return <type>void</>. The inline handler
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will be called when a <command>DO</> statement is executed specifying
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the procedural language. The parameter actually passed is a pointer
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to an <structname>InlineCodeBlock</> struct, which contains information
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about the <command>DO</> statement's parameters, in particular the
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text of the anonymous code block to be executed. The inline handler
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should execute this code and return.
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</para>
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<para>
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It's recommended that you wrap all these function declarations,
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as well as the <command>CREATE LANGUAGE</> command itself, into
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an <firstterm>extension</> so that a simple <command>CREATE EXTENSION</>
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command is sufficient to install the language. See
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<xref linkend="extend-extensions"> for information about writing
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extensions.
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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 language handler.
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Look into the <filename>src/pl</> subdirectory of the source tree.
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The <xref linkend="sql-createlanguage">
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reference page also has some useful details.
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</para>
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</chapter>
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