190 lines
6.9 KiB
Plaintext
190 lines
6.9 KiB
Plaintext
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$PostgreSQL: pgsql/doc/src/sgml/xplang.sgml,v 1.26 2003/11/29 19:51:38 pgsql Exp $
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<chapter id="xplang">
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<title id="xplang-title">Procedural Languages</title>
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<indexterm zone="xplang">
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<primary>procedural language</primary>
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</indexterm>
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<para>
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<productname>PostgreSQL</productname> allows users to add new
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programming languages to be available for writing functions and
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procedures. These are called <firstterm>procedural
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languages</firstterm> (PL). In the case of a function or trigger
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procedure written in a procedural language, the database server has
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no built-in knowledge about how to interpret the function's source
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text. Instead, the task is passed to a special handler that knows
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the details of the language. The handler could either do all the
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work of parsing, syntax analysis, execution, etc. itself, or it
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could serve as <quote>glue</quote> between
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<productname>PostgreSQL</productname> and an existing implementation
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of a programming language. The handler itself is a special
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C language function compiled into a shared object and
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loaded on demand.
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</para>
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<para>
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Writing a handler for a new procedural language is described in
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<xref linkend="plhandler">. Several procedural languages are
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available in the standard <productname>PostgreSQL</productname>
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distribution, which can serve as examples.
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</para>
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<sect1 id="xplang-install">
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<title>Installing Procedural Languages</title>
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<para>
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A procedural language must be <quote>installed</quote> into each
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database where it is to be used. But procedural languages installed in
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the database <literal>template1</> are automatically available in all
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subsequently created databases. So the database administrator can
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decide which languages are available in which databases and can make
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some languages available by default if he chooses.
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</para>
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<para>
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For the languages supplied with the standard distribution, the
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program <command>createlang</command> may be used to install the
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language instead of carrying out the details by hand. For
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example, to install the language
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<application>PL/pgSQL</application> into the database
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<literal>template1</>, use
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<programlisting>
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createlang plpgsql template1
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</programlisting>
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The manual procedure described below is only recommended for
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installing custom languages that <command>createlang</command>
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does not know about.
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</para>
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<procedure>
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<title>
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Manual Procedural Language Installation
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</title>
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<para>
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A procedural language is installed in a database in three steps,
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which must be carried out by a database superuser. The
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<command>createlang</command> program automates <xref
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linkend="xplang-install-cr1"> and <xref
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linkend="xplang-install-cr2">.
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</para>
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<step performance="required">
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<para>
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The shared object for the language handler must be compiled and
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installed into an appropriate library directory. This works in the same
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way as building and installing modules with regular user-defined C
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functions does; see <xref linkend="dfunc">.
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</para>
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</step>
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<step performance="required" id="xplang-install-cr1">
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<para>
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The handler must be declared with the command
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<synopsis>
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CREATE FUNCTION <replaceable>handler_function_name</replaceable>()
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RETURNS language_handler
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AS '<replaceable>path-to-shared-object</replaceable>'
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LANGUAGE C;
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</synopsis>
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The special return type of <type>language_handler</type> tells
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the database system that this function does not return one of
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the defined <acronym>SQL</acronym> data types and is not directly usable
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in <acronym>SQL</acronym> statements.
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</para>
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</step>
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<step performance="required" id="xplang-install-cr2">
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<para>
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The PL must be declared with the command
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<synopsis>
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CREATE <optional>TRUSTED</optional> <optional>PROCEDURAL</optional> LANGUAGE <replaceable>language-name</replaceable>
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HANDLER <replaceable>handler_function_name</replaceable>;
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</synopsis>
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The optional key word <literal>TRUSTED</literal> specifies that
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ordinary database users that have no superuser privileges should
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be allowed to use this language to create functions and trigger
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procedures. Since PL functions are executed inside the database
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server, the <literal>TRUSTED</literal> flag should only be given
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for languages that do not allow access to database server
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internals or the file system. The languages
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<application>PL/pgSQL</application>,
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<application>PL/Tcl</application>, and
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<application>PL/Perl</application>
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are considered trusted; the languages
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<application>PL/TclU</application>,
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<application>PL/PerlU</application>, and
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<application>PL/PythonU</application>
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are designed to provide unlimited functionality and should
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<emphasis>not</emphasis> be marked trusted.
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</para>
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</step>
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</procedure>
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<para>
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<xref linkend="xplang-install-example"> shows how the manual
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installation procedure would work with the language
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<application>PL/pgSQL</application>.
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</para>
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<example id="xplang-install-example">
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<title>Manual Installation of <application>PL/pgSQL</application></title>
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<para>
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The following command tells the database server where to find the
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shared object for the <application>PL/pgSQL</application> language's call handler function.
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<programlisting>
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CREATE FUNCTION plpgsql_call_handler() RETURNS language_handler AS
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'$libdir/plpgsql' LANGUAGE C;
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</programlisting>
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</para>
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<para>
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The command
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<programlisting>
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CREATE TRUSTED PROCEDURAL LANGUAGE plpgsql
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HANDLER plpgsql_call_handler;
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</programlisting>
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then defines that the previously declared call handler function
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should be invoked for functions and trigger procedures where the
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language attribute is <literal>plpgsql</literal>.
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</para>
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</example>
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<para>
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In a default <productname>PostgreSQL</productname> installation,
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the handler for the <application>PL/pgSQL</application> language
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is built and installed into the <quote>library</quote>
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directory. If <application>Tcl/Tk</> support is configured in, the handlers for
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<application>PL/Tcl</> and <application>PL/TclU</> are also built and installed in the same
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location. Likewise, the <application>PL/Perl</> and <application>PL/PerlU</> handlers are built
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and installed if Perl support is configured, and <application>PL/PythonU</> is
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installed if Python support is configured.
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</para>
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</sect1>
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</chapter>
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End:
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-->
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