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c2903fb3d2
Recommend use of CREATE EXTENSION rather than plain CREATE LANGUAGE where relevant. Encourage PL authors to provide extension wrappers for their PLs.
240 lines
9.6 KiB
Plaintext
240 lines
9.6 KiB
Plaintext
<!-- doc/src/sgml/xplang.sgml -->
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<chapter id="xplang">
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<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 user-defined functions
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to be written in other languages besides SQL and C. These other
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languages are generically called <firstterm>procedural
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languages</firstterm> (<acronym>PL</>s). For a function
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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
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C language function compiled into a shared object and
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loaded on demand, just like any other C function.
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</para>
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<para>
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There are currently four procedural languages available in the
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standard <productname>PostgreSQL</productname> distribution:
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<application>PL/pgSQL</application> (<xref linkend="plpgsql">),
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<application>PL/Tcl</application> (<xref linkend="pltcl">),
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<application>PL/Perl</application> (<xref linkend="plperl">), and
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<application>PL/Python</application> (<xref linkend="plpython">).
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There are additional procedural languages available that are not
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included in the core distribution. <xref linkend="external-projects">
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has information about finding them. In addition other languages can
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be defined by users; the basics of developing a new procedural
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language are covered in <xref linkend="plhandler">.
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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, since their entries in
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<literal>template1</> will be copied by <command>CREATE DATABASE</>.
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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, it is
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only necessary to execute <command>CREATE EXTENSION</>
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<replaceable>language_name</> to install the language into the
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current database. Alternatively, the program <xref
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linkend="app-createlang"> can be used to do this from the shell
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command line. For example, to install the language
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<application>PL/Perl</application> into the database
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<literal>template1</>, use:
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<programlisting>
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createlang plperl template1
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</programlisting>
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The manual procedure described below is only recommended for
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installing languages that have not been packaged as extensions.
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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 five steps,
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which must be carried out by a database superuser. In most cases
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the required SQL commands should be packaged as the installation script
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of an <quote>extension</>, so that <command>CREATE EXTENSION</> can be
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used to execute them.
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</para>
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<step performance="required" id="xplang-install-cr1">
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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">. Often, the language
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handler will depend on an external library that provides the actual
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programming language engine; if so, that must be installed as well.
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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 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="optional" id="xplang-install-cr3">
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<para>
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Optionally, the language handler can provide an <quote>inline</>
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handler function that executes anonymous code blocks
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(<xref linkend="sql-do"> commands)
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written in this language. If an inline handler function
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is provided by the language, declare it with a command like
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<synopsis>
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CREATE FUNCTION <replaceable>inline_function_name</replaceable>(internal)
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RETURNS void
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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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</para>
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</step>
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<step performance="optional" id="xplang-install-cr4">
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<para>
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Optionally, the language handler can provide a <quote>validator</>
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function that checks a function definition for correctness without
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actually executing it. The validator function is called by
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<command>CREATE FUNCTION</> if it exists. If a validator function
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is provided by the language, declare it with a command like
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<synopsis>
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CREATE FUNCTION <replaceable>validator_function_name</replaceable>(oid)
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RETURNS void
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AS '<replaceable>path-to-shared-object</replaceable>'
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LANGUAGE C STRICT;
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</synopsis>
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</para>
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</step>
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<step performance="required" id="xplang-install-cr5">
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<para>
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Finally, 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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<optional>INLINE <replaceable>inline_function_name</replaceable></optional>
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<optional>VALIDATOR <replaceable>validator_function_name</replaceable></optional> ;
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</synopsis>
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The optional key word <literal>TRUSTED</literal> specifies that
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the language does not grant access to data that the user would
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not otherwise have. Trusted languages are designed for ordinary
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database users (those without superuser privilege) and allows them
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to safely 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/Perl</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/Perl</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/Perl</application> language's call
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handler function:
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<programlisting>
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CREATE FUNCTION plperl_call_handler() RETURNS language_handler AS
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'$libdir/plperl' LANGUAGE C;
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</programlisting>
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</para>
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<para>
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<application>PL/Perl</application> has an inline handler function
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and a validator function, so we declare those too:
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<programlisting>
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CREATE FUNCTION plperl_inline_handler(internal) RETURNS void AS
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'$libdir/plperl' LANGUAGE C;
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CREATE FUNCTION plperl_validator(oid) RETURNS void AS
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'$libdir/plperl' LANGUAGE C STRICT;
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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 plperl
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HANDLER plperl_call_handler
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INLINE plperl_inline_handler
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VALIDATOR plperl_validator;
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</programlisting>
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then defines that the previously declared functions
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should be invoked for functions and trigger procedures where the
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language attribute is <literal>plperl</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; furthermore, the <application>PL/pgSQL</application> language
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itself is installed in all databases.
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If <application>Tcl</> support is configured in, the handlers for
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<application>PL/Tcl</> and <application>PL/TclU</> are built and installed
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in the library directory, but the language itself is not installed in any
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database by default.
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Likewise, the <application>PL/Perl</> and <application>PL/PerlU</>
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handlers are built and installed if Perl support is configured, and the
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<application>PL/PythonU</> handler is installed if Python support is
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configured, but these languages are not installed by default.
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</para>
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</sect1>
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</chapter>
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