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diff --git a/doc/src/sgml/xplang.sgml b/doc/src/sgml/xplang.sgml new file mode 100644 index 0000000..31d403c --- /dev/null +++ b/doc/src/sgml/xplang.sgml @@ -0,0 +1,230 @@ +<!-- doc/src/sgml/xplang.sgml --> + + <chapter id="xplang"> + <title>Procedural Languages</title> + + <indexterm zone="xplang"> + <primary>procedural language</primary> + </indexterm> + + <para> + <productname>PostgreSQL</productname> allows user-defined functions + to be written in other languages besides SQL and C. These other + languages are generically called <firstterm>procedural + languages</firstterm> (<acronym>PL</acronym>s). For a function + written in a procedural language, the database server has + no built-in knowledge about how to interpret the function's source + text. Instead, the task is passed to a special handler that knows + the details of the language. The handler could either do all the + work of parsing, syntax analysis, execution, etc. itself, or it + could serve as <quote>glue</quote> between + <productname>PostgreSQL</productname> and an existing implementation + of a programming language. The handler itself is a + C language function compiled into a shared object and + loaded on demand, just like any other C function. + </para> + + <para> + There are currently four procedural languages available in the + standard <productname>PostgreSQL</productname> distribution: + <application>PL/pgSQL</application> (<xref linkend="plpgsql"/>), + <application>PL/Tcl</application> (<xref linkend="pltcl"/>), + <application>PL/Perl</application> (<xref linkend="plperl"/>), and + <application>PL/Python</application> (<xref linkend="plpython"/>). + There are additional procedural languages available that are not + included in the core distribution. <xref linkend="external-projects"/> + has information about finding them. In addition other languages can + be defined by users; the basics of developing a new procedural + language are covered in <xref linkend="plhandler"/>. + </para> + + <sect1 id="xplang-install"> + <title>Installing Procedural Languages</title> + + <para> + A procedural language must be <quote>installed</quote> into each + database where it is to be used. But procedural languages installed in + the database <literal>template1</literal> are automatically available in all + subsequently created databases, since their entries in + <literal>template1</literal> will be copied by <command>CREATE DATABASE</command>. + So the database administrator can + decide which languages are available in which databases and can make + some languages available by default if desired. + </para> + + <para> + For the languages supplied with the standard distribution, it is + only necessary to execute <command>CREATE EXTENSION</command> + <replaceable>language_name</replaceable> to install the language into the + current database. + The manual procedure described below is only recommended for + installing languages that have not been packaged as extensions. + </para> + + <procedure> + <title>Manual Procedural Language Installation</title> + + <para> + A procedural language is installed in a database in five steps, + which must be carried out by a database superuser. In most cases + the required SQL commands should be packaged as the installation script + of an <quote>extension</quote>, so that <command>CREATE EXTENSION</command> can be + used to execute them. + </para> + + <step performance="required" id="xplang-install-cr1"> + <para> + The shared object for the language handler must be compiled and + installed into an appropriate library directory. This works in the same + way as building and installing modules with regular user-defined C + functions does; see <xref linkend="dfunc"/>. Often, the language + handler will depend on an external library that provides the actual + programming language engine; if so, that must be installed as well. + </para> + </step> + + <step performance="required" id="xplang-install-cr2"> + <para> + The handler must be declared with the command +<synopsis> +CREATE FUNCTION <replaceable>handler_function_name</replaceable>() + RETURNS language_handler + AS '<replaceable>path-to-shared-object</replaceable>' + LANGUAGE C; +</synopsis> + The special return type of <type>language_handler</type> tells + the database system that this function does not return one of + the defined <acronym>SQL</acronym> data types and is not directly usable + in <acronym>SQL</acronym> statements. + </para> + </step> + + <step performance="optional" id="xplang-install-cr3"> + <para> + Optionally, the language handler can provide an <quote>inline</quote> + handler function that executes anonymous code blocks + (<link linkend="sql-do"><command>DO</command></link> commands) + written in this language. If an inline handler function + is provided by the language, declare it with a command like +<synopsis> +CREATE FUNCTION <replaceable>inline_function_name</replaceable>(internal) + RETURNS void + AS '<replaceable>path-to-shared-object</replaceable>' + LANGUAGE C; +</synopsis> + </para> + </step> + + <step performance="optional" id="xplang-install-cr4"> + <para> + Optionally, the language handler can provide a <quote>validator</quote> + function that checks a function definition for correctness without + actually executing it. The validator function is called by + <command>CREATE FUNCTION</command> if it exists. If a validator function + is provided by the language, declare it with a command like +<synopsis> +CREATE FUNCTION <replaceable>validator_function_name</replaceable>(oid) + RETURNS void + AS '<replaceable>path-to-shared-object</replaceable>' + LANGUAGE C STRICT; +</synopsis> + </para> + </step> + + <step performance="required" id="xplang-install-cr5"> + <para> + Finally, the PL must be declared with the command +<synopsis> +CREATE <optional>TRUSTED</optional> LANGUAGE <replaceable>language_name</replaceable> + HANDLER <replaceable>handler_function_name</replaceable> + <optional>INLINE <replaceable>inline_function_name</replaceable></optional> + <optional>VALIDATOR <replaceable>validator_function_name</replaceable></optional> ; +</synopsis> + The optional key word <literal>TRUSTED</literal> specifies that + the language does not grant access to data that the user would + not otherwise have. Trusted languages are designed for ordinary + database users (those without superuser privilege) and allows them + to safely create functions and + procedures. Since PL functions are executed inside the database + server, the <literal>TRUSTED</literal> flag should only be given + for languages that do not allow access to database server + internals or the file system. The languages + <application>PL/pgSQL</application>, + <application>PL/Tcl</application>, and + <application>PL/Perl</application> + are considered trusted; the languages + <application>PL/TclU</application>, + <application>PL/PerlU</application>, and + <application>PL/PythonU</application> + are designed to provide unlimited functionality and should + <emphasis>not</emphasis> be marked trusted. + </para> + </step> + </procedure> + + <para> + <xref linkend="xplang-install-example"/> shows how the manual + installation procedure would work with the language + <application>PL/Perl</application>. + </para> + + <example id="xplang-install-example"> + <title>Manual Installation of <application>PL/Perl</application></title> + + <para> + The following command tells the database server where to find the + shared object for the <application>PL/Perl</application> language's call + handler function: + +<programlisting> +CREATE FUNCTION plperl_call_handler() RETURNS language_handler AS + '$libdir/plperl' LANGUAGE C; +</programlisting> + </para> + + <para> + <application>PL/Perl</application> has an inline handler function + and a validator function, so we declare those too: + +<programlisting> +CREATE FUNCTION plperl_inline_handler(internal) RETURNS void AS + '$libdir/plperl' LANGUAGE C STRICT; + +CREATE FUNCTION plperl_validator(oid) RETURNS void AS + '$libdir/plperl' LANGUAGE C STRICT; +</programlisting> + </para> + + <para> + The command: +<programlisting> +CREATE TRUSTED LANGUAGE plperl + HANDLER plperl_call_handler + INLINE plperl_inline_handler + VALIDATOR plperl_validator; +</programlisting> + then defines that the previously declared functions + should be invoked for functions and procedures where the + language attribute is <literal>plperl</literal>. + </para> + </example> + + <para> + In a default <productname>PostgreSQL</productname> installation, + the handler for the <application>PL/pgSQL</application> language + is built and installed into the <quote>library</quote> + directory; furthermore, the <application>PL/pgSQL</application> language + itself is installed in all databases. + If <application>Tcl</application> support is configured in, the handlers for + <application>PL/Tcl</application> and <application>PL/TclU</application> are built and installed + in the library directory, but the language itself is not installed in any + database by default. + Likewise, the <application>PL/Perl</application> and <application>PL/PerlU</application> + handlers are built and installed if Perl support is configured, and the + <application>PL/PythonU</application> handler is installed if Python support is + configured, but these languages are not installed by default. + </para> + + </sect1> + +</chapter> |