PostgreSQL 8.4.17 Documentation
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# 37.1. Installing Procedural Languages

A procedural language must be "installed" into each database where it is to be used. But procedural languages installed in the database `template1` are automatically available in all subsequently created databases, since their entries in `template1` will be copied by `CREATE DATABASE`. So the database administrator can decide which languages are available in which databases and can make some languages available by default if he chooses.

For the languages supplied with the standard distribution, it is only necessary to execute `CREATE LANGUAGE` `language_name` to install the language into the current database. Alternatively, the program createlang can be used to do this from the shell command line. For example, to install the language PL/pgSQL into the database `template1`, use:

    createlang plpgsql template1

The manual procedure described below is only recommended for installing custom languages that `CREATE LANGUAGE` does not know about.

Manual Procedural Language Installation

A procedural language is installed in a database in four steps, which must be carried out by a database superuser. (For languages known to `CREATE LANGUAGE`, the second and third steps can be omitted, because they will be carried out automatically if needed.)

1. 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 Section 34.9.6. 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.

2. The handler must be declared with the command

       CREATE FUNCTION handler_function_name()
           RETURNS language_handler
           AS 'path-to-shared-object'
           LANGUAGE C;

   The special return type of `language_handler` tells the database system that this function does not return one of the defined SQL data types and is not directly usable in SQL statements.

3. Optionally, the language handler can provide a "validator" function that checks a function definition for correctness without actually executing it. The validator function is called by `CREATE FUNCTION` if it exists. If a validator function is provided by the handler, declare it with a command like

       CREATE FUNCTION validator_function_name(oid)
           RETURNS void
           AS 'path-to-shared-object'
           LANGUAGE C;

4. The PL must be declared with the command

       CREATE [TRUSTED] [PROCEDURAL] LANGUAGE language-name
           HANDLER handler_function_name
           [VALIDATOR validator_function_name] ;

   The optional key word `TRUSTED` specifies that ordinary database users that have no superuser privileges should be allowed to use this language to create functions and trigger procedures. Since PL functions are executed inside the database server, the `TRUSTED` flag should only be given for languages that do not allow access to database server internals or the file system. The languages PL/pgSQL, PL/Tcl, and PL/Perl are considered trusted; the languages PL/TclU, PL/PerlU, and PL/PythonU are designed to provide unlimited functionality and should not be marked trusted.

Example 37-1 shows how the manual installation procedure would work with the language PL/pgSQL.

Example 37-1. Manual Installation of PL/pgSQL

The following command tells the database server where to find the shared object for the PL/pgSQL language's call handler function.

    CREATE FUNCTION plpgsql_call_handler() RETURNS language_handler AS
        '$libdir/plpgsql' LANGUAGE C;

PL/pgSQL has a validator function, so we declare that too:

    CREATE FUNCTION plpgsql_validator(oid) RETURNS void AS
        '$libdir/plpgsql' LANGUAGE C;

The command:

    CREATE TRUSTED PROCEDURAL LANGUAGE plpgsql
        HANDLER plpgsql_call_handler
        VALIDATOR plpgsql_validator;

then defines that the previously declared functions should be invoked for functions and trigger procedures where the language attribute is `plpgsql`.

In a default PostgreSQL installation, the handler for the PL/pgSQL language is built and installed into the "library" directory. If Tcl support is configured in, the handlers for PL/Tcl and PL/TclU are also built and installed in the same location. Likewise, the PL/Perl and PL/PerlU handlers are built and installed if Perl support is configured, and the PL/PythonU handler is installed if Python support is configured.

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