PostgreSQL 可以讓使用者自行定義的函數除了 SQL 和 C 之外還能用其他語言編寫。這些其他語言通常稱為程序語言（PL）。對於用程序語言編寫的函數，資料庫伺服器並沒有關於如何解釋函數原始程式碼的能力。所以相關的任務會被傳遞給一個瞭解語言細節的特殊處理程序來處理。處理程序可以自己完成內容過濾，語法分析，程序執行等所有工作，也可以作為 PostgreSQL 與現有程序語言實作之間的「粘合劑」。處理程序本身是一個 C 語言函數，編譯成一個共享物件並按需要載入，就像任何其他 C 函數一樣。

目前標準的 PostgreSQL 發行版中有四種可用的程序語言：PL/pgSQL（第 43 章），PL/Tcl（第 44 章），PL/Perl（第 45 章）和 PL/Python（第 46 章）。 還有其他可用的程序語言未包含在主要發行版中。附錄 H 包含相關的其他訊息。此外，使用者可以定義其他語言；第 58 章介紹了開發新程序語言的基礎知識。

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 desired.

For the languages supplied with the standard distribution, it is only necessary to execute CREATE EXTENSION language_name 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.

Manual Procedural Language Installation

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 “extension”, so that CREATE EXTENSION can be used to execute them.

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 38.10.5. 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

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   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 an “inline” handler function that executes anonymous code blocks (DO commands) written in this language. If an inline handler function is provided by the language, declare it with a command like

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

4. 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 language, declare it with a command like

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

5. Finally, the PL must be declared with the command

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   CREATE [TRUSTED] LANGUAGE language_name
       HANDLER handler_function_name
       [INLINE inline_function_name]
       [VALIDATOR validator_function_name] ;

   The optional key word TRUSTED 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 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 42.1 shows how the manual installation procedure would work with the language PL/Perl.

Example 42.1. Manual Installation of PL/Perl

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

CREATE FUNCTION plperl_call_handler() RETURNS language_handler AS
    '$libdir/plperl' LANGUAGE C;

PL/Perl has an inline handler function and a validator function, so we declare those too:

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;

The command:

CREATE TRUSTED LANGUAGE plperl
    HANDLER plperl_call_handler
    INLINE plperl_inline_handler
    VALIDATOR plperl_validator;

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

In a default PostgreSQL installation, the handler for the PL/pgSQL language is built and installed into the “library” directory; furthermore, the PL/pgSQL language itself is installed in all databases. If Tcl support is configured in, the handlers for PL/Tcl and PL/TclU are built and installed in the library directory, but the language itself is not installed in any database by default. 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, but these languages are not installed by default.