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Chapter 35. PL/pgSQL - SQL Procedural Language

PL/pgSQL is a loadable procedural language for the PostgreSQL database system. The design goals of PL/pgSQL were to create a loadable procedural language that

  • can be used to create functions and trigger procedures,

  • adds control structures to the SQL language,

  • can perform complex computations,

  • inherits all user-defined types, functions, and operators,

  • can be defined to be trusted by the server,

  • is easy to use.

Except for input/output conversion and calculation functions for user-defined types, anything that can be defined in C language functions can also be done with PL/pgSQL. For example, it is possible to create complex conditional computation functions and later use them to define operators or use them in index expressions.

35.1. Overview

The PL/pgSQL call handler parses the function's source text and produces an internal binary instruction tree the first time the function is called (within each session). The instruction tree fully translates the PL/pgSQL statement structure, but individual SQL expressions and SQL commands used in the function are not translated immediately.

As each expression and SQL command is first used in the function, the PL/pgSQL interpreter creates a prepared execution plan (using the SPI manager's SPI_prepare and SPI_saveplan functions). Subsequent visits to that expression or command reuse the prepared plan. Thus, a function with conditional code that contains many statements for which execution plans might be required will only prepare and save those plans that are really used during the lifetime of the database connection. This can substantially reduce the total amount of time required to parse, and generate execution plans for the statements in a PL/pgSQL function. A disadvantage is that errors in a specific expression or command may not be detected until that part of the function is reached in execution.

Once PL/pgSQL has made an execution plan for a particular command in a function, it will reuse that plan for the life of the database connection. This is usually a win for performance, but it can cause some problems if you dynamically alter your database schema. For example:

CREATE FUNCTION populate() RETURNS integer AS $$
DECLARE
    -- declarations
BEGIN
    PERFORM my_function();
END;
$$ LANGUAGE plpgsql;

If you execute the above function, it will reference the OID for my_function() in the execution plan produced for the PERFORM statement. Later, if you drop and recreate my_function(), then populate() will not be able to find my_function() anymore. You would then have to recreate populate(), or at least start a new database session so that it will be compiled afresh. Another way to avoid this problem is to use CREATE OR REPLACE FUNCTION when updating the definition of my_function (when a function is "replaced", its OID is not changed).

Because PL/pgSQL saves execution plans in this way, SQL commands that appear directly in a PL/pgSQL function must refer to the same tables and columns on every execution; that is, you cannot use a parameter as the name of a table or column in an SQL command. To get around this restriction, you can construct dynamic commands using the PL/pgSQL EXECUTE statement — at the price of constructing a new execution plan on every execution.

Note: The PL/pgSQL EXECUTE statement is not related to the EXECUTE SQL statement supported by the PostgreSQL server. The server's EXECUTE statement cannot be used within PL/pgSQL functions (and is not needed).

35.1.1. Advantages of Using PL/pgSQL

SQL is the language PostgreSQL and most other relational databases use as query language. It's portable and easy to learn. But every SQL statement must be executed individually by the database server.

That means that your client application must send each query to the database server, wait for it to be processed, receive the results, do some computation, then send other queries to the server. All this incurs interprocess communication and may also incur network overhead if your client is on a different machine than the database server.

With PL/pgSQL you can group a block of computation and a series of queries inside the database server, thus having the power of a procedural language and the ease of use of SQL, but saving lots of time because you don't have the whole client/server communication overhead. This can make for a considerable performance increase.

Also, with PL/pgSQL you can use all the data types, operators and functions of SQL.

35.1.2. Supported Argument and Result Data Types

Functions written in PL/pgSQL can accept as arguments any scalar or array data type supported by the server, and they can return a result of any of these types. They can also accept or return any composite type (row type) specified by name. It is also possible to declare a PL/pgSQL function as returning record, which means that the result is a row type whose columns are determined by specification in the calling query, as discussed in Section 7.2.1.4.

PL/pgSQL functions may also be declared to accept and return the polymorphic types anyelement and anyarray. The actual data types handled by a polymorphic function can vary from call to call, as discussed in Section 31.2.5. An example is shown in Section 35.4.1.

PL/pgSQL functions can also be declared to return a "set", or table, of any data type they can return a single instance of. Such a function generates its output by executing RETURN NEXT for each desired element of the result set.

Finally, a PL/pgSQL function may be declared to return void if it has no useful return value.

PL/pgSQL does not currently have full support for domain types: it treats a domain the same as the underlying scalar type. This means that constraints associated with the domain will not be enforced. This is not an issue for function arguments, but it is a hazard if you declare a PL/pgSQL function as returning a domain type.

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