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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).
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 and process the results, do some computation, then send further queries to the server. All this incurs interprocess communication and will 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 with considerable savings because you don't have the whole client/server communication overhead.
Elimination of additional round trips between client and server
Intermediate results that the client does not need do not need to be marshaled or transferred between server and client
There is no need for additional rounds of query parsing
This can allow for a considerable performance increase as compared to an application that does not use stored functions.
Also, with PL/pgSQL you can use all the data types, operators and functions of SQL.
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 33.2.5. An example is shown in Section 37.4.1.
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 functions can also be declared with output parameters in place of an explicit specification of the return type. This does not add any fundamental capability to the language, but it is often convenient, especially for returning multiple values.
Specific examples appear in Section 37.4.1 and Section 37.7.1.
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