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>Chapter 39. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> - <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Procedural Language</TD
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><H1
CLASS="SECT1"
><A
NAME="PLPGSQL-OVERVIEW"
>39.1. Overview</A
></H1
><P
> <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> is a loadable procedural
language for the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> database
system. The design goals of <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> were to create
a loadable procedural language that
<P
></P
></P><UL
><LI
><P
> can be used to create functions and trigger procedures,
</P
></LI
><LI
><P
> adds control structures to the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> language,
</P
></LI
><LI
><P
> can perform complex computations,
</P
></LI
><LI
><P
> inherits all user-defined types, functions, and operators,
</P
></LI
><LI
><P
> can be defined to be trusted by the server,
</P
></LI
><LI
><P
> is easy to use.
</P
></LI
></UL
><P>
</P
><P
> Functions created with <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> can be
used anywhere that built-in functions could be used.
For example, it is possible to
create complex conditional computation functions and later use
them to define operators or use them in index expressions.
</P
><P
> In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.0 and later,
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> is installed by default.
However it is still a loadable module, so especially security-conscious
administrators could choose to remove it.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-ADVANTAGES"
>39.1.1. Advantages of Using <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
></A
></H2
><P
> <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> is the language <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
and most other relational databases use as query language. It's
portable and easy to learn. But every <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
>
statement must be executed individually by the database server.
</P
><P
> 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.
</P
><P
> With <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> you can group a block of
computation and a series of queries <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>inside</I
></SPAN
>
the database server, thus having the power of a procedural
language and the ease of use of SQL, but with considerable
savings of client/server communication overhead.
</P
><P
></P
><UL
><LI
><P
> Extra round trips between
client and server are eliminated </P
></LI
><LI
><P
> Intermediate results that the client does not
need do not have to be marshaled or transferred between server
and client </P
></LI
><LI
><P
> Multiple rounds of query
parsing can be avoided </P
></LI
></UL
><P
> This can result in a considerable performance increase as
compared to an application that does not use stored functions.
</P
><P
> Also, with <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> you can use all
the data types, operators and functions of SQL.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-ARGS-RESULTS"
>39.1.2. Supported Argument and Result Data Types</A
></H2
><P
> Functions written in <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> 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 <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>
function as returning <TT
CLASS="TYPE"
>record</TT
>, which means that the result
is a row type whose columns are determined by specification in the
calling query, as discussed in <A
HREF="queries-table-expressions.html#QUERIES-TABLEFUNCTIONS"
>Section 7.2.1.4</A
>.
</P
><P
> <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> functions can be declared to accept a variable
number of arguments by using the <TT
CLASS="LITERAL"
>VARIADIC</TT
> marker. This
works exactly the same way as for SQL functions, as discussed in
<A
HREF="xfunc-sql.html#XFUNC-SQL-VARIADIC-FUNCTIONS"
>Section 35.4.5</A
>.
</P
><P
> <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> functions can also be declared to accept
and return the polymorphic types
<TT
CLASS="TYPE"
>anyelement</TT
>, <TT
CLASS="TYPE"
>anyarray</TT
>, <TT
CLASS="TYPE"
>anynonarray</TT
>,
<TT
CLASS="TYPE"
>anyenum</TT
>, and <TT
CLASS="TYPE"
>anyrange</TT
>. The actual
data types handled by a polymorphic function can vary from call to
call, as discussed in <A
HREF="extend-type-system.html#EXTEND-TYPES-POLYMORPHIC"
>Section 35.2.5</A
>.
An example is shown in <A
HREF="plpgsql-declarations.html#PLPGSQL-DECLARATION-PARAMETERS"
>Section 39.3.1</A
>.
</P
><P
> <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> functions can also be declared to return
a <SPAN
CLASS="QUOTE"
>"set"</SPAN
> (or table) of any data type that can be returned as
a single instance. Such a function generates its output by executing
<TT
CLASS="COMMAND"
>RETURN NEXT</TT
> for each desired element of the result
set, or by using <TT
CLASS="COMMAND"
>RETURN QUERY</TT
> to output the result of
evaluating a query.
</P
><P
> Finally, a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function can be declared to return
<TT
CLASS="TYPE"
>void</TT
> if it has no useful return value.
</P
><P
> <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> 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.
The <TT
CLASS="LITERAL"
>RETURNS TABLE</TT
> notation can also be used in place
of <TT
CLASS="LITERAL"
>RETURNS SETOF</TT
>.
</P
><P
> Specific examples appear in
<A
HREF="plpgsql-declarations.html#PLPGSQL-DECLARATION-PARAMETERS"
>Section 39.3.1</A
> and
<A
HREF="plpgsql-control-structures.html#PLPGSQL-STATEMENTS-RETURNING"
>Section 39.6.1</A
>.
</P
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