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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
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><A
NAME="PLPGSQL-STRUCTURE"
>39.2. Structure of <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
></A
></H1
><P
> Functions written in <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> are defined
to the server by executing <A
HREF="sql-createfunction.html"
>CREATE FUNCTION</A
> commands.
Such a command would normally look like, say,
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION somefunc(integer, text) RETURNS integer
AS '<TT
CLASS="REPLACEABLE"
><I
>function body text</I
></TT
>'
LANGUAGE plpgsql;</PRE
><P>
The function body is simply a string literal so far as <TT
CLASS="COMMAND"
>CREATE
FUNCTION</TT
> is concerned. It is often helpful to use dollar quoting
(see <A
HREF="sql-syntax-lexical.html#SQL-SYNTAX-DOLLAR-QUOTING"
>Section 4.1.2.4</A
>) to write the function
body, rather than the normal single quote syntax. Without dollar quoting,
any single quotes or backslashes in the function body must be escaped by
doubling them. Almost all the examples in this chapter use dollar-quoted
literals for their function bodies.
</P
><P
> <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> is a block-structured language.
The complete text of a function body must be a
<I
CLASS="FIRSTTERM"
>block</I
>. A block is defined as:
</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> <<<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>>> </SPAN
>]
[<SPAN
CLASS="OPTIONAL"
> DECLARE
<TT
CLASS="REPLACEABLE"
><I
>declarations</I
></TT
> </SPAN
>]
BEGIN
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P>
</P
><P
> Each declaration and each statement within a block is terminated
by a semicolon. A block that appears within another block must
have a semicolon after <TT
CLASS="LITERAL"
>END</TT
>, as shown above;
however the final <TT
CLASS="LITERAL"
>END</TT
> that
concludes a function body does not require a semicolon.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> A common mistake is to write a semicolon immediately after
<TT
CLASS="LITERAL"
>BEGIN</TT
>. This is incorrect and will result in a syntax error.
</P
></BLOCKQUOTE
></DIV
><P
> A <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> is only needed if you want to
identify the block for use
in an <TT
CLASS="LITERAL"
>EXIT</TT
> statement, or to qualify the names of the
variables declared in the block. If a label is given after
<TT
CLASS="LITERAL"
>END</TT
>, it must match the label at the block's beginning.
</P
><P
> All key words are case-insensitive.
Identifiers are implicitly converted to lower case
unless double-quoted, just as they are in ordinary SQL commands.
</P
><P
> Comments work the same way in <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> code as in
ordinary SQL. A double dash (<TT
CLASS="LITERAL"
>--</TT
>) starts a comment
that extends to the end of the line. A <TT
CLASS="LITERAL"
>/*</TT
> starts a
block comment that extends to the matching occurrence of
<TT
CLASS="LITERAL"
>*/</TT
>. Block comments nest.
</P
><P
> Any statement in the statement section of a block
can be a <I
CLASS="FIRSTTERM"
>subblock</I
>. Subblocks can be used for
logical grouping or to localize variables to a small group
of statements. Variables declared in a subblock mask any
similarly-named variables of outer blocks for the duration
of the subblock; but you can access the outer variables anyway
if you qualify their names with their block's label. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION somefunc() RETURNS integer AS $$
<< outerblock >>
DECLARE
quantity integer := 30;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 30
quantity := 50;
--
-- Create a subblock
--
DECLARE
quantity integer := 80;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 80
RAISE NOTICE 'Outer quantity here is %', outerblock.quantity; -- Prints 50
END;
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 50
RETURN quantity;
END;
$$ LANGUAGE plpgsql;</PRE
><P>
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> There is actually a hidden <SPAN
CLASS="QUOTE"
>"outer block"</SPAN
> surrounding the body
of any <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function. This block provides the
declarations of the function's parameters (if any), as well as some
special variables such as <TT
CLASS="LITERAL"
>FOUND</TT
> (see
<A
HREF="plpgsql-statements.html#PLPGSQL-STATEMENTS-DIAGNOSTICS"
>Section 39.5.5</A
>). The outer block is
labeled with the function's name, meaning that parameters and special
variables can be qualified with the function's name.
</P
></BLOCKQUOTE
></DIV
><P
> It is important not to confuse the use of
<TT
CLASS="COMMAND"
>BEGIN</TT
>/<TT
CLASS="COMMAND"
>END</TT
> for grouping statements in
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> with the similarly-named SQL commands
for transaction
control. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>'s <TT
CLASS="COMMAND"
>BEGIN</TT
>/<TT
CLASS="COMMAND"
>END</TT
>
are only for grouping; they do not start or end a transaction.
Functions and trigger procedures are always executed within a transaction
established by an outer query — they cannot start or commit that
transaction, since there would be no context for them to execute in.
However, a block containing an <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause effectively
forms a subtransaction that can be rolled back without affecting the
outer transaction. For more about that see <A
HREF="plpgsql-control-structures.html#PLPGSQL-ERROR-TRAPPING"
>Section 39.6.6</A
>.
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