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>Chapter 19. <SPAN
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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="PLPGSQL-CONTROL-STRUCTURES"
>19.6. Control Structures</A
></H1
><P
> Control structures are probably the most useful (and
important) part of <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>. With
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>'s control structures,
you can manipulate <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> data in a very
flexible and powerful way.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-STATEMENTS-RETURNING"
>19.6.1. Returning from a function</A
></H2
><P
></P><PRE
CLASS="SYNOPSIS"
>RETURN <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>;</PRE
><P>
RETURN with an expression is used to return from a
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function that does not return a set.
The function terminates and the value of
<TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> is returned to the caller.
</P
><P
> To return a composite (row) value, you must write a record or row
variable as the <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>. When
returning a scalar type, any expression can be used.
The expression's result will be automatically cast into the
function's return type as described for assignments.
(If you have declared the function to return <TT
CLASS="TYPE"
>void</TT
>,
then the expression can be omitted, and will be ignored in any case.)
</P
><P
> The return value of a function cannot be left undefined. If control
reaches the end of the top-level block of
the function without hitting a RETURN statement, a run-time error
will occur.
</P
><P
> When a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function is declared to return
<TT
CLASS="LITERAL"
>SETOF</TT
> <TT
CLASS="REPLACEABLE"
><I
>sometype</I
></TT
>, the procedure
to follow is slightly different. In that case, the individual
items to return are specified in RETURN NEXT commands, and then a
final RETURN command with no arguments is used to indicate that
the function has finished executing. RETURN NEXT can be used with
both scalar and composite data types; in the later case, an
entire "table" of results will be returned. Functions that use
RETURN NEXT should be called in the following fashion:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM some_func();</PRE
><P>
That is, the function is used as a table source in a FROM clause.
</P><PRE
CLASS="SYNOPSIS"
>RETURN NEXT <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
>;</PRE
><P>
RETURN NEXT does not actually return from the function; it simply
saves away the value of the expression (or record or row variable,
as appropriate for the data type being returned).
Execution then continues with the next statement in the
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function. As successive RETURN NEXT
commands are executed, the result set is built up. A final
RETURN, which need have no argument, causes control to exit
the function.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> The current implementation of RETURN NEXT for
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> stores the entire result set before
returning from the function, as discussed above. That means that
if a <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function produces a very large result set,
performance may be poor: data will be written to disk to avoid
memory exhaustion, but the function itself will not return until
the entire result set has been generated. A future version of
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> may allow users to allow users to define set-returning
functions that do not have this limitation. Currently, the point
at which data begins being written to disk is controlled by the
<TT
CLASS="VARNAME"
>SORT_MEM</TT
> configuration variable. Administrators who
have sufficient memory to store larger result sets in memory
should consider increasing this parameter.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-CONDITIONALS"
>19.6.2. Conditionals</A
></H2
><P
> <TT
CLASS="LITERAL"
>IF</TT
> statements let you execute commands based on
certain conditions. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> has four forms of
<TT
CLASS="LITERAL"
>IF</TT
>:
<P
></P
></P><UL
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSE</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSE IF</TT
> and</P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>IF ... THEN ... ELSIF ... THEN ... ELSE</TT
></P
></LI
></UL
><P>
</P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN35954"
>19.6.2.1. <TT
CLASS="LITERAL"
>IF-THEN</TT
></A
></H3
><P
></P><PRE
CLASS="SYNOPSIS"
>IF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END IF;</PRE
><P>
IF-THEN statements are the simplest form of IF. The
statements between THEN and END IF will be executed if
the condition is true. Otherwise, they are skipped.
</P><PRE
CLASS="PROGRAMLISTING"
>IF v_user_id <> 0 THEN
UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN35962"
>19.6.2.2. <TT
CLASS="LITERAL"
>IF-THEN-ELSE</TT
></A
></H3
><P
></P><PRE
CLASS="SYNOPSIS"
>IF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
ELSE
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END IF;</PRE
><P>
IF-THEN-ELSE statements add to IF-THEN by letting you
specify an alternative set of statements that should be executed if
the condition evaluates to FALSE.
</P><PRE
CLASS="PROGRAMLISTING"
>IF parentid IS NULL or parentid = ''''
THEN
return fullname;
ELSE
return hp_true_filename(parentid) || ''/'' || fullname;
END IF;
IF v_count > 0 THEN
INSERT INTO users_count(count) VALUES(v_count);
return ''t'';
ELSE
return ''f'';
END IF;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN35971"
>19.6.2.3. <TT
CLASS="LITERAL"
>IF-THEN-ELSE IF</TT
></A
></H3
><P
> IF statements can be nested, as in the following example:
</P><PRE
CLASS="PROGRAMLISTING"
>IF demo_row.sex = ''m'' THEN
pretty_sex := ''man'';
ELSE
IF demo_row.sex = ''f'' THEN
pretty_sex := ''woman'';
END IF;
END IF;</PRE
><P>
</P
><P
> When you use this form, you are actually
nesting an IF statement inside the ELSE part of an outer IF
statement. Thus you need one END IF statement for each
nested IF and one for the parent IF-ELSE.
This is workable but grows tedious when there are many
alternatives to be checked.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN35977"
>19.6.2.4. <TT
CLASS="LITERAL"
>IF-THEN-ELSIF-ELSE</TT
></A
></H3
><P
></P><PRE
CLASS="SYNOPSIS"
>IF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
[<SPAN
CLASS="OPTIONAL"
> ELSIF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
[<SPAN
CLASS="OPTIONAL"
> ELSIF <TT
CLASS="REPLACEABLE"
><I
>boolean-expression</I
></TT
> THEN
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
...</SPAN
>]</SPAN
>]
[<SPAN
CLASS="OPTIONAL"
> ELSE
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
> </SPAN
>]
END IF;</PRE
><P>
<TT
CLASS="LITERAL"
>IF-THEN-ELSIF-ELSE</TT
> provides a more convenient
method of checking many alternatives in one statement.
Formally it is equivalent to nested
<TT
CLASS="LITERAL"
>IF-THEN-ELSE-IF-THEN</TT
> commands, but only one
<TT
CLASS="LITERAL"
>END IF</TT
> is needed.
</P
><P
> Here is an example:
</P><PRE
CLASS="PROGRAMLISTING"
>IF number = 0 THEN
result := ''zero'';
ELSIF number > 0 THEN
result := ''positive'';
ELSIF number < 0 THEN
result := ''negative'';
ELSE
-- hmm, the only other possibility is that number IS NULL
result := ''NULL'';
END IF;</PRE
><P>
</P
><P
> The final ELSE section is optional.
</P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-CONTROL-STRUCTURES-LOOPS"
>19.6.3. Simple Loops</A
></H2
><P
> With the LOOP, EXIT, WHILE and FOR statements, you can arrange
for your <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function to repeat
a series of commands.
</P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN36002"
>19.6.3.1. LOOP</A
></H3
><P
></P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
><<label>></SPAN
>]
LOOP
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP;</PRE
><P>
LOOP defines an unconditional loop that is repeated indefinitely
until terminated by an EXIT or RETURN statement.
The optional label can be used by
EXIT statements in nested loops to specify which level of
nesting should be terminated.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN36008"
>19.6.3.2. EXIT</A
></H3
><P
></P><PRE
CLASS="SYNOPSIS"
>EXIT [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>] [<SPAN
CLASS="OPTIONAL"
> WHEN <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> </SPAN
>];</PRE
><P>
If no <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> is given,
the innermost loop is terminated and the
statement following END LOOP is executed next.
If <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> is given, it
must be the label of the current or some outer level of nested loop
or block. Then the named loop or block is terminated and control
continues with the statement after the loop's/block's corresponding
END.
</P
><P
> If WHEN is present, loop exit occurs only if the specified condition
is true, otherwise control passes to the statement after EXIT.
</P
><P
> Examples:
</P><PRE
CLASS="PROGRAMLISTING"
>LOOP
-- some computations
IF count > 0 THEN
EXIT; -- exit loop
END IF;
END LOOP;
LOOP
-- some computations
EXIT WHEN count > 0;
END LOOP;
BEGIN
-- some computations
IF stocks > 100000 THEN
EXIT; -- illegal. Can't use EXIT outside of a LOOP
END IF;
END;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN36021"
>19.6.3.3. WHILE</A
></H3
><P
></P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
><<label>></SPAN
>]
WHILE <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> LOOP
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP;</PRE
><P>
The WHILE statement repeats a
sequence of statements so long as the condition expression
evaluates to true. The condition is checked just before
each entry to the loop body.
</P
><P
> For example:
</P><PRE
CLASS="PROGRAMLISTING"
>WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
-- some computations here
END LOOP;
WHILE NOT boolean_expression LOOP
-- some computations here
END LOOP;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="AEN36030"
>19.6.3.4. FOR (integer for-loop)</A
></H3
><P
></P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
><<label>></SPAN
>]
FOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> IN [<SPAN
CLASS="OPTIONAL"
> REVERSE </SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> .. <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> LOOP
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP;</PRE
><P>
This form of FOR creates a loop that iterates over a range of integer
values. The variable
<TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> is automatically defined as type
integer and exists only inside the loop. The two expressions giving
the lower and upper bound of the range are evaluated once when entering
the loop. The iteration step is normally 1, but is -1 when REVERSE is
specified.
</P
><P
> Some examples of integer FOR loops:
</P><PRE
CLASS="PROGRAMLISTING"
>FOR i IN 1..10 LOOP
-- some expressions here
RAISE NOTICE ''i is %'',i;
END LOOP;
FOR i IN REVERSE 10..1 LOOP
-- some expressions here
END LOOP;</PRE
><P>
</P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPGSQL-RECORDS-ITERATING"
>19.6.4. Looping Through Query Results</A
></H2
><P
> Using a different type of FOR loop, you can iterate through
the results of a query and manipulate that data
accordingly. The syntax is:
</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
><<label>></SPAN
>]
FOR <TT
CLASS="REPLACEABLE"
><I
>record | row</I
></TT
> IN <TT
CLASS="REPLACEABLE"
><I
>select_query</I
></TT
> LOOP
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP;</PRE
><P>
The record or row variable is successively assigned all the rows
resulting from the SELECT query and the loop body is executed
for each row. Here is an example:
</P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION cs_refresh_mviews () RETURNS INTEGER AS '
DECLARE
mviews RECORD;
BEGIN
PERFORM cs_log(''Refreshing materialized views...'');
FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP
-- Now "mviews" has one record from cs_materialized_views
PERFORM cs_log(''Refreshing materialized view '' || quote_ident(mviews.mv_name) || ''...'');
EXECUTE ''TRUNCATE TABLE '' || quote_ident(mviews.mv_name);
EXECUTE ''INSERT INTO '' || quote_ident(mviews.mv_name) || '' '' || mviews.mv_query;
END LOOP;
PERFORM cs_log(''Done refreshing materialized views.'');
RETURN 1;
end;
' LANGUAGE 'plpgsql';</PRE
><P>
If the loop is terminated by an EXIT statement, the last
assigned row value is still accessible after the loop.
</P
><P
> The FOR-IN-EXECUTE statement is another way to iterate over
records:
</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
><<label>></SPAN
>]
FOR <TT
CLASS="REPLACEABLE"
><I
>record | row</I
></TT
> IN EXECUTE <TT
CLASS="REPLACEABLE"
><I
>text_expression</I
></TT
> LOOP
<TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END LOOP;</PRE
><P>
This is like the previous form, except that the source SELECT
statement is specified as a string expression, which is evaluated
and re-planned on each entry to the FOR loop. This allows the
programmer to choose the speed of a pre-planned query or the
flexibility of a dynamic query, just as with a plain EXECUTE
statement.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> The <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> parser presently distinguishes the
two kinds of FOR loops (integer or record-returning) by checking
whether the target variable mentioned just after FOR has been
declared as a record/row variable. If not, it's presumed to be
an integer FOR loop. This can cause rather nonintuitive error
messages when the true problem is, say, that one has
misspelled the FOR variable name.
</P
></BLOCKQUOTE
></DIV
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