Control structures are probably the most useful (and
important) part of PL/pgSQL. With
PL/pgSQL's control structures,
you can manipulate PostgreSQL data in a very
flexible and powerful way.
RETURN expression;
RETURN with an expression is used to return from a
PL/pgSQL function that does not return a set.
The function terminates and the value of
expression is returned to the caller.
To return a composite (row) value, you must write a record or row
variable as the expression. 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 void,
then the expression can be omitted, and will be ignored in any case.)
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.
When a PL/pgSQL function is declared to return
SETOF sometype, 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:
SELECT * FROM some_func();
That is, the function is used as a table source in a FROM clause.
RETURN NEXT expression;
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
PL/pgSQL 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.
Note: The current implementation of RETURN NEXT for
PL/pgSQL stores the entire result set before
returning from the function, as discussed above. That means that
if a PL/pgSQL 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
PL/pgSQL 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
SORT_MEM configuration variable. Administrators who
have sufficient memory to store larger result sets in memory
should consider increasing this parameter.
IF statements let you execute commands based on
certain conditions. PL/pgSQL has four forms of
IF:
IF boolean-expression THEN
statements
END IF;
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.
IF v_user_id <> 0 THEN
UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;
IF boolean-expression THEN
statements
ELSE
statements
END IF;
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.
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;
IF statements can be nested, as in the following example:
IF demo_row.sex = ''m'' THEN
pretty_sex := ''man'';
ELSE
IF demo_row.sex = ''f'' THEN
pretty_sex := ''woman'';
END IF;
END IF;
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.
IF boolean-expression THEN
statements
[ ELSIF boolean-expression THEN
statements
[ ELSIF boolean-expression THEN
statements
...]]
[ ELSE
statements ]
END IF;
IF-THEN-ELSIF-ELSE provides a more convenient
method of checking many alternatives in one statement.
Formally it is equivalent to nested
IF-THEN-ELSE-IF-THEN commands, but only one
END IF is needed.
Here is an example:
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;
The final ELSE section is optional.
With the LOOP, EXIT, WHILE and FOR statements, you can arrange
for your PL/pgSQL function to repeat
a series of commands.
[<<label>>]
LOOP
statements
END LOOP;
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.
EXIT [ label ] [ WHEN expression ];
If no label is given,
the innermost loop is terminated and the
statement following END LOOP is executed next.
If label 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.
If WHEN is present, loop exit occurs only if the specified condition
is true, otherwise control passes to the statement after EXIT.
Examples:
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;
[<<label>>]
WHILE expression LOOP
statements
END LOOP;
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.
For example:
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;
[<<label>>]
FOR name IN [ REVERSE ] expression .. expression LOOP
statements
END LOOP;
This form of FOR creates a loop that iterates over a range of integer
values. The variable
name 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.
Some examples of integer FOR loops:
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;
Using a different type of FOR loop, you can iterate through
the results of a query and manipulate that data
accordingly. The syntax is:
[<<label>>]
FOR record | row IN select_query LOOP
statements
END LOOP;
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:
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';
If the loop is terminated by an EXIT statement, the last
assigned row value is still accessible after the loop.
The FOR-IN-EXECUTE statement is another way to iterate over
records:
[<<label>>]
FOR record | row IN EXECUTE text_expression LOOP
statements
END LOOP;
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.
Note: The PL/pgSQL 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.
