ffunc called multiple for same value

I posted a message a couple weeks ago abou having a problem with a
user-defined C language aggregate and the ffunc being called multiple
times with the same state. I came up with a test case which shows the
problem with plpgsql functions. It occurs with an aggregate in an inner
query, when a nested loop is used. ANALYZE the tables with zero rows
causes it to use a nested loop. We first discovered the problem when we
analyzed a test database and our ffunc started failing because we
assumed the ffunc was called once and could free memory.

CREATE TABLE foo (a integer);

CREATE TABLE bar (a integer, b integer, c integer);

ANALYZE foo;
ANALYZE bar;

INSERT INTO foo VALUES (1);
INSERT INTO foo VALUES (2);
INSERT INTO foo VALUES (3);

INSERT INTO bar VALUES (1, 5, 19);
INSERT INTO bar VALUES (2, 7, 23);
INSERT INTO bar VALUES (2, 9, 29);
INSERT INTO bar VALUES (3, 11, 31);
INSERT INTO bar VALUES (3, 13, 37);
INSERT INTO bar VALUES (3, 17, 41);

CREATE OR REPLACE FUNCTION custom_agg_sfunc(integer, integer) RETURNS
integer
LANGUAGE 'plpgsql'
AS '
BEGIN
RAISE NOTICE ''custom_agg_sfunc: state: % value % '', $1, $2;
RETURN $1 * $2;
END;
';

CREATE OR REPLACE FUNCTION custom_agg_ffunc(integer) RETURNS integer
LANGUAGE 'plpgsql'
AS '
BEGIN
RAISE NOTICE ''custom_agg_ffunc: % '', $1;
RETURN $1;
END;
';

CREATE AGGREGATE custom_agg (
sfunc = custom_agg_sfunc,
basetype = integer,
stype = integer,
finalfunc = custom_agg_ffunc,
initcond = 1
);

SELECT foo.a, comp
FROM foo, (
SELECT a, custom_agg(c) AS comp
FROM bar
GROUP BY a
) x
WHERE foo.a = x.a;

The results are:

NOTICE: custom_agg_sfunc: state: 1 value 31
NOTICE: custom_agg_sfunc: state: 1 value 37
NOTICE: custom_agg_sfunc: state: 37 value 41
NOTICE: custom_agg_sfunc: state: 1 value 43
NOTICE: custom_agg_sfunc: state: 43 value 47
NOTICE: custom_agg_sfunc: state: 2021 value 53
NOTICE: custom_agg_ffunc: 31
NOTICE: custom_agg_ffunc: 1517
NOTICE: custom_agg_ffunc: 107113
NOTICE: custom_agg_ffunc: 31
NOTICE: custom_agg_ffunc: 1517
NOTICE: custom_agg_ffunc: 107113
NOTICE: custom_agg_ffunc: 31
NOTICE: custom_agg_ffunc: 1517
NOTICE: custom_agg_ffunc: 107113
a | comp
---+--------
3 | 31
5 | 1517
7 | 107113
(3 rows)

- Ian