Mysterious performance of query because of plsql function in where condition

Hi there,

i have a problem with a query that uses the result of a plsql function
In the where clause:

SELECT
   assignments.assignment_id,
   assignments.package_id AS package_id,
   assignments.title AS title,
   COUNT(*) AS Count
FROM
   assignments INNER JOIN submissions ON
   (assignments.assignment_id=submissions.assignment_id)
WHERE
   package_id=949589 AND
   submission_status(submissions.submission_id)='closed'
GROUP BY
   assignments.assignment_id, assignments.package_id, assignments.title
ORDER BY
   assignments.title;

Postgres seems to execute the function "submission_status" for every row
of the submissions table (~1500 rows). The query therefore takes quite a
lot time, although in fact no row is returned from the assignments table
when the condition package_id=949589 is used.

                                                          QUERY PLAN

------------------------------------------------------------------------
------------------------------------------------------
 Sort  (cost=41.21..41.21 rows=1 width=35) (actual
time=4276.978..4276.978 rows=0 loops=1)
   Sort Key: assignments.title
   ->  HashAggregate  (cost=41.19..41.20 rows=1 width=35) (actual
time=4276.970..4276.970 rows=0 loops=1)
         ->  Hash Join  (cost=2.40..41.18 rows=1 width=35) (actual
time=4276.966..4276.966 rows=0 loops=1)
               Hash Cond: ("outer".assignment_id =
"inner".assignment_id)
               ->  Seq Scan on submissions  (cost=0.00..38.73 rows=9
width=4) (actual time=10.902..4276.745 rows=38 loops=1)
                     Filter: (submission_status(submission_id) =
'closed'::text)
               ->  Hash  (cost=2.40..2.40 rows=2 width=35) (actual
time=0.058..0.058 rows=0 loops=1)
                     ->  Seq Scan on assignments  (cost=0.00..2.40
rows=2 width=35) (actual time=0.015..0.052 rows=2 loops=1)
                           Filter: (package_id = 949589)
 Total runtime: 4277.078 ms
(11 rows)

I therefore tried to rephrase the query, to make sure that the function
is only used for the rows returned by the join but not even the
following does help (the subselect t1 does not return a single row):

select * from (
    SELECT
      a.assignment_id, a.package_id, a.title, s.submission_id,
COUNT(*) AS Count
    FROM
      assignments a INNER JOIN submissions s ON
(a.assignment_id=s.assignment_id)
    WHERE
        a.package_id=949589
    GROUP BY
        a.assignment_id, a.package_id, a.title, s.submission_id
) t1
where
   submission_status(t1.submission_id)='closed'
order by
   title;

                                                              QUERY PLAN

------------------------------------------------------------------------
--------------------------------------------------------------
 Sort  (cost=41.21..41.22 rows=1 width=188) (actual
time=4114.251..4114.251 rows=0 loops=1)
   Sort Key: title
   ->  Subquery Scan t1  (cost=41.20..41.20 rows=1 width=188) (actual
time=4114.242..4114.242 rows=0 loops=1)
         ->  HashAggregate  (cost=41.20..41.20 rows=1 width=39) (actual
time=4114.238..4114.238 rows=0 loops=1)
               ->  Hash Join  (cost=2.40..41.18 rows=1 width=39) (actual
time=4114.235..4114.235 rows=0 loops=1)
                     Hash Cond: ("outer".assignment_id =
"inner".assignment_id)
                     ->  Seq Scan on submissions s  (cost=0.00..38.73
rows=9 width=8) (actual time=7.179..4113.984 rows=38 loops=1)
                           Filter: (submission_status(submission_id) =
'closed'::text)
                     ->  Hash  (cost=2.40..2.40 rows=2 width=35) (actual
time=0.100..0.100 rows=0 loops=1)
                           ->  Seq Scan on assignments a
(cost=0.00..2.40 rows=2 width=35) (actual time=0.045..0.094 rows=2
loops=1)
                                 Filter: (package_id = 949589)
 Total runtime: 4114.356 ms
(12 rows)

The function is nevertheless executed for every row in the submissions
table. A simple "select *, submission_status(submission_id) from
submissions" takes about the same time as the 2 queries stated above.

The whole database has been vacuum analysed right before the explain
analyse output has been captured.

What can I do to reduce the time this query takes? And why is the
function executed although there is no row in the result set of t1 in my
rephrased query?

TIA, peter

--
peter.alberer@wu-wien.ac.at  Tel: +43/1/31336/4341
Abteilung für Wirtschaftsinformatik, Wirtschaftsuniversitaet Wien,
Austria