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

Ps: table definitions:

Table "public.assignments"
Column | Type | Modifiers
---------------+-----------------------------+------------------------
assignment_id | integer | not null
title | character varying(100) | not null
max_grade | smallint | not null
start_date | timestamp without time zone | not null default now()
end_date | timestamp without time zone | not null
over_due_date | timestamp without time zone |
score_release | smallint | not null default 1
package_id | integer | not null
cal_item_id | integer |
Indexes:
"assignments_pk" primary key, btree (assignment_id)
Check constraints:
"assignments_sr_ck" CHECK (score_release = 1 OR score_release = 2 OR
score_release = 3)
Foreign-key constraints:
"cal_item_id" FOREIGN KEY (cal_item_id) REFERENCES
cal_items(cal_item_id) ON DELETE SET NULL
"package_id_fk" FOREIGN KEY (package_id) REFERENCES
apm_packages(package_id)
"assignment_id_fk" FOREIGN KEY (assignment_id) REFERENCES
acs_objects(object_id) ON DELETE CASCADE

Table "public.submissions"
Column | Type | Modifiers
---------------+-----------------------------+-----------
submission_id | integer | not null
person_id | integer | not null
assignment_id | integer | not null
last_modified | timestamp without time zone | not null
recovery_date | timestamp without time zone |
grading | smallint |
grading_date | timestamp without time zone |
Indexes:
"submissions_pk" primary key, btree (submission_id)
"submissions_person_ass_un" unique, btree (person_id, assignment_id)
Foreign-key constraints:
"assignment_id_fk" FOREIGN KEY (assignment_id) REFERENCES
assignments(assignment_id)
"person_id_fk" FOREIGN KEY (person_id) REFERENCES persons(person_id)

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