Need help with query

Don't know if this would be the correct place to ask this. We have a 
postgresql server running version 6.5.3
It hosts a 100 or so different databases, and performs just great. I do 
however have one query which takes about 20 seconds to complete. The 
database only has 200-300 records. Don't know much about SQL other than 
simple selects and inserts. I'm the one who created the query in 
question, and even today when I look at it, I'm not sure I understand 
it, but it gives us the results we want, it's just very slow.

I'm sure someone with some SQL know-how could probably shorten this 
query down some. But it's way over my head.

Basically I've got a database with these tables:
client (list of clients)
project (each project belongs to a client)
subproject (each subproject belongs to a project)
status (each project has a status)
users (each project and subproject are assigned to a user)
timelog (each timelog record indicates time worked on a particular 
subproject and who worked on it)

A sample row would look like this:


project_id|project_title|clientid|status|percentcomplete|duedate   
|usersid|client_id|name       
|users_id|users_name|status_id|status_name|hoursworked
----------+-------------+--------+------+---------------+----------+-------+
---------+-----------+--------+----------+---------+-----------+-----------
        143|project 1    |      79|     3|            
100|04-06-2001|      8|       79|Client Name|       8|     Kevin|        
3|status     |          1



The query is below. Again if this is not the appropriate list for such a 
question, my apologies.

Kevin


*******************
SELECT
p1.project_id, p1.project_title, p1.clientid, p1.status, 
p1.percentcomplete, p1.duedate, p1.usersid,
c2.client_id, c2.name,
u3.users_id, u3.users_name,
s4.status_id, s4.status_name,
sum(case when t.jobid=s.subproject_id then t.timespent else 0::float4 
end) as hoursworked

FROM
project p1, client c2, users u3, status s4, timelog t, subproject s

WHERE
(s4.status_id=2 OR s4.status_id=3) AND
(c2.client_id=p1.clientid) AND (u3.users_id=p1.usersid) AND
(s4.status_id=p1.status) AND (p1.project_id=s.projectid)

GROUP BY
p1.project_id, p1.project_title, p1.clientid, p1.status, 
p1.percentcomplete, p1.duedate, p1.usersid,
c2.client_id, c2.name, u3.users_id, u3.users_name, s4.status_id, 
s4.status_name

UNION SELECT
p1.project_id, p1.project_title, p1.clientid, p1.status, 
p1.percentcomplete, p1.duedate, p1.usersid,
c2.client_id, c2.name,
u3.users_id, u3.users_name,
s4.status_id, s4.status_name,
0 as hoursworked

FROM
project p1,client c2,users u3,status s4, subproject s

WHERE
(s4.status_id=2 OR s4.status_id=3) AND
(c2.client_id=p1.clientid) AND (u3.users_id=p1.usersid) AND
(s4.status_id=p1.status) AND

NOT EXISTS
(Select s.subproject_id from subproject s where 
s.projectid=p1.project_id)

GROUP BY
p1.project_id, p1.project_title, p1.clientid, p1.status, 
p1.percentcomplete, p1.duedate, p1.usersid,
c2.client_id, c2.name, u3.users_id, u3.users_name, s4.status_id, 
s4.status_name

ORDER BY p1.project_title