Re: query optimization question

Jack Coates <jack(at)lyris(dot)com> writes:
> jackdb=# explain SELECT DISTINCT members_.memberid_
> jackdb-# FROM members_
> jackdb-# WHERE ( members_.List_='list1'
> jackdb(# AND members_.MemberType_='normal'
> jackdb(# AND members_.SubType_='mail'
> jackdb(# AND members_.emailaddr_ IS NOT NULL )
> jackdb-# GROUP BY memberid_ HAVING (

Um, that's not what I had in mind at all. Does GROUP BY actually do
anything at all here? (You didn't answer me as to whether memberid_
is a unique identifier or not, but if it is, this GROUP BY is just an
expensive no-op.)

What I was envisioning was pulling the sub-selects up to the top level
and using grouping to calculate the count(*) values for all memberids
in parallel. Roughly speaking it would look like (again assuming
memberid_ is unique)

SELECT memberid_ FROM
(
SELECT memberid_ FROM lyrActiveRecips, members_, outmail
WHERE (all the conditions for this case)
UNION ALL
SELECT memberid_ FROM lyrCompletedRecips, members_, outmail
WHERE (all the conditions for this case)
)
GROUP BY memberid_ HAVING count(*) = 3;

However, if you can't change the boilerplate part of your query then
this is all blue-sky speculation anyway. What I'm actually more
interested in is your statement that MSSQL can do the original query
quickly. I find that a bit hard to believe because I don't see any
relevant optimization techniques. Do they have any equivalent to
EXPLAIN that would give some hint how they're doing it?

regards, tom lane