Skip site navigation (1) Skip section navigation (2)

Re: query optimization question

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Jack Coates <jack(at)lyris(dot)com>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: query optimization question
Date: 2004-01-29 19:31:11
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
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)
  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

In response to


pgsql-performance by date

Next:From: Bill MoranDate: 2004-01-29 19:44:28
Subject: Re: [PERFORM] Set-Returning Functions WAS: On the performance of
Previous:From: lndDate: 2004-01-29 19:29:59
Subject: Explain plan for 2 column index

Privacy Policy | About PostgreSQL
Copyright © 1996-2017 The PostgreSQL Global Development Group