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mysterious difference in speed when combining two queries with OR

From: Hans Ekbrand <hans(dot)ekbrand(at)sociology(dot)gu(dot)se>
To: pgsql-performance(at)postgresql(dot)org
Subject: mysterious difference in speed when combining two queries with OR
Date: 2008-04-23 07:23:07
Message-ID: 20080423072303.GF11886@amin (view raw or flat)
Thread:
Lists: pgsql-performance
I cannot understand why the following two queries differ so much in execution time (almost ten times)

Query A (two queries)

select distinct moment.mid from moment,timecard where parent = 45 and (pid=17 and timecard.mid = moment.mid) order by moment.mid;
select distinct moment.mid from moment,timecard where parent = 45 and (pbar = 0) order by moment.mid;

Query B (combining the two with OR)

select distinct moment.mid from moment,timecard where parent = 45 and ((pid=17 and timecard.mid = moment.mid) or (pbar = 0)) order by moment.mid;

$ time psql -o /dev/null -f query-a.sql fektest

real    0m2.016s
user    0m1.532s
sys     0m0.140s

$ time psql -o /dev/null -f query-b.sql fektest

real    0m28.534s
user    0m1.516s
sys     0m0.156s

I have tested this in two different computers with different amount of
RAM, fast or slow CPU, and the difference is persistent, almost ten
times.

I should say that this is on postgresql 7.4.16 (debian stable).

Can query B be rewritten so that it would execute faster?

TIA

-- 
Hans Ekbrand (http://sociologi.cjb.net) <hans(at)sociologi(dot)cjb(dot)net>
GPG Fingerprint: 1408 C8D5 1E7D 4C9C C27E 014F 7C2C 872A 7050 614E

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Next:From: A. KretschmerDate: 2008-04-23 07:58:10
Subject: Re: mysterious difference in speed when combining two queries with OR
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Subject: SELECT 'DBD::Pg ping test'

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