On 23 Apr 2008, at 9:23AM, Hans Ekbrand wrote:
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?
Try
select distinct moment.mid from moment,timecard where parent = 45 and
(pid=17 and timecard.mid = moment.mid) order by moment.mid
union all
select distinct moment.mid from moment,timecard where parent = 45 and
(pbar = 0) order by moment.mid;
--
Regards
Theo
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