am Wed, dem 23.04.2008, um 9:23:07 +0200 mailte Hans Ekbrand folgendes:
> I cannot understand why the following two queries differ so much in
execution time (almost ten times)
wild guess: different execution plans.
Can you show us the plans? (EXPLAIN ANALYSE SELECT ...)
>
> 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;
>
> [ snip ]
>
> I should say that this is on postgresql 7.4.16 (debian stable).
Uhh. Why not a recent version? We have 8.3.0...
>
> Can query B be rewritten so that it would execute faster?
Quick and dirty: use both selects (query A) combined with UNION.
I guess, with a recent version the planner can use a bitmap index scan
to perform Query B faster.
Andreas
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Andreas Kretschmer
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