Re: mysterious difference in speed when combining two queries with OR

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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