Re: query uses index but takes too much time?

Le 24 novembre 2011 18:20, MirrorX <mirrorx(at)gmail(dot)com> a écrit :
> hello to all,
>
> i would like your advice on the following matter. i have a table with 150
> million rows. there are some indexes on this table but the one that is
> really important is one that has 3 columns (a,b,c). one application
> constantly makes queries and the query planner uses this index to narrow
> down the final set of results. so usually from 150 millions, when the 3
> conditions have been applied, the remaining rows to be checked are about
> 20-300. So these queries are very fast, and take from 10-100 ms usually.
> There is a special case where these 3 conditions narrow down the final set
> to 15.000 rows so the server must check all these rows. The result is that
> the query takes around 1 minute to complete. Is that a normal time for the
> execution of the query?
>
> i know that most of you will send me the link with the guide to reporting
> slow queries but that's not the point at the moment. i am not looking for a
> specific answer why this is happening.
> i just want to know if that seems strange to more people than just me and if
> i should look into that.
>
> but if for the above you need to have a clearer picture of the server then:
> -red hat 5.6
> -32 cores,
> -96GB ram
> -fiber storage (4GBps)
> -postgresql 9.0.5
> -shared_buffers : 25 GB
> -not i/o bound (too many disks, different partitions for backup, archives,
> xlogs, indexes)
> -not cpu bound (the cpu util was about 5% when i performed the tests)
> -the query planner values on postgresql.conf are the default
> -i also performed the tests on the hot-standby with the same results
> -the query plan is the correct one, indicating that it should use the
> correct index
> -i forced index_scan to off and then it used bitmap heap scan with similar
> results.
> -i forced bitmap heap scan to off and then it did a seq scan
>
> any ideas? thx in advance for your insight
>

not it is not that strange. It can be several things that lead you to
this situation.

>
>
>
>
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Cédric Villemain +33 (0)6 20 30 22 52
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