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Re: Slow query: bitmap scan troubles - Mailing list pgsql-performance

From Vitalii Tymchyshyn
Subject Re: Slow query: bitmap scan troubles
Date
Msg-id CABWW-d2gK6=AOBWxJij8GYS6CQC4+p7xWDsr4FSs2-BaXDQMsA@mail.gmail.com
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In response to Re: Slow query: bitmap scan troubles  (<postgresql@foo.me.uk>)
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Re: Slow query: bitmap scan troubles
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Well, you don't need to put anything down. Most settings that change planner decisions can be tuned on per-quey basis by issuing set commands in given session. This should not affect other queries more than it is needed to run query in the way planner chooses.

Best regards, Vitalii Tymchyshyn


2012/12/4 <postgresql@foo.me.uk>

>> But the row estimates are not precise at the top of the join/filter.
>> It thinks there will 2120 rows, but there are only 11.

>Ah... I didn't spot that one...

Yes, you are right there - this is probably a slightly atypical query of
this sort actually, 2012 is a pretty good guess.

On Claudio's suggestion I have found lots more things to read up on and am
eagerly awaiting 6pm when I can bring the DB down and start tweaking. The
effective_work_mem setting is going from 6Gb->88Gb which I think will make
quite a difference.

I still can't quite wrap around my head why accessing an index is expected
to use more disk access than doing a bitmap scan of the table itself, but I
guess it does make a bit of sense if postgres assumes the table is more
likely to be cached.

It's all quite, quite fascinating :)

I'll let you know how it goes.

- Phil



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Best regards,
 Vitalii Tymchyshyn

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