Re: Slow query: bitmap scan troubles

Ah okay, thanks. I knew I could set various things but not
effective_work_mem (I tried reloading the edited config file but it didn't
seem to pick it up)

From: Vitalii Tymchyshyn [mailto:tivv00(at)gmail(dot)com]
Sent: 04 December 2012 18:51
To: postgresql(at)foo(dot)me(dot)uk
Cc: postgres performance list
Subject: Re: [PERFORM] Slow query: bitmap scan troubles

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(at)foo(dot)me(dot)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