From: Mathieu De Zutter <mathieu@dezutter.org>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: pgsql-performance@postgresql.org
Subject: Re: Choice of bitmap scan over index scan
Date: Mon, 11 Jan 2010 09:36:40 +0100
Message-ID: <d4468d971001110036s22796d2bud01f8ec38fd21ce0@mail.gmail.com> (raw)
In-Reply-To: <603c8f071001101852n7ce69d9atfc0b7aa65d8e357d@mail.gmail.com>
References: <4B49A3AF020000250002E138@gw.wicourts.gov>
<603c8f071001101852n7ce69d9atfc0b7aa65d8e357d@mail.gmail.com>
On Mon, Jan 11, 2010 at 3:52 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Sun, Jan 10, 2010 at 10:53 AM, Kevin Grittner
> <Kevin.Grittner@wicourts.gov> wrote:
> > seq_page_cost = 0.1
> > random_page_cost = 0.1
>
> These might not even be low enough. The reason why bitmap index scans
> win over plain index scans, in general, is because you make one pass
> through the heap to get all the rows you need instead of bouncing
> around doing lots of random access. But of course if all the data is
> in RAM then this argument falls down.
>
> If these aren't enough to get the query planner to DTRT, then the OP
> might want to try lowering them further and seeing how low he has to
> go to flip the plan...
>
So if this query usually does *not* hit the cache, it will be probably
faster if I leave it like that? While testing a query I execute it that much
that it's always getting into the cache. However, since other applications
run on the same server, I think that infrequently used data gets flushed
after a while, even if the DB could fit in the RAM.
--
Mathieu
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To: pgsql-performance@postgresql.org
Cc: mathieu@dezutter.org, robertmhaas@gmail.com, Kevin.Grittner@wicourts.gov
Subject: Re: Choice of bitmap scan over index scan
In-Reply-To: <d4468d971001110036s22796d2bud01f8ec38fd21ce0@mail.gmail.com>
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