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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: Jeff Janes <jeff.janes@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: [PERFORM] Slow query: bitmap scan troubles
Date: Mon, 14 Jan 2013 12:56:37 -0500
Message-ID: <24605.1358186197@sss.pgh.pa.us> (raw)
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Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Jan 14, 2013 at 12:23 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> The old code definitely had an unreasonably large charge for indexes
>> exceeding 1e8 or so tuples.  This wouldn't matter that much for simple
>> single-table lookup queries, but I could easily see it putting the
>> kibosh on uses of an index on the inside of a nestloop.

> The reported behavior was that the planner would prefer to
> sequential-scan the table rather than use the index, even if
> enable_seqscan=off.  I'm not sure what the query looked like, but it
> could have been something best implemented as a nested loop w/inner
> index-scan.

Remember also that "enable_seqscan=off" merely adds 1e10 to the
estimated cost of seqscans.  For sufficiently large tables this is not
exactly a hard disable, just a thumb on the scales.  But I don't know
what your definition of "extremely large indexes" is.

			regards, tom lane


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