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Re: [PERFORM] Slow query: bitmap scan troubles

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [PERFORM] Slow query: bitmap scan troubles
Date: 2013-01-14 17:56:37
Message-ID: 24605.1358186197@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Jan 14, 2013 at 12:23 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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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