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Re: [PERFORM] Slow query: bitmap scan troubles

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>,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: 2013-01-15 19:46:39
Message-ID: 20130115194639.GG27934@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On Mon, Jan 14, 2013 at 12:56:37PM -0500, Tom Lane wrote:
> > 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.

Wow, do we need to bump up that value based on larger modern hardware?

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +


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