To: Gary Warner <gar@cis.uab.edu>
cc: pgsql-performance@postgresql.org
Subject: Re: Seq Scan used instead of Index Scan 
In-reply-to: <1930791595.3607.1322087076188.JavaMail.root@zimbra.cis.uab.edu> 
References: <1930791595.3607.1322087076188.JavaMail.root@zimbra.cis.uab.edu>
Comments: In-reply-to Gary Warner <gar@cis.uab.edu>
	message dated "Wed, 23 Nov 2011 16:24:36 -0600"
Date: Wed, 23 Nov 2011 22:06:10 -0500
Message-ID: <10512.1322103970@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Gary Warner <gar@cis.uab.edu> writes:
> Recently my database stopped respecting one of my indexes, which took a query that should run in "subsecond response time" and turning it into something that with small data sets runs in the 7-10 minute range and with large data sets runs in the 30 minute - eternity range.

> Explain Analyze tells me that what used to be an Index Scan has become a Seq Scan, doing a full table scan through 140 million records.

> Any thoughts on why that happens?

I'd bet it has a lot to do with the nigh-three-orders-of-magnitude
overestimates of the numbers of matching rows.  You might find that
increasing the statistics targets for the indexed columns helps ---
I'm guessing that these particular key values are out in the long
tail of a highly skewed distribution, and the planner needs a larger MCV
list to convince it that non-MCV values will not occur very many times.

If that is an accurate guess, then trying to force the matter with
something like enable_seqscan = off is not a good production solution,
because it will result in horrid plans whenever you decide to query
a not-so-infrequent value.

			regards, tom lane