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From: Tom Lane <[email protected]>
To: George Pavlov <[email protected]>
Cc: [email protected]
Subject: Re: index vs. seq scan choice?
Date: Thu, 24 May 2007 21:08:11 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <8C5B026B51B6854CBE88121DBF097A86C3A30D@ehost010-33.exch010.intermedia.net>
References: <8C5B026B51B6854CBE88121DBF097A86C3A30D@ehost010-33.exch010.intermedia.net>

"George Pavlov" <[email protected]> writes:
> I am curious what could make the PA query to ignore the index. What are
> the specific stats that are being used to make this decision?

The frequency of the specific value being searched for, and the overall
order-correlation of the column.  Since the latter is not dependent on a
particular value, my guess at the reason for the inconsistent results is
that you don't have the column's statistics target set high enough to
track all the interesting values --- or maybe just not high enough to
acquire sufficiently accurate frequency estimates for them.  Take a look
at the pg_stats row for the column ...

(The default statistics target is 10, which is widely considered too
low --- you might find 100 more suitable.)

			regards, tom lane

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