From: Tom Lane <tgl@sss.pgh.pa.us>
To: Sai Sadashiva Kundurmutt <saisadashiva2000@gmail.com>
cc: pgsql-admin@lists.postgresql.org
Subject: Re: Are the estimates made for number of pages accurate
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Comments: In-reply-to Sai Sadashiva Kundurmutt <saisadashiva2000@gmail.com>
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Sai Sadashiva Kundurmutt <saisadashiva2000@gmail.com> writes:
> I wanted to know whether the numbers calculated by ANALYZE for number of
> pages are exact(the relpages field in pg_class).

If memory serves, relpages is accurate as of the time of the last
VACUUM or ANALYZE on the table, but of course it could be arbitrarily
out of date later.

> I had the impression that
> it is exact  because during random sampling the number of pages is assumed
> to be known.

What makes you think that?  And even more to the point, what makes
you think that we'd rely on relpages for such a purpose?  I'd expect
a fresh RelationGetNumberOfBlocks call in any scan that had a need
to know the table size.

			regards, tom lane