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To: Robert Haas <robertmhaas@gmail.com>
cc: Merlin Moncure <mmoncure@gmail.com>, Ivan Voras <ivoras@freebsd.org>,
	pgsql-performance@postgresql.org
Subject: Re: Query performance with disabled hashjoin and mergejoin 
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Tue, 08 Mar 2011 15:57:46 -0500"
Date: Tue, 08 Mar 2011 16:24:25 -0500
Message-ID: <28147.1299619465@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Robert Haas <robertmhaas@gmail.com> writes:
> The reason I thought cross-column correlations might be relevant is
> that the bitmap index scan on news_visible_from is quite accurate
> (19976 estimated vs. 19932 actual) and the bitmap index scan on
> news_visible_to is tolerably accurate (151 estimated vs. 41 actual)
> but the estimate on the BitmapOr is somehow totally wrong (20127
> estimated vs. 0 actual).  But on further reflection that doesn't make
> much sense.  How can the BitmapOr produce fewer rows than the sum of
> its constituent inputs?

That's not an estimation bug, that's a measurement bug.  We don't try to
count the actual number of rows present in the result of a BitmapOr or
BitmapAnd node.  (It would be impractical in lossy cases anyway, not to
mention expensive.)

			regards, tom lane