From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Scara Maccai <m_lists(at)yahoo(dot)it> |
Cc: | postgresql <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: Nested Loop Left Join always shows rows=1 |
Date: | 2008-12-01 21:09:40 |
Message-ID: | 1135.1228165780@sss.pgh.pa.us |
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Lists: | pgsql-general pgsql-hackers |
Scara Maccai <m_lists(at)yahoo(dot)it> writes:
> I ran ANALYZE on both tables, but the plan shows rows=1 in the inner
> table results.
> If I change the "left outer join" into an "inner join" the bitmap index
> scan of the inner table of the nested loop shows a correct number of
> rows (not the exact same thing as "actual rows", but very close).
Yeah, this is a bug: it's effectively double-counting the selectivity of
the index clause. The error isn't enormously critical, since the join
size estimate is right; but it could perhaps lead to choosing a plain
indexscan when a bitmap scan would be better. I've applied a patch.
regards, tom lane
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