From: | Anuradha Ratnaweera <anuradha(at)gnu(dot)com> |
---|---|
To: | pgsql-general(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org |
Cc: | anuradha(at)gnu(dot)org |
Subject: | Group by count() and indexes |
Date: | 2003-02-17 15:43:35 |
Message-ID: | 20030217154335.GA25666@aratnaweera.virtusa.com |
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Lists: | pgsql-general pgsql-hackers |
Consider the following query on a large table with lots of different
`id's:
SELECT id FROM my_table GROUP BY id ORDER BY count(id) LIMIT 10;
It has an (usually unique) index on id. Obviously, the index helps to
evaluate count(id) for a given value of id, but count()s for all the
`id's should be evaluated, so sort() will take most of the time.
Is there a way to improve performance of this query? If not, please
give some indication to do a workaround on the source itself, so perhaps
I may be able to work out a patch.
Thanks in advance.
Anuradha
--
Debian GNU/Linux (kernel 2.4.21-pre4)
It is contrary to reasoning to say that there is a vacuum or space in
which there is absolutely nothing.
-- Descartes
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