On 05/06/2011 05:13 PM, Thomas HÃgi wrote:
the query "SELECT * FROM data.logs ORDER BY re_timestamp DESC LIMIT 100" does use seq scans on all tables instead of using the existing indexes which takes ages. when issuing the the same query to one of the child tables directly ("SELECT * FROM data.logs_2011 ORDER BY re_timestamp DESC LIMIT 100") the index is used as expected and the data returned quickly.
Let's see, cut and paste http://archives.postgresql.org/message-id/4DB8CE7D.8030503@xxxxxxxxxxxxxxx and:
This is probably the limitation that's fixed in PostgreSQL 9.1 by this commit (following a few others leading up to it): http://archives.postgresql.org/pgsql-committers/2010-11/msg00028.php
There was a good example showing what didn't work as expected before (along with an earlier patch that didn't everything the larger 9.1 improvement does) at http://archives.postgresql.org/pgsql-hackers/2009-07/msg01115.php ; "ORDER BY x DESC LIMIT 1" returns the same things as MAX(x).
It's a pretty serious issue with the partitioning in earlier versions. I know of multiple people, myself included, who have been compelled to apply this change to an earlier version of PostgreSQL to make larger partitioned databases work correctly. The other option is to manually decompose the queries into ones that target each of the child tables individually, then combine the results, which is no fun either.
(Am thinking about a documentation backpatch pointing out this limitation) -- Greg Smith 2ndQuadrant US greg@xxxxxxxxxxxxxxx Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
