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Optimizer regression 8.2.1 -> 8.2.3 on TSEARCH2 queries with ORDER BY and LIMIT

From: "Brendan McMahon" <Brendan(dot)McMahon(at)esb(dot)ie>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: Optimizer regression 8.2.1 -> 8.2.3 on TSEARCH2 queries with ORDER BY and LIMIT
Date: 2007-11-28 15:30:22
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Hi folks,

An apparent optimizer regression between 8.2.1 & 8.2.3 ? : 

	select pk,... from tbl where tsv @@ to_tsquery(...) order by pk limit 10 

disadvantageously uses PK index scan against a 2.5 million row (vacuum analysed) table whenever limit<=16 , leading to an increase in query time from sub 100ms to 4 seconds typically.

With identical freshly vaccuum analyzed table, 8.2.1 does the same only when limit <= 3

Although it's not a difference in principle, the later behaviour is more problematic as it is much more likely to be encountered in practice as part of a results paging scheme (with OFFSET N) 

Changing the ORDER BY clause to pk ||'' seems to get around the problem without any substantial execution overhead.

Anyone aware of any alternate workaround or info on likely behaviour in 8.3 ?


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