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8.4 semi-join slows down query performance (EXISTS)

From: vacuum(at)quantentunnel(dot)de
To: pgsql-hackers(at)postgresql(dot)org
Subject: 8.4 semi-join slows down query performance (EXISTS)
Date: 2009-04-21 09:38:06
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Hello folk,

I migrate a pg 8.3 database to a pg 8.4 backend for testing. All works fine except changes or new features of the planner.

There are two tables. The first (A) stores "data" - second table (B) holds (forinstance) "structure" information and references to A by defining foreign-key constraint(s).

I've queries returning tupels of A, with a (sub-)selected constant-expression that indicates whether a referenced tupel exists in B or not. For this issue the EXISTS clause is used.

In past (8.3) the planner resolves this into index-scans using existing foreign-key indices -> fast query (1.5 seconds for comparison). Now (in 8.4) the planner wants "semi-joins". Index-scans are not longer used and my query needs 600 seconds to return.

I attached two plans of the identical query - executed in 8.3 and 8.4 as well as the query itself.

You will see some more differences between planning in 8.3 and 8.4. The differences relating this mail you can find at the end of the plans.


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