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Re: Performance regression 8.3.8 -> 8.4.1 with NOT EXISTS

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: wiktor(dot)wodecki(at)net-m(dot)de
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Performance regression 8.3.8 -> 8.4.1 with NOT EXISTS
Date: 2009-11-17 16:30:14
Message-ID: 29752.1258475414@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Wiktor Wodecki <wiktor(dot)wodecki(at)net-m(dot)de> writes:
> As you can see the 8.4 run is 16 times slower. It was even worse before
> we added the index idx_b_b_date which we didn't have initially.
> Is there anything we can do about this issue? Do you need more information?

You could prevent flattening of the EXISTS subquery by adding an OFFSET
0 or some such to it.  A real fix involves being able to handle nestloop
indexscans where the parameter comes from more than one join level up;
there's been some discussion about that but it's not going to happen
in 8.4.x.

			regards, tom lane

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