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Re: 7.4 vs 7.3 ( hash join issue )

From: Dennis Bjorklund <db(at)zigo(dot)dhs(dot)org>
To: Gaetano Mendola <mendola(at)bigfoot(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: 7.4 vs 7.3 ( hash join issue )
Date: 2004-09-22 09:32:19
Message-ID: Pine.LNX.4.44.0409221128210.9559-100000@zigo.dhs.org (view raw or flat)
Thread:
Lists: pgsql-patchespgsql-performance
On Wed, 22 Sep 2004, Gaetano Mendola wrote:

>   Limit  (cost=10.21..10.21 rows=1 width=24) (actual time=0.885..0.885 rows=0 loops=1)
>   Limit  (cost=10.28..10.28 rows=1 width=24) (actual time=0.429..0.429 rows=0 loops=1)

These estimated costs are almost the same, but the runtime differs a bit. 
This means that maybe you need to alter settings like random_page_cost, 
effective_cache and maybe some others to make the cost reflect the runtime 
better.

Since the costs are so close to each other very small changes can make it 
choose the other plan. It's also very hard to make an estimate that is 
correct in all situations. That's why it's called an estimate after all.

-- 
/Dennis Björklund


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