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Re: IN or EXISTS

From: "Tomas Vondra" <tv(at)fuzzy(dot)cz>
To: "Andy Colson" <andy(at)squeakycode(dot)net>
Cc: "Craig Ringer" <ringerc(at)ringerc(dot)id(dot)au>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: IN or EXISTS
Date: 2011-08-31 14:19:36
Message-ID: cf12201c9c3aa4a29a5c2ed26c7853ce.squirrel@sq.gransy.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 31 Srpen 2011, 15:59, Andy Colson wrote:
> I assume:
> Buckets: 16384  Batches: 1  Memory Usage: 4531kB
>
> That means a total of 4.5 meg of ram was used for the hash, so if my
> work_mem was lower than that it would swap?  (or choose a different plan?)

Why don't you try that? Just set the work_mem to 1MB or so and run the query.

I think it'll use the same plan but multiple batches - read just part of
the inner table so that the hash table fits into work_mem, scan the outer
table etc. The downside is it'd rescan the outer table several times.

Tomas


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