From: Tom Lane <tgl@sss.pgh.pa.us>
To: Shrirang Chitnis <Shrirang.Chitnis@hovservices.com>
Cc: Bryce Nesbitt <bryce2@obviously.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: hashed subplan 5000x slower than two sequential operations
Date: Wed, 08 Dec 2010 15:12:26 -0500
Message-ID: <20170.1291839146@sss.pgh.pa.us> (raw)
In-Reply-To: <E6DB850FDAD49A459E3C217442489C9225FB9256AB@HOV-MAIL.hovservices.com>
References: <4CFFD446.6040609@obviously.com>
<E6DB850FDAD49A459E3C217442489C9225FB9256AB@HOV-MAIL.hovservices.com>
Shrirang Chitnis <Shrirang.Chitnis@hovservices.com> writes:
> Bryce,
> The two queries are different:
I suspect the second one is a typo and not what he really wanted.
> WHERE (contexts.parent_key = 392210
> OR contexts.context_key IN
> (SELECT collection_data.context_key
> FROM collection_data
> WHERE collection_data.collection_context_key = 392210)
The only really effective way the planner knows to optimize an
"IN (sub-SELECT)" is to turn it into a semi-join, which is not possible
here because of the unrelated OR clause. You might consider replacing
this with a UNION of two scans of "contexts". (And yes, I know it'd be
nicer if the planner did that for you.)
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, Shrirang.Chitnis@hovservices.com, bryce2@obviously.com
Subject: Re: hashed subplan 5000x slower than two sequential operations
In-Reply-To: <20170.1291839146@sss.pgh.pa.us>
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