Cc: "pgsql-performance" <pgsql-performance@postgresql.org>
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Date: Thu, 13 Dec 2012 10:47:14 -0500
From: "Kevin Grittner" <kgrittn@mail.com>
Message-ID: <20121213154714.80050@gmx.com>
MIME-Version: 1.0
Subject: Re: Limit & offset effect on query plans
To: "Pavan Deolasee" <pavan.deolasee@gmail.com>,"Amitabh Kant"
	<amitabhkant@gmail.com>
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Sender: pgsql-performance-owner@postgresql.org

Pavan Deolasee wrote:
> Amitabh Kant <amitabhkant@gmail.com> wrote:

>> Our scripts automatically add "LIMIT ALL" & "OFFSET 0" to every
>> select query if no values are passed on for these parameters. I
>> remember reading through the mailing list that it's better not
>> to pass them if they are not needed as they add a cost to the
>> query plan. Is this the case, or am i looking at a very minor
>> optimization.
>>
> 
> I would tend to think that is the latter. While undoubtedly
> limit/offset clause will add another node during query planning
> and execution, AFAICS the OFFSET 0 and LIMIT ALL cases are
> optimized to a good extent. So the overhead of having them will
> not be significant.

I ran some quick tests on my i7 under Linux. Plan time was
increased by about 40 microseconds (based on EXPLAIN runtime) and
added a limit node to the plan. Execution time on a SELECT * FROM
tenk1 in the regression database went up by 1.35 ms on fully cached
runs.

-Kevin


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