From: | Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> |
---|---|
To: | Jay Levitt <jay(dot)levitt(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Ants Aasma <ants(at)cybertec(dot)at>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Francois Deliege <fdeliege(at)gmail(dot)com> |
Subject: | Re: [PATCH] Lazy hashaggregate when no aggregation is needed |
Date: | 2012-06-15 10:55:41 |
Message-ID: | 4.3.2-J.20120615194845.03a6fa20@129.60.53.11 |
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Lists: | pgsql-hackers |
Hi,
I would like to ask a question before looking into the patch.
At 21:56 12/03/30 -0400, Jay Levitt wrote:
>Tom Lane wrote:
>>Ants Aasma<ants(at)cybertec(dot)at> writes:
>>>A user complained on pgsql-performance that SELECT col FROM table
>>>GROUP BY col LIMIT 2; performs a full table scan. ISTM that it's safe
>>>to return tuples from hash-aggregate as they are found when no
>>>aggregate functions are in use. Attached is a first shot at that.
>>
>>As I commented in the other thread, the user would be a lot better off
>>if he'd had an index on the column in question. I'm not sure it's worth
>>complicating the hashagg logic when an indexscan + groupagg would
>>address the case better.
>
>Would this patch help in the case where "table" is actually a
>set-returning function, and thus can't have an index?
ISTM that in many cases, the result size of a set-returning function is
not so large compared with that of a full plain table scan. So, in such a
case a full hash aggregation is not so time consuming. Am I wrong?
Best regards,
Etsuro Fujita
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