Message-ID: <4F766454.9040909@gmail.com>
Date: Fri, 30 Mar 2012 21:56:36 -0400
From: Jay Levitt <jay.levitt@gmail.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Ants Aasma <ants@cybertec.at>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>,
	Francois Deliege <fdeliege@gmail.com>
Subject: Re: [PATCH] Lazy hashaggregate when no aggregation is needed
References: 
 <CA+CSw_uE-RCyQd_bXJNe=usrXkq+keFrQrahkc+8ou+Ws4Y=Vw@mail.gmail.com>
	<22709.1332944158@sss.pgh.pa.us>
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Tom Lane wrote:
> Ants Aasma<ants@cybertec.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? (I don't yet know enough about the 
tree to know when hashaggs get used). I'm wondering if this is a useful 
exception to the "restrictions can't get pushed down through GROUP BYs" rule.

Jay