Re: [PATCH] Lazy hashaggregate when no aggregation is needed

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@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?

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