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<a...@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
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