Tom Lane wrote:
Ants Aasma<> 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.


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