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

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? (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