Re: POC: GROUP BY optimization

On 2/10/22 10:00, Andrey V. Lepikhov wrote:
> On 1/22/22 01:34, Tomas Vondra wrote:
>>
>
> I rebased (with minor fixes) this patch onto current master.
>
> Also, second patch dedicated to a problem of "varno 0" (fake_var).
> I think, this case should make the same estimations as in the case of
> varno != 0, but no any stats found. So I suppose to restrict number of
> groups with min of a number of incoming tuples and DEFAULT_NUM_DISTINCT
> value.
>

Thanks for the rebase. The two proposed changes (tweaked costing and
simplified fake_var handling) seem fine to me. I think the last thing
that needs to be done is cleanup of the debug GUCs, which I added to
allow easier experimentation with the patch.

I probably won't remove the GUCs entirely, though. I plan to add a
single GUC that would enable/disable this optimization. I'm not a huge
fan of adding more and more GUCs, but in this case it's probably the
right thing to do given the complexity of estimating cost with
correlated columns etc.

regards

--
Tomas Vondra
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