Re: Allow pushdown of HAVING clauses with grouping sets

On Wed, Sep 11, 2024 at 11:43 AM Richard Guo <guofenglinux@gmail.com> wrote:
> In some cases, we may want to transfer a HAVING clause into WHERE in
> hopes of eliminating tuples before aggregation instead of after.
>
> Previously, we couldn't do this if there were any nonempty grouping
> sets, because we didn't have a way to tell if the HAVING clause
> referenced any columns that were nullable by the grouping sets, and
> moving such a clause into WHERE could potentially change the results.
>
> Now, with expressions marked nullable by grouping sets with the RT
> index of the RTE_GROUP RTE, it is much easier to identify those
> clauses that reference any nullable-by-grouping-sets columns: we just
> need to check if the RT index of the RTE_GROUP RTE is present in the
> clause.  For other HAVING clauses, they can be safely pushed down.
>
> I'm not sure how common it is in practice to have a HAVING clause
> where all referenced columns are present in all the grouping sets.
> But it seems to me that this optimization doesn't cost too much.  Not
> implementing it seems like leaving money on the table.

I've gone ahead and pushed this.

Thanks
Richard