Re: POC: GROUP BY optimization

On 4/12/24 06:44, Tom Lane wrote:
> * The very first hunk causes get_eclass_for_sort_expr to have
> side-effects on existing EquivalenceClass data structures.
> What is the argument that that's not going to cause problems?
> At the very least it introduces asymmetry, in that the first
> caller wins the chance to change the EC's ec_sortref and later
> callers won't be able to.
Let me resolve issues bit by bit.
Addressing your first note, 'asymmetry,' I discovered this part of the
code. Before the 8d83a5d, it could cause some artefacts, but since then,
a kind of asymmetry has been introduced into the GROUP-BY clause list.
As you can see in the attached patch, GROUP-BY reordering works even
when the asymmetry of the 8d83a5d chooses different keys.

At the same time, I agree that implicitly setting anything in an
exporting function, which should just look up for EC, is a questionable
coding style. To avoid possible usage issues (in extensions, for
example), I propose setting it up afterwards, explicitly forcing this
action by input parameter - see my attempt in the attachment.

--
regards,
Andrei Lepikhov
Postgres Professional