Re: POC, WIP: OR-clause support for indexes

Hi, Alexander!
d4378c0005e61b1bb7

On Fri, 28 Mar 2025 at 03:18, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>
> Hi!
>
> On Mon, Mar 24, 2025 at 2:46 PM Alena Rybakina
> <a(dot)rybakina(at)postgrespro(dot)ru> wrote:
> > I agree with Andrey's changes and think we should fix this, because otherwise it might be inconvenient.
> > For example, without this changes we will have to have different test output files for the same query for different versions of Postres in extensions if the whole change is only related to the order of column output for a transformation that was not applied.
>
> I agree with problem spotted by Andrei: it should be preferred to
> preserve original order of clauses as much as possible. The approach
> implemented in Andrei's patch seems fragile for me. Original order is
> preserved if we didn't find any group. But once we find a single
> group original order might be destroyed completely.
>
> The attached patch changes the reordering algorithm of
> group_similar_or_args() in the following way. We reorder each group
> of similar clauses so that the first item of the group stays in place,
> but all the other items are moved after it. So, if there are no
> similar clauses, the order of clauses stays the same. When there are
> some groups, only required reordering happens while the rest of the
> clauses remain in their places.

With your patch, I've re-checked that there are no changes in the
order of evaluation in plans compared to d4378c0005e61b1bb7

It might be good to also include Andrei's test from his last patch. i.e:

+-- No OR-clause groupings should happen - no clause permutations in
+-- the filtering conditions we should see in the EXPLAIN.
+EXPLAIN (COSTS OFF)
+SELECT * FROM tenk1 WHERE unique1 < 1 OR hundred < 2;
+
+-- OR clauses on the 'unique' column is grouped. So, clause
permutation happened
+-- We see it in the 'Recheck Cond' and order of BitmapOr subpaths:
index scan on
+-- the 'hundred' column occupies the first position.
+EXPLAIN (COSTS OFF)
+SELECT * FROM tenk1 WHERE unique1 < 1 OR unique1 < 3 OR hundred < 2;

I propose small changes for comments:

s/To have this property,/To do so,/g
s/in place, but all the/in place, and all the/g
s/some groups, only/some groups,/g
s/Resort/Re-sort/п

The patch overall looks good to me.

Regards,
Pavel Borisov
Supabase