I noticed that this query worked correctly in the main branch with the inequality operator:
postgres=# select distinct on (a, b) a, b from (values (3, 1), (2, 2)) as t (a, b) where a > b group by grouping sets((a, b), (a)); a | b ---+--- 3 | 1 3 | (2 rows)
So, I think you are right)
Thanks for taking an interest in this patch and verifying it.
I looked at your patch and noticed a few things:
1. I think you should add a test with the cube operator, because I noticed that the order of the query in the result has also changed:
Hmm, I'm not sure if that's necessary. The wrong result order you saw here is caused by the same reason explained above: the planner fails to realize that Var 'a' and 'b' are nullable by the grouping sets, making them no longer always equal to each other. This issue should have been covered in the tests added by v1 patch.
