I think the root cause is that when we generate distinct_pathkeys, we failed to realize that Var 'b' might be nullable by the grouping sets, so it's no longer always equal to Var 'a'. It's not correct to deem that the PathKey for 'b' is redundant and thus remove it from the pathkeys list.
We have the same issue when generating sort_pathkeys. As a result, we may have the final output in the wrong order. There were several reports about this issue before, such as [1][2].
To fix this issue, I'm thinking that we mark the grouping expressions nullable by grouping sets with a dummy RTE for grouping sets, something like attached.
Hi Tom, I'm wondering if you've got a chance to look into this issue. What do you think about the fix?
