I wrote: > I also notice that setrefs.c can elide Append and MergeAppend nodes > too in some cases, but AFAICS costing of those node types doesn't > take that into account.
I took a closer look at this point and realized that in fact, create_append_path and create_merge_append_path do attempt to account for this. But they get it wrong! Somebody changed the rules in setrefs.c to account for parallelism, and did not update the costing side of things.
The attached v2 is the same as v1 plus adding a fix for this point. No regression test results change from that AFAICT.
The new fix looks good to me. Seems setrefs.c added a new logic to check parallel_aware when removing single-child Appends/MergeAppends in f9a74c14, but it neglected to update the related costing logic. And I can see this patch fixes the costing for that.
BTW, not related to this patch, the new lines for parallel_aware check in setrefs.c are very wide. How about wrap them to keep consistent with arounding codes?
