Re: Server crashed with TRAP: FailedAssertion("!(parallel_workers >0)" when partitionwise_aggregate true.

In the reported testcase, parallel_workers is set to 0 for all partition (child) relations. Which means partial parallel paths are not possible for child rels. However, the parent can have partial parallel paths. Thus, when we have a full partitionwise aggregate possible (as the group by clause matches with the partition key), we end-up in a situation where we do create a partially_grouped_rel for the parent but there won't be any partially_grouped_live_children.

The current code was calling add_paths_to_append_rel() without making sure of any live children present or not (sorry, it was my fault). This causes an Assertion failure in add_paths_to_append_rel() as this function assumes that it will have non-NIL live_childrels list.

I have fixed partitionwise aggregate code which is calling add_paths_to_append_rel() by checking the live children list correctly. And for robustness, I have also added an Assert() in add_paths_to_append_rel().

I have verified the callers of add_paths_to_append_rel() and except one, all are calling it by making sure that they have a non-NIL live children list. The one which is calling add_paths_to_append_rel() directly is set_append_rel_pathlist(). And I think, at that place, we will never have an empty live children list, I may be wrong though. And if that's the case then newly added Assert() in add_paths_to_append_rel() will fail anyway to catch any programming error in that code path.

Attached patch fixing the crash and also added a simple test-coverage for that.

Let me know if I missed any.

Rajkumar offlist reported another issue related to data-loss. If few of the partitions has parallel_workers = 0, not all, then PWA plan ended up with a plan having children which has parallel_workers != 0. So the partitions with parallel_workers = 0; were not scanned.

Fixed this in attached version of the patch.