Re: Fix BUG #17335: Duplicate result rows in Gather node

Good day, hackers.

Problem:
- Append path is created with explicitely parallel_aware = true
- It has two child, one is trivial, other is parallel_aware = false .
  Trivial child is dropped.
- Gather/GatherMerge path takes Append path as a child and thinks
  its child is parallel_aware = true.
- But Append path is removed at the last since it has only one child.
- Now Gather/GatherMerge thinks its child is parallel_aware, but it
  is not.
  Gather/GatherMerge runs its child twice: in a worker and in a leader,
  and gathers same rows twice.

Reproduction code attached (repro.sql. Included as a test as well).

Suggested quick (and valid) fix in the patch attached:
- If Append has single child, then copy its parallel awareness.

Bug were introduced with commit 8edd0e79460b414b1d971895312e549e95e12e4f
"Suppress Append and MergeAppend plan nodes that have a single child."

During discussion, it were supposed [1] those fields should be copied:

> I haven't looked into whether this does the right things for parallel
> planning --- possibly create_[merge]append_path need to propagate up
> parallel-related path fields from the single child?

But it were not so obvious [2].

Better fix could contain removing Gather/GatherMerge node as well if
its child is not parallel aware.