Re: Parallel append plan instability/randomness

I wrote:
>> In the regression test case at hand, the startup costs are all zero so
>> this change wouldn't improve the test case's stability. So I'm thinking
>> that in addition, it would be a good idea for these functions to break
>> exact compare_path_costs ties in some arbitrary but deterministic way,
>> rather than letting qsort() have the final say on what happens. If the
>> subplans were all simple relation scans we could order them by planner
>> relid, but I'm not sure what to do if they're not.

> Ah, I have an idea --- let's create a bms_compare() qsort comparator for
> bitmapsets, and use that on the paths' relid sets. It hardly matters
> what the exact semantics of the comparator are, as long as it behaves
> sanely for equal sets.

Concretely, I propose the attached. I spent a little bit of extra effort
on the comparator in hopes that it might be useful for something else.

regards, tom lane