Re: UNION ALL has higher cost than inheritance

I wrote:
> The oversight here is that we don't use appendrel planning for
> a top-level UNION ALL construct. That didn't use to matter,
> because you always got the same stupid Append plan either way.
> Now it seems like we ought to have some more intelligence for the
> top-level SetOp case. I smell some code refactoring coming up.

I did some hacking on this and came up with the attached patch, which
could use a bit more work on the comments but passes regression tests.
However, this just solves the issue of being smart about top-level
UNION ALL cases. It might be worth looking into using MergeAppend
for the sorting required for other types of set operations. That would
involve quite a different patch, and I'm not sure if it'd remove the
need for this one or not.

regards, tom lane