Re: Why does create_gather_merge_plan need make_sort?

Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
> On 11/20/20 11:24 PM, James Coleman wrote:
>> While looking at another issue I noticed that create_gather_merge_plan
>> calls make_sort if the subplan isn't sufficiently sorted. In all of
>> the cases I've seen where a gather merge path (not plan) is created
>> the input path is expected to be properly sorted, so I was wondering
>> if anyone happened to know what case is being handled by the make_sort
>> call. Removing it doesn't seem to break any tests.

> Yeah, I think you're right this is dead code, essentially. We're only
> ever calling create_gather_merge_path() with pathkeys matching the
> subpath. And it's like that on REL_12_STABLE too, i.e. before the
> incremental sort was introduced.

It's probably there by analogy to the other callers of
prepare_sort_from_pathkeys, which all do at least a conditional
make_sort(). I'd be inclined to leave it there; it's cheap insurance
against somebody weakening the existing behavior.

regards, tom lane