On Sun, Nov 22, 2020 at 5:07 PM Tomas Vondra <tomas.von...@enterprisedb.com> wrote: > > On 11/22/20 10:31 PM, Tom Lane wrote: > > Tomas Vondra <tomas.von...@enterprisedb.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. > > > > But how do we know it's safe to actually do the sort there, e.g. in > light of the volatility/parallel-safety issues discussed in other threads? > > I agree the check may be useful, but maybe we should just do elog(ERROR) > instead.
That was my thought. I'm not a big fan of maintaining a "this might be useful" path particularly when there isn't any case in the code or tests at all that exercises it. In other words, not only is it not useful [yet], but also we don't actually have any signal to know that it works (or keeps working) -- whether through tests or production use. So I'm +1 on turning it into an ERROR log instead, since it seems to me that encountering this case would almost certainly represent a bug in path generation. James
