> On Tue, 9 Oct 2018 at 15:43, Pavel Stehule <pavel.steh...@gmail.com> wrote: > > Hi > > I tested last patch and I have some notes: > > 1. > > postgres=# explain select distinct a10000 from foo; > +-------------------------------------------------------------------------------------------+ > | QUERY PLAN > | > +-------------------------------------------------------------------------------------------+ > | Unique (cost=0.43..4367.56 rows=9983 width=4) > | > | -> Index Skip Scan using foo_a10000_idx on foo (cost=0.43..4342.60 > rows=9983 width=4) | > +-------------------------------------------------------------------------------------------+ > (2 rows) > > In this case Unique node is useless and can be removed
Just to clarify which exactly version were you testing? If index-skip-fallback.patch, then the Unique node was added there to address the situation when ndistinct is underestimated, with an idea to fallback to original plan (and to tolerate that I suggested to use Unique, since we don't know if fallback will happen or not during the planning). > 2. Can be nice COUNT(DISTINCT support) similarly like MIN, MAX suppport Yep, as far as I understand MIN/MAX is going to be the next step after this patch will be accepted. > 3. Once time patched postgres crashed, but I am not able to reproduce it. Maybe you have at least some ideas what could cause that or what's the possible way to reproduce that doesn't work anymore?
