Re: Index Skip Scan - Mailing list pgsql-hackers

út 9. 10. 2018 v 15:59 odesílatel Dmitry Dolgov <9erthalion6@gmail.com> napsal:

> On Tue, 9 Oct 2018 at 15:43, Pavel Stehule <pavel.stehule@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).

I tested index-skip-fallback.patch.

It looks like good idea, but then the node should be named "index scan" and other info can be displayed in detail parts. It can be similar like "sort".

The combination of unique and index skip scan looks strange :)

> 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.

ok

Now, the development cycle is starting - maybe it can use same infrastructure like MIN/MAX and this part can be short.

more if you use dynamic index scan

> 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?

I think it was query like

select count(*) from (select distinct x from tab) s