Re: Use LIMIT instead of Unique for DISTINCT when all distinct pathkeys are redundant

On Thu, Oct 13, 2022 at 6:43 PM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Thu, 13 Oct 2022 at 21:17, Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> >
> > On Thu, Oct 13, 2022 at 2:48 PM David Rowley <dgrowleyml(at)gmail(dot)com>
> wrote:
> >> To stop the planner from adding that final sort, I opted to hack the
> >> LimitPath's pathkeys to say that it's already sorted by the
> >> PlannerInfo's sort_pathkeys. That feels slightly icky, but it does
> >> seem a little wasteful to initialise a sort node on every execution of
> >> the plan to sort a single tuple.
> >
> >
> > I don't get how this plan comes out. It seems not correct because Limit
> > node above an unsorted path would give us an unpredictable row.
>
> Actually, you're right. That manual setting of the pathkeys is an
> unneeded remanent from a bug I fixed before sending out v2. It can
> just be removed.
>
> I've attached the v3 patch.

The v3 patch looks good to me.

Thanks
Richard