Re: Considering additional sort specialisation functions for PG16

On Tue, Aug 23, 2022 at 9:18 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
>
> I have the following dimensions in mind for consideration:
>
> 1. Specialisations to handle sorting of non-null datums (eliminates
> checking for nulls in the comparison function)
> 2. Specialisations to handle single column sorts (eliminates
> tiebreaker function call or any checks for existence of tiebreaker)
> 3. ASC sort (No need for if (ssup->ssup_reverse) INVERT_COMPARE_RESULT(compare))
>
> If we did all of the above then we'd end up with 3 * 2 * 2 * 2 = 24
> specialization functions. That seems a bit excessive. So here I'd
> like to discuss which ones we should add, if any.
>
> I've attached a very basic implementation of #1 which adds 3 new
> functions for sorting non-null datums.

Did you happen to see

https://www.postgresql.org/message-id/CAFBsxsFhq8VUSkUL5YO17cFXbCPwtbbxBu%2Bd9MFrrsssfDXm3Q%40mail.gmail.com

where I experimented with removing all null handling? What I had in
mind was pre-partitioning nulls and non-nulls when populating the
SortTuple array, then calling qsort twice, once with the non-null
partition with comparators that assume non-null, and (only if there
are additional sort keys) once on the null partition. And the
pre-partitioning would take care of nulls first/last upfront. I
haven't looked into the feasibility of this yet, but the good thing
about the concept is that it removes null handling in the comparators
without additional sort specializations.

--
John Naylor
EDB: http://www.enterprisedb.com