Horton

Re: A qsort template

John Naylor <john.naylor@enterprisedb.com>

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From: John Naylor <john.naylor@enterprisedb.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Thomas Munro <thomas.munro@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2022-01-19T16:08:51Z
Lists: pgsql-hackers 
On Tue, Jan 18, 2022 at 9:58 PM Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Tue, Jan 18, 2022 at 6:39 PM John Naylor
> <john.naylor@enterprisedb.com> wrote:
> > Editorializing the null position in queries is not very common in my
> > experience. Not null is interesting since it'd be trivial to pass
> > constant false to the same Apply[XYZ]SortComparator() and let the
> > compiler remove all those branches for us. On the other hand, those
> > branches would be otherwise predicted well, so it might make little or
> > no difference.
>
> If you were going to do this, maybe you could encode NULL directly in
> an abbreviated key. I think that that could be made to work if it was
> limited to opclasses with abbreviated keys encoded as unsigned
> integers. Just a thought.

Now that you mention that, I do remember reading about this technique
in the context of b-tree access, so it does make sense. If we had that
capability, it would be trivial to order the nulls how we want while
building the sort tuple datums, and the not-null case would be handled
automatically. And have a smaller code footprint, I think.

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

Commits

  1. Remove debug messages from tuplesort_sort_memtuples()

  2. Fix performance regression in tuplesort specializations

  3. Fix tuplesort optimization for CLUSTER-on-expression.

  4. Initial pgindent and pgperltidy run for v14.

  5. Specialize checkpointer sort functions.

  6. Use sort_template.h for qsort() and qsort_arg().

  7. Use sort_template.h for qsort_tuple() and qsort_ssup().

  8. Add sort_template.h for making sort functions.

  9. Use abbreviated keys for faster sorting of text datums.