Horton

Re: A qsort template

John Naylor <john.naylor@enterprisedb.com>

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From: John Naylor <john.naylor@enterprisedb.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Thomas Munro <thomas.munro@gmail.com>, Andres Freund <andres@anarazel.de>, Justin Pryzby <pryzby@telsasoft.com>, Peter Geoghegan <pg@bowt.ie>, pgsql-hackers <pgsql-hackers@postgresql.org>, Robert Haas <rhaas@postgresql.org>
Date: 2022-04-18T14:11:22Z
Lists: pgsql-hackers 
On Tue, Apr 12, 2022 at 7:58 AM David Rowley <dgrowleyml@gmail.com> wrote:
>
> I've attached the patch I tested. It was thrown together very quickly
> just to try out the performance. If it's interesting I can polish it
> up a bit. If not, I didn't waste too much time.

@@ -959,6 +965,10 @@ tuplesort_begin_batch(Tuplesortstate *state)

  state->tapeset = NULL;

+ /* check if specialized sorts can skip calling the tiebreak function */
+ state->oneKeySort = state->nKeys == 1 &&
+ !state->sortKeys[0].abbrev_converter;
+

IIUC, this function is called by tuplesort_begin_common, which in turn
is called by tuplesort_begin_{heap, indexes, etc}. The latter callers
set the onlyKey and now oneKeySort variables as appropriate, and
sometimes hard-coded to false. Is it intentional to set them here
first?

Falling under the polish that you were likely thinking of above:

We might rename oneKeySort to skipTiebreaker to avoid confusion.
SInce the test for these variable is the same, we could consolidate
them into a block and reword this existing comment  (which I find a
little confusing anyway):

/*
* The "onlyKey" optimization cannot be used with abbreviated keys, since
* tie-breaker comparisons may be required.  Typically, the optimization
* is only of value to pass-by-value types anyway, whereas abbreviated
* keys are typically only of value to pass-by-reference types.
*/

I can take a stab at this, unless you had something else in mind.

-- 
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.