Horton

Re: [PATCH] Incremental sort

Peter Geoghegan <pg@bowt.ie>

View thread View on postgr.es
From: Peter Geoghegan <pg@bowt.ie>
To: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Mithun Cy <mithun.cy@enterprisedb.com>, Robert Haas <robertmhaas@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2017-04-26T16:56:30Z
Lists: pgsql-hackers 
On Wed, Apr 26, 2017 at 8:39 AM, Alexander Korotkov
<a.korotkov@postgrespro.ru> wrote:
> That appears to be wrong.  I intended to make cost_sort prefer plain sort
> over incremental sort for this dataset size.  But, that appears to be not
> always right solution.  Quick sort is so fast only on presorted data.

As you may know, I've often said that the precheck for sorted input
added to our quicksort implementation by a3f0b3d is misguided. It
sometimes throws away a ton of work if the presorted input isn't
*perfectly* presorted. This happens when the first out of order tuple
is towards the end of the presorted input.

I think that it isn't fair to credit our qsort with doing so well on a
100% presorted case, because it doesn't do the necessary bookkeeping
to not throw that work away completely in certain important cases.

-- 
Peter Geoghegan

VMware vCenter Server
https://www.vmware.com/

Commits

  1. Further adjustments to Hashagg EXPLAIN ANALYZE output

  2. Rework EXPLAIN format for incremental sort

  3. Fix typos and improve incremental sort comments

  4. Stabilize incremental_sort tests

  5. Minor improvements in Incremental Sort explain

  6. Consider Incremental Sort paths at additional places

  7. Fix representation of SORT_TYPE_STILL_IN_PROGRESS.

  8. Fix failures in incremental_sort due to number of workers

  9. Fix show_incremental_sort_info with force_parallel_mode

  10. Implement Incremental Sort

  11. Fix handling of "Subplans Removed" field in EXPLAIN output.

  12. Fix EXPLAIN (SETTINGS) to follow policy about when to print empty fields.

  13. Ensure plpgsql result tuples have the right composite type marking.

  14. Propagate sort instrumentation from workers back to leader.

  15. Make new regression test case parallel-safe, and improve its output.

  16. Push limit through subqueries to underlying sort, where possible.

  17. Fix inappropriate printing of never-measured times in EXPLAIN.

  18. Fix some infelicities in EXPLAIN output for parallel query plans.