Horton

Re: [PATCH] Incremental sort (was: PoC: Partial sort)

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndquadrant.com>
To: Rafia Sabih <rafia.pghackers@gmail.com>
Cc: James Coleman <jtc331@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Shaun Thomas <shaun.thomas@2ndquadrant.com>, Dmitry Dolgov <9erthalion6@gmail.com>, Alexander Korotkov <a.korotkov@postgrespro.ru>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2019-06-24T16:51:16Z
Lists: pgsql-hackers 
On Wed, 5 Jun 2019 at 17:14, Rafia Sabih <rafia.pghackers@gmail.com> wrote:


> Regarding this, I came across this,
> /*
>   * Incremental sort can't be used with either EXEC_FLAG_REWIND,
>   * EXEC_FLAG_BACKWARD or EXEC_FLAG_MARK, because we hold only current
>   * bucket in tuplesortstate.
>   */
> I think that is quite understandable. How are you planning to support
> backwards scan for this? In other words, when will incremental sort be
> useful for backward scan.
>

We stopped materializing the sort by default about 15 years ago because it
wasn't a common use case and it was very expensive for large sorts.

It's no real problem if incremental sorts don't support backwards scans -
we just won't use incremental in that case.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Solutions for the Enterprise

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.