Horton

Re: [HACKERS] [PATCH] Incremental sort

Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>

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From: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
To: Alexander Korotkov <a.korotkov@postgrespro.ru>, Andres Freund <andres@anarazel.de>
Cc: Teodor Sigaev <teodor@sigaev.ru>, Darafei Komяpa Praliaskouski <me@komzpa.net>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Antonin Houska <ah@cybertec.at>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-03-29T19:13:08Z
Lists: pgsql-hackers

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Hi Alexander,

I took a quick look at the patch. Some things I fixed myself in the 
attached patch v.21. Here is the summary:

Typo in compare_fractional_path_costs() should be fixed as a separate patch.
Remove unused function estimate_pathkeys_groups.
Extra MemoryContextReset before tuplesort_end() shouldn't be a big deal, 
so we don't have to add a parameter to tuplesoft_free().
Add comment to maincontext declaration.
Fix typo in INITIAL_MEMTUPSIZE.
Remove trailing whitespace.

Some other things I found:

In tuplesort_reset:
if (state->memtupsize < INITIAL_MEMTUPSIZE)
     <reallocate memtuples to INITIAL_MEMTUPSIZE>
I'd add a comment explaining when and why we have to do this. Also maybe 
a comment to other allocations of memtuples in tuplesort_begin() and 
mergeruns(), explaining why it is reallocated and why in maincontext.


In tuplesort_updatemax:
     /* XXX */
     if (spaceUsedOnDisk > state->maxSpaceOnDisk ||
         (spaceUsedOnDisk == state->maxSpaceOnDisk && spaceUsed > 
state->maxSpace))
XXX. Also comparing bools with '>' looks confusing to me.


We should add a comment on top of tuplesort.c, explaining that we now 
have a faster way to sort multiple batches of data using the same sort 
conditions.


The name 'main context' sounds somewhat vague. Maybe 'top context'? Not 
sure.


In ExecSupportBackwardsScan:
         case T_IncrementalSort:
             return false;
This separate case looks useless, I'd either add a comment explaining 
why it can't scan backwards, or just return false by default.



That's all I have for today; tomorrow I'll continue with reviewing the 
planner part of the patch.

-- 
Alexander Kuzmenkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

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.