Re: Asynchronous MergeAppend

Alexander Pyhalov писал(а) 2026-04-13 18:09:
> Hi.
> 
> Looked at it. Overall I agree that when we wait for data from one slot 
> after node initialization, we can't get data from other slots - they 
> are already either exhausted, or have already received data which is 
> not needed for now. So it seems that async machinery on later stages is 
> useless.
> 
> Was a bit surprised that asyncresults field is not used in async merge 
> append anymore, as we save result directly in ms_slots. However, as we 
> do it only on initial stage, this seems to be OK.
> 
> classify_matching_subplans() comment still refers to ms_valid_subplans.
> 
> Will look at it once again tomorrow.

Haven't found anything suspicious (besides redundant empty line after 
enable_async_merge_append GUC description in 
src/backend/utils/misc/guc_parameters.dat). Tested it and was a bit 
surprised that new async Merge Append version (without async machinery 
after nodeMergeAppend initialization) was about 5% faster than the old 
one with concurrent queries (-j 16) and 10-15% faster with single-thread 
load (when there was a lot of CPU capacity)  in my tests (test was 
basically the same as in [1]). So, async machinery after Merge Append 
node is initialized was not only useless, but even harmful.

Test results:

patch version  | concurrency  | async_capable off | async_capable on
old            |  -c 1 -j 1   | 880  tps          | 1428 tps  (+62%)
old            |  -c 16 -j 16 | 5190 tps          | 4933 tps  (-5%)
new            |  -c 1 -j 1   | 888  tps          | 1582 tps  (+78%)
new            |  -c 16 -j 16 | 5020 tps          | 5256 tps  (+4%)

1. 
https://www.postgresql.org/message-id/159b591411bb2c81332018927acbd509%40postgrespro.ru
-- 
Best regards,
Alexander Pyhalov,
Postgres Professional