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