RE: Perform streaming logical transactions by background workers and parallel apply

Hi,

I did some performance tests for this patch, based on v59-0001 and v59-0002
patch.

This test used synchronous logical replication, and compared SQL execution times
before and after applying the patch.

Two cases are tested by varying logical_decoding_work_mem:
a) Bulk insert.
b) Rollback to savepoint. (Different percentage of changes in the transaction
are rolled back).

The test was performed ten times, and the average of the middle eight was taken.

The results are as follows. The bar charts are attached.
(The steps are the same as before.[1])

RESULT - bulk insert (5kk)
---------------------------------------------------------------
logical_decoding_work_mem   64kB        256kB       64MB
HEAD                        51.655      51.694      51.262
patched                     31.104      31.234      31.711
Compare with HEAD           -39.79%     -39.58%     -38.14%

RESULT - rollback 10% (5kk)
---------------------------------------------------------------
logical_decoding_work_mem   64kB        256kB       64MB
HEAD                        43.908      43.358      42.874
patched                     31.924      31.343      29.102
Compare with HEAD           -27.29%     -27.71%     -32.12%

RESULT - rollback 20% (5kk)
---------------------------------------------------------------
logical_decoding_work_mem   64kB        256kB       64MB
HEAD                        40.561      40.599      40.015
patched                     31.562      32.116      29.680
Compare with HEAD           -22.19%     -20.89%     -25.83%

RESULT - rollback 30% (5kk)
---------------------------------------------------------------
logical_decoding_work_mem   64kB        256kB       64MB
HEAD                        38.092      37.756      37.142
patched                     31.631      31.236      28.783
Compare with HEAD           -16.96%     -17.27%      -22.50%

RESULT - rollback 50% (5kk)
---------------------------------------------------------------
logical_decoding_work_mem   64kB        256kB       64MB
HEAD                        33.387      33.056      32.638
patched                     31.272      31.279      29.876
Compare with HEAD           -6.34%      -5.38%      -8.46%

(If "Compare with HEAD" is a positive number, it means worse than HEAD; if it is
a negative number, it means better than HEAD.)

Summary:
In the case of bulk insert, it takes about 30% ~ 40% less time, which looks good
to me.
In the case of rollback to savepoint, the larger the amount of data rolled back,
the smaller the improvement compared to HEAD. But as such cases won't be often,
this should be okay.

[1]
https://www.postgresql.org/message-id/OSZPR01MB63103AA97349BBB858E27DEAFD499%40OSZPR01MB6310.jpnprd01.prod.outlook.com

Regards,
Shi yu