RE: Selectively invalidate caches in pgoutput module

Dear hackers,

I did a performance testing with HEAD and v2-0001, and confirmed that it could
improve performance around 20% in the typical case.

Workload
======
I emulated a scenario that there are many tables to be published and only one
table is stop and resume publishing. In HEAD, ALTER PUBLICATION DROP/ADD command
invalidates all entries in the RelSyncCache, but v2 invalidates only a needed
one - this means the decoding time would be reduced after patching.

1. Initialized an instance
2. Created a root table and 5000 leaf tables.
3. Created another table which did not have parent-child relationship.
4. Created a publication which included a root table and another table
5. Created a replication slot with pgoutput plugin.
6. Executed a transaction which would be decoded. In the transaction:
a. Inserted tuples to all the leaf tables
b. Altered publication to drop another table
c. Altered publication again to add the dropped one
d. Inserted tuples to all the leaf tables again.
7. measured decoding time via SQL interfaces, five times.

Attached script automated above.

Results
=====
Each cell is a median value of 5 runs. Compared with HEAD, V2-0001 can reduce
the decoding time, relatively 20%.

head [ms] v2-0001 [ms]
252 198

I'm planning to do further tests to prove the benefit for 0002, 0003 patches.

Best regards,
Hayato Kuroda
FUJITSU LIMITED