Re: In-placre persistance change of a relation

The following review has been posted through the commitfest application:
make installcheck-world:  tested, passed
Implements feature:       tested, passed
Spec compliant:           tested, passed
Documentation:            not tested

I've retested v15 of the patch with everything that came to my mind. The patch passes all my tests (well, there's this
justwindows / cfbot issue). Patch looks good to me. I haven't looked in-depth at the code, so patch might still need
review.

FYI, about potential usage of this patch: the most advanced test that I did was continually bouncing wal_level - it
works.So chain of :
 
1. wal_level=replica->minimal
2. alter table set unlogged and load a lot of data, set logged
3. wal_level=minimal->replica
4. barman incremental backup # rsync(1) just backups changed files in steps 2 and 3 (not whole DB)
5. some other (logged) work
The idea is that when performing mass-alterations to the DB (think nightly ETL/ELT on TB-sized DBs), one could skip
backingup most of DB and then just quickly backup only the changed files - during the maintenance window - e.g. thanks
tolocal-rsync barman mode. This is the output of barman show-backups after loading data to unlogged table each such
cycle:
mydb 20220110T100236 - Mon Jan 10 10:05:14 2022 - Size: 144.1 GiB - WAL Size: 16.0 KiB
mydb 20220110T094905 - Mon Jan 10 09:50:12 2022 - Size: 128.5 GiB - WAL Size: 80.2 KiB
mydb 20220110T094016 - Mon Jan 10 09:40:17 2022 - Size: 109.1 GiB - WAL Size: 496.3 KiB
And dedupe ratio of the last one: Backup size: 144.1 GiB. Actual size on disk: 36.1 GiB (-74.96% deduplication ratio).


The only thing I've found out that bouncing wal_level also forces max_wal_senders=X -> 0 -> X which in turn requires
droppingreplication slot for pg_receievewal (e.g. barman receive-wal --create-slot/--drop-slot/--reset). I have tested
therestore using barman recover afterwards to backup 20220110T094905  and indeed it worked OK using this patch too. 

The new status of this patch is: Needs review