min_safe_lsn column in pg_replication_slots view

Hi,

Per the docs, pg_replication_slots.min_safe_lsn inedicates "the minimum
LSN currently available for walsenders". When I executed pg_walfile_name()
with min_safe_lsn, the function returned the name of the last removed
WAL file instead of minimum available WAL file name. This happens because
min_safe_lsn actually indicates the ending position (the boundary byte)
of the last removed WAL file.

I guess that some users would want to calculate the minimum available
WAL file name from min_safe_lsn by using pg_walfile_name(), but the result
would be incorrect. Isn't this confusing? min_safe_lsn should indicate
the bondary byte + 1, instead?

BTW, I just wonder why each row in pg_replication_slots needs to have
min_safe_lsn column? Basically min_safe_lsn should be the same between
every replication slots.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION