To: pgsql-hackers@postgresql.org
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Subject: min_safe_lsn column in pg_replication_slots view
Message-ID: <9ddfbf8c-2f67-904d-44ed-cf8bc5916228@oss.nttdata.com>
Date: Mon, 15 Jun 2020 12:40:03 +0900
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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