From: | Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> |
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To: | pgsql-hackers(at)postgresql(dot)org |
Subject: | min_safe_lsn column in pg_replication_slots view |
Date: | 2020-06-15 03:40:03 |
Message-ID: | 9ddfbf8c-2f67-904d-44ed-cf8bc5916228@oss.nttdata.com |
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Lists: | pgsql-hackers |
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
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