Re: pg_stat_wal_receiver and flushedUpto/writtenUpto

On Tue, May 19, 2020 at 11:38:52PM +0900, Fujii Masao wrote:
> I found that "received_lsn" is still used in high-availability.sgml.
> We should apply the following change in high-availability?
>
> - view's <literal>received_lsn</literal> indicates that WAL is being
> + view's <literal>flushed_lsn</literal> indicates that WAL is being

Oops, thanks. Will fix.

> BTW, we have pg_last_wal_receive_lsn() that returns the same lsn as
> pg_stat_wal_receiver.flushed_lsn. Previously both used the term "receive"
> in their names, but currently not. IMO it's better to use the same term in
> those names for the consistency, but it's not good idea to rename
> pg_last_wal_receive_lsn() to something like pg_last_wal_receive_lsn().
> I have no better idea for now. So I'm ok with the current names.

I think you mean renaming pg_last_wal_receive_lsn() to something like
pg_last_wal_flushed_lsn(), no? This name may become confusing because
we lose the "receive" idea in the function, that we have with the
"receiver" part of pg_stat_wal_receiver. Maybe something like that,
though that's long:
- pg_last_wal_receive_flushed_lsn()
- pg_last_wal_receive_written_lsn()

Anyway, a rename of this function does not strike me as strongly
necessary, as that's less tied with the shared memory structure, and
we document that pg_last_wal_receive_lsn() tracks the current LSN
received and flushed. I am actually wondering if in the future it may
not be better to remove this function, but it has no maintenance
cost either so I would just let it as-is.
--
Michael