Re: Add last commit LSN to pg_last_committed_xact()

On 2022-Jan-14, James Coleman wrote:

> The logical slot can't flush past the
> last commit, so even if there's 100s of megabytes of unflushed WAL on
> the slot there may be zero lag (in terms of what's possible to
> process).
>
> I've attached a simple patch (sans tests and documentation) to get
> feedback early. After poking around this afternoon it seemed to me
> that the simplest approach was to hook into the commit timestamps
> infrastructure and store the commit's XLogRecPtr in the cache of the
> most recent value (but of course don't write it out to disk).

Maybe it would work to have a single LSN in shared memory, as an atomic
variable, which uses monotonic advance[1] to be updated. Whether this is
updated or not would depend on a new GUC, maybe track_latest_commit_lsn.
Causing performance pain during transaction commit is not great, but at
least this way it shouldn't be *too* a large hit.

[1] part of a large patch at
https://www.postgresql.org/message-id/202111222156.xmo2yji5ifi2%40alvherre.pgsql

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