Re: Non-null values of recovery functions after promote or crash of primary

Greetings,

* Martín Marqués (martin(at)2ndquadrant(dot)com) wrote:
> pg_last_wal_receive_lsn()
> pg_last_wal_replay_lsn()
> pg_last_xact_replay_timestamp()
>
> Under normal circumstances we would expect to receive NULLs from all
> three functions on a primary node, and code comments back up my thoughts.

Agreed.

> The problem is, what if the node is a standby which was promoted without
> restarting, or that had to perform crash recovery?
>
> So during the time it's recovering the values in ` XLogCtl` are updated
> with recovery information, and once the recovery finishes, due to crash
> recovery reaching a consistent state, or a promotion of a standby
> happening, those values are not reset to startup defaults.
>
> That's when you start seeing non-null values returned by
> `pg_last_wal_replay_lsn()`and `pg_last_xact_replay_timestamp()`.
>
> Now, I don't know if we should call this a bug, or an undocumented
> anomaly. We could fix the bug by resetting the values from ` XLogCtl`
> after finishing recovery, or document that we might see non-NULL values
> in certain cases.

IMV, and not unlike other similar cases I've talked about on another
thread, these should be cleared when the system is promoted as they're
otherwise confusing and nonsensical.

Thanks,

Stephen