Re: Add last_commit_lsn to pg_stat_database

Hi,

> [...]
> As I was thinking about how to improve things, I realized that this
> information (since it's for monitoring anyway) fits more naturally
> into the stats system. I'd originally thought of exposing it in
> pg_stat_wal, but that's per-cluster rather than per-database (indeed,
> this is a flaw I hadn't considered in the original patch), so I think
> pg_stat_database is the correct location.
>
> I've attached a patch to track the latest commit's LSN in pg_stat_database.

Thanks for the patch. It was marked as "Needs Review" so I decided to
take a brief look.

All in all the code seems to be fine but I have a couple of nitpicks:

- If you are modifying pg_stat_database the corresponding changes to
the documentation are needed.
- pg_stat_get_db_last_commit_lsn() has the same description as
pg_stat_get_db_xact_commit() which is confusing.
- pg_stat_get_db_last_commit_lsn() is marked as STABLE which is
probably correct. But I would appreciate a second opinion on this.
- Wouldn't it be appropriate to add a test or two?
- `if (!XLogRecPtrIsInvalid(commit_lsn))` - I suggest adding
XLogRecPtrIsValid() macro for better readability

--
Best regards,
Aleksander Alekseev