Re: Combine pg_walinspect till_end_of_wal functions with others

On Fri, Mar 10, 2023 at 12:24 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Wed, Mar 08, 2023 at 01:40:46PM +0530, Bharath Rupireddy wrote:
> > 1. When start_lsn is NULL or invalid ('0/0'), emit an error. There was
> > a comment on the functions automatically determining start_lsn to be
> > the oldest WAL LSN. I'm not implementing this change now, as it
> > requires extra work to traverse the pg_wal directory. I'm planning to
> > do it in the next set of improvements where I'm planning to make the
> > functions timeline-aware, introduce functions for inspecting
> > wal_buffers and so on.
> >
> > [.. long description ..]
> >
> > 9. Refactored the tests according to the new behaviours.
>
> Hmm. I think this patch ought to have a result simpler than what's
> proposed here.
>
> First, do we really have to begin marking the functions as non-STRICT
> to abide with the treatment of NULL as a special value? The part that
> I've found personally the most annoying with these functions is that
> an incorrect bound leads to a random failure, particularly when such
> queries are used for monitoring.

As long as we provide a sensible default value (so I guess '0/0' to
mean "no upper bound") and that we therefore don't have to manually
specify an upper bound if we don't want one I'm fine with keeping the
functions marked as STRICT.

> I would simplify the whole with two
> simple rules, as of:
> - Keeping all the functions strict.
> - When end_lsn is a LSN in the future of the current LSN inserted or
> replayed, adjust its value to be the exactly GetXLogReplayRecPtr() or
> GetFlushRecPtr(). This way, monitoring tools can use a value ahead,
> at will.
> - Failing if start_lsn > end_lsn.
> - Failing if start_lsn refers to a position older than what exists is
> still fine by me.

+1

> I would also choose to remove
> pg_get_wal_records_info_till_end_of_wal() from the SQL interface in
> 1.1 to limit the confusion arount it, but keep a few lines of code so
> as we are still able to use it when pg_walinspect 1.0 is the version
> enabled with CREATE EXTENSION.

Yeah the SQL function should be removed no matter what.