Re: pg_walinspect - a new extension to get raw WAL data and WAL stats

On Thu, Mar 10, 2022 at 9:38 PM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
> It seems to me too rigorous that pg_get_wal_records_info/stats()
> reject future LSNs as end-LSN and I think WARNING or INFO and stop at
> the real end-of-WAL is more kind to users. I think the same with the
> restriction that start and end LSN are required to be different.

In his review just yesterday, Jeff suggested this: "Don't issue
WARNINGs or other messages for ordinary situations, like when
pg_get_wal_records_info() hits the end of WAL." I think he's entirely
right, and I don't think any patch that does otherwise should get
committed. It is worth remembering that the results of queries are
often examined by something other than a human being sitting at a psql
terminal. Any tool that uses this is going to want to understand what
happened from the result set, not by parsing strings that may show up
inside warning messages.

I think that the right answer here is to have a function that returns
one row per record parsed, and each row should also include the start
and end LSN of the record. If for some reason the WAL records return
start after the specified start LSN (e.g. because we skip over a page
header) or end before the specified end LSN (e.g. because we reach
end-of-WAL) the user can figure it out from looking at the LSNs in the
output rows and comparing them to the LSNs provided as input.

--
Robert Haas
EDB: http://www.enterprisedb.com