On Mon, Nov 13, 2023 at 5:01 PM Peter Geoghegan <p...@bowt.ie> wrote: > > On Mon, Nov 13, 2023 at 4:43 PM Bruce Momjian <br...@momjian.us> wrote: > > I looked into this and all the 4-byte xid functions are marked as > > deprecated for the 8-byte variants. I don't think documenting 4-byte > > mxid_age() and age(xid) makes sense anymore, and I don't see their value > > enough to create 8-byte versions, so I just added C comments that they > > were undocumented, in the attached patch. > > I'm sympathetic to the goal of making 4 byte XIDs an on-disk > implementation detail that is all but completely hidden from users. > However, there are practical problems with taking that to its logical > extreme. At least right now. > > These functions are in fact documented -- albeit only partially. There > are references to both in "Routine Vacuuming". Moreover, those > references are rather useful; they're the basis of many > monitoring/alerting queries. If anything, I'd recommend adding more > documentation for these two functions. > > We also continue to show 32-bit XIDs (alongside 32-bit relfrozenxid) > in the output of VACUUM VERBOSE/autovacuum log messages. (Though that > issue can be fixed fairly easily.) > > The bottom line is that there is only one way to figure out the age of > a table right now, and it involves 32-bit XIDs/MXIDs, and these two > functions. And, if we were to change something in this area, we'd > definitely need to provide for the needs of those monitoring queries I > mentioned.
Also, the doc page [1] mentions mxid_age(), but doesn't provide a snippet to use it. The regular XID wraparound section above has such a snippet. As a consequence, almost nobody monitors for MultiXact wraparound – I noticed it recently once again, exploring numerous blog posts and tools on this topic to write a howto [2] in my collection. In other words, maybe there should be not only a reference doc for the function itself present in the doc (the lack of it seems to be an issue for all older versions), but also a snippet to control MultiXact ID wraparound – while it's still a potential problem, it would be good to have both a function reference doc and a how-to-use-it doc. [1] https://www.postgresql.org/docs/current/routine-vacuuming.html#VACUUM-FOR-MULTIXACT-WRAPAROUND [2] https://gitlab.com/postgres-ai/postgresql-consulting/postgres-howtos/-/blob/main/0044_how_to_monitor_transaction_id_wraparound_risks.md
