On Tue, 2025-09-02 at 08:22 +0000, PG Doc comments form wrote: > Page: https://www.postgresql.org/docs/17/routine-vacuuming.html > > This is a most pedantic point, but since the postgres documentation is > incredibly accurate and well written I indulge my pedantry this one time: > > Regarding the last sentence of the first paragraph of 24.1.5: I sure hope > vacuuming every table in every database at least once every two billion > transactions is not only necessary to avoid catastrophic data loss, but also > sufficient. Indeed if I understand the subsequent explanation, it is > sufficient but not necessary. > > Here is the full paragraph: > > 24.1.5. Preventing Transaction ID Wraparound Failures > PostgreSQL's MVCC transaction semantics depend on being able to compare > transaction ID (XID) numbers: a row version with an insertion XID greater > than the current transaction's XID is “in the future” and should not be > visible to the current transaction. But since transaction IDs have limited > size (32 bits) a cluster that runs for a long time (more than 4 billion > transactions) would suffer transaction ID wraparound: the XID counter wraps > around to zero, and all of a sudden transactions that were in the past > appear to be in the future — which means their output become invisible. In > short, catastrophic data loss. (Actually the data is still there, but that's > cold comfort if you cannot get at it.) To avoid this, it is necessary to > vacuum every table in every database at least once every two billion > transactions. > > Suggested change for the last sentence: > To avoid this, it suffices to vacuum every table in every database at least > once every two billion transactions.
I don't think that that would be an improvement. Yes, it is sufficient, but it is also necessary. And the "necessary" part is the more important one. As reader, I would implicitly assume that VACUUM is sufficient, otherwise the nice writers of the documentation would surely have told me what else I have to do to avoid that scary eventuality. I'd be OK with writing "necessary and sufficient". Or is that too much legalese? Yours, Laurenz Albe
