Обсуждение: Minor necessary/sufficient slip-up?
The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/17/routine-vacuuming.html Description: 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. Bets regards, Knut Bjarte Haus
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
On 03.09.25 09:52, Laurenz Albe wrote: > 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? I think this introductory sentence establishes the necessity only. The rest of the section and chapter establishes the sufficiency.