On Tue, 2024-02-20 at 12:57 +0530, Rajesh Kumar wrote: > On Tue, 20 Feb 2024, 12:53 Laurenz Albe, <laurenz.albe@cybertec.at> wrote: > > On Tue, 2024-02-20 at 10:02 +0530, Rajesh Kumar wrote: > > > Is autovacuum wraparound issue is when transaction id reaches > > > autovacuum_freeze_max_age or if transaction Reaches 2^32. > > > > > > My autovacuum_freeze_age setting is 200million. > > > > > > Db size is 150gb. > > > > 2^31 is the magical number at which very old transactions become > > transactions from the future, which would cause data loss unless > > all old rows have been frozen. > > Sorry, I could not understand. If possible, please elaborate. Otherwise, no issues
The (32-bit) transaction numbers are stored in "xmin" and "xmax" in each row. You have to imagine these numbers as a closed ring: if you get past 2^32-1, you wrap around to 0 and continue.
Now the 2^31 numbers before the current 32-bit transaction id are the past, and the 2^31 numbers after it are the future. This includes wraparound, so initially numbers close to 2^32 will also be past.
As the current transaction id crosses 2^31, very old transactions suddenly become future transactions. All rows with a low "xmin" will suddenly appear to have been created in the future and become invisible. Some rows with a low "xmax" that have been deleted long ago can suddenly rise from the dead, since the transaction that deleted them now appears to be in the future.
