PostgreSQL 10 High Performance - Third Edition

Transaction ID wraparound

The implementation of MVCC in PostgreSQL uses a transaction ID that is 32 bits in size. It's impractical to make it any longer because as you've just seen, visibility information is stored in each row by as xmin and xmax values. Having a larger ID would therefore increase the size of each row by a significant amount. A signed 32-bit number can only handle a range of about two billion transactions before rolling over to zero. When it exceeds its range, transactions that used to appear in the past will now appear to be from the future, which as you might imagine will wreak havoc. If this transaction wraparound ever happens to your database, it will fail to operate sanely, and therefore the database will go far out ...