freeze

Transaction ids are 32-bit and wrap. A row whose xmin falls more than 2^31 transactions behind the current xid would appear to be in the future, so Postgres must mark old rows frozen (permanently visible, their xmin no longer compared) before that happens. Freezing is vacuum’s second job, and the quiet one: it happens per page, it dirties the page, and its schedule is controlled by three age settings that only make sense together.