Birth stamp, horizon flag, pin rod, vacuum broomLive version cards on the page bed (xmin)Dead cards waiting past the horizon gate (xmax)Row versions are stamped metal cards on a page bench. The birth press stamps xmin, the death punch marks xmax, the snapshot camera decides who can see a card, and the horizon gate blocks the vacuum sweeper until the oldest live reader advances. Pin the horizon (long REPEATABLE READ, idle logical slot, or hot_standby_feedback) and the dead bin piles up — dequeue scan cost rises with every uncleared dead card.
This is a simulation. Times and counts are fixed model costs, not live Postgres measurements.