The vacuum lag
One transaction has been open for an hour and nobody noticed. Why is the table three times the size of its data?
Every update leaves the old row behind, so a sweeper comes along later and clears out the versions nobody can still see. One slider sets how far back the oldest open reader sits. The sweeper cannot clear anything that reader might still need. Watch the pinned versions pile up and the table grow.
Board: MVCC & Vacuum →Twenty transactions, six rows. Step through them yourself. Nothing plays automatically.
try this Press Commit next transaction to the end, then drag How long the oldest reader has been sitting there right and watch The bill.
Included with a plan
This lab is included with a plan. The first three lessons of every track are free, with their boards and videos. A plan unlocks every lesson, board, video, lab and paper for one price.
see also A Critique of ANSI SQL Isolation Levels in Papers.