/explain/xmax

xmax

xmax is the second half of every visibility check: the transaction id that deleted or superseded a row version. A DELETE stamps it on the current version; an UPDATE stamps it on the old version while writing the new one. It is set and never cleared: the version stays on disk, invisible to new snapshots, until vacuum removes it. A row with an empty xmax is simply alive.

The demo below needs JavaScript. Every definition, every formula and the snapshot query are plain text and are on this page already.

DEMO — toy table, 4 rowsnext xid 4711

ctidxminxmaxpayloadvisibility

(0,1)4702—'checkout'visible

(0,2)4703—'signup'visible

(0,3)4707—'refund'visible

(0,4)4710—'login'visible

LIVE / DEAD TUPLES

4 / 0

HEAP VERSIONS ON DISK

4

OPEN SNAPSHOT

none

UPDATE writes a new version and stamps the old row’s xmax. Open a snapshot first to see vacuum blocked.

SEE ALSO

xmin dead tuple ← back to start

xmax also stores lock information: SELECT … FOR UPDATE writes a locker xid (or a multixact id) into xmax without killing the row. The demo shows only the delete/supersede meaning.