/explain/dead-tuple

dead tuple

A dead tuple is a row version whose xmax is committed and older than every live snapshot: nobody can see it anymore, but it still occupies its bytes on the page and its entries in every index. Vacuum exists to find these and free the space for reuse. n_dead_tup in pg_stat_user_tables is the running estimate, and the trigger formula compares exactly this number against the threshold.

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 bloat autovacuum_vacuum_scale_factor ← back to start

The estimate comes from the statistics collector, not a count: it drifts between analyzes and resets on crash. Treat it as a signal, not an inventory.