Dead Tuple
Dead Row
A Dead Tuple is a row in a PostgreSQL table that has been updated (UPDATE) or deleted (DELETE) but has not yet been physically removed. It remains in the data pages, occupying disk space and slowing down scans.
How it works #
When PostgreSQL executes an UPDATE, it does not overwrite the original row: it creates a new version and marks the old one as “dead.” The old row remains physically in the data page until VACUUM cleans it up. Dead tuples are the price of the MVCC model — necessary to guarantee transactional isolation.
What it’s for #
Dead tuples are a key indicator of table health. The pg_stat_user_tables view shows n_dead_tup and last_autovacuum — if dead tuples grow faster than autovacuum can clean, the table has a problem. A dead_tuple_percent above 20-30% is a warning sign.
What can go wrong #
On a table with 500,000 updates per day and autovacuum defaults (scale_factor 0.2), VACUUM triggers every 4 days. Meanwhile dead tuples accumulate, tables bloat, and queries slow down progressively — the “Monday fine, Friday disaster” pattern.