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Autovacuum thresholds: the dead-tuple trigger formula and what drives it

Simple terms

Autovacuum is on. Your biggest table is still bloating. Both of those are true at the same time, and that is the part that trips people up. The trigger isn't a fixed number of dead rows, it's a share of the table, twenty percent by default. On a small table that's nothing. On a billion-row table you are waiting for two hundred million dead rows before anything happens. Lower that share on the tables that churn and they finally get cleaned when you expected them to.

You might be asked

Explain exactly when autovacuum decides to vacuum a table. What is the threshold formula, which counters feed it, and why can a high-churn table still bloat even though autovacuum is enabled?

Topicvacuum / autovacuum / MVCC maintenance
PostgreSQL14, 15, 16, 17, 18
Tools usedDocker pgi17 (PostgreSQL 17.10), pg_stat_user_tables, pg_class.reltuples, ALTER SYSTEM autovacuum_naptime, source-cache @REL_17_10 (autovacuum.c)
Last reviewed2026-06-19

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