Vacuum & wraparoundsenior🔒 Pro concept

Tuple freezing & transaction-ID wraparound

Simple terms

Transaction ids are 32-bit counters. Eventually they run out and wrap around, and when they do, transactions from long ago start looking like they came from the future. Rows that were perfectly visible just vanish. Freezing is the defense: VACUUM stamps old rows as older than everything, permanently visible, immune to the counter rolling over. It is also the one job Postgres will not let you skip. Let a table's ids drift old enough and anti-wraparound autovacuum comes for it whether you switched autovacuum off or not.

You might be asked

What does VACUUM FREEZE actually change on a tuple, and how does freezing prevent transaction-ID wraparound from making old rows disappear? Does it overwrite t_xmin?

TopicHeap storage / VACUUM / MVCC xid space
PostgreSQL14, 15, 16, 17, 18
Tools usedpageinspect (heap_page_items / get_raw_page), pg_visibility (pg_visibility_map_summary, pg_visibility), VACUUM (FREEZE, VERBOSE), pg_class.relfrozenxid + age(), source navigation @REL_17_10
Last reviewed2026-06-19

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