Answer
Each transaction gets a unique XID. Because XIDs are 32-bit, they eventually wrap around, risking marking live rows as expired. VACUUM prevents wraparound by freezing tuples.
Quick Answer
Transaction ID (XID) is a 32-bit integer assigned to every transaction. PostgreSQL uses XIDs to determine tuple visibility in MVCC. Wraparound danger: after ~2 billion transactions, XID wraps around to 0. Old rows could suddenly appear to be "in the future" and become invisible - catastrophic data loss. Autovacuum prevents this by freezing old tuples (replacing real XIDs with FrozenXID that's always visible).
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SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ