How does Multi-Version Concurrency Control (MVCC) work?

MVCC maintains multiple versions of data to allow concurrent reads without blocking writes. Each transaction sees a snapshot at its start time. PostgreSQL: old versions kept in same table (requires VACUUM cleanup). MySQL InnoDB: undo logs store old versions, rolled back after oldest transaction completes. Versions tracked by transaction IDs. Readers never block writers and vice versa. Trade-off: storage overhead, cleanup complexity. Enables READ COMMITTED and SNAPSHOT isolation efficiently.