Describe MySQL's multi-version concurrency control (MVCC).

MVCC allows multiple transactions to access same data simultaneously without locking.

How it works:
• Each row has multiple versions
• Transactions see consistent snapshot
• Old versions kept in undo log
• Readers don't block writers

Benefits:
• High concurrency
• Consistent reads
• No read locks needed

Used in InnoDB with REPEATABLE READ isolation level.