Explain the principle of Multi-Version Concurrency Control in PostgreSQL.

Multi-Version Concurrency Control (MVCC) in PostgreSQL is a concurrency control mechanism that allows multiple transactions to operate on the database simultaneously without read locks.

Working principle:

• Each transaction sees a snapshot of the data at the moment it started.
• When a row is modified, a new version with a new transaction ID is created.
• Old versions remain accessible to transactions that started earlier, providing them with a consistent view of the data.
• This allows reading data without waiting for locks, increasing performance and reducing mutual blocking.

Thus, MVCC ensures transaction isolation and efficient parallel query execution.