How PostgreSQL Uses MVCC to Handle Concurrent Transactions Safely

Multi-Version Concurrency Control (MVCC) is a protocol PostgreSQL uses to ensure that concurrent transactions produce results as if they had run sequentially. Instead of modifying data in place, each update creates a new row version linked to the previous one, forming a version chain per record. Transactions read data by walking this chain and selecting the version visible to them based on metadata. Each transaction carries a unique ID and a snapshot of in-flight transactions at the time of its first statement, while each row version is stamped with the ID of the transaction that created it. These mechanisms together allow PostgreSQL to maintain consistency across different isolation levels without blocking reads with locks.