What is the role of Multi-Version Concurrency Control in the PostgreSQL database management system?

Multi-Version Concurrency Control (MVCC) in PostgreSQL provides high concurrency of transactions without read locks. Instead of locking rows during reads, MVCC keeps multiple versions of data, allowing transactions to see a consistent state of the database at the start of their work. This is achieved by storing old versions of rows with indicators of which transactions created or deleted them.

Thus, readers do not block writers and vice versa, which increases performance and reduces conflicts. PostgreSQL periodically cleans up outdated versions (vacuum) to prevent accumulation of garbage.

Example: if one transaction updates a row, another transaction that started before this update will see the old version of the row, while the new transaction will see the changed version.