How does MVCC work in PostgreSQL? Why is versioning needed?

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

How it works:

• When data is modified, a new version of the row is created with a new transaction ID (XID).
• Transactions see only the versions of rows that were committed before their start, ensuring read consistency.
• Old versions of rows are kept until they are no longer needed (removed by the VACUUM process).

Why versioning is needed:

• It enables isolation levels like "Repeatable Read" and "Serializable" without locks.
• It reduces contention between transactions, increasing performance.
• It provides consistent data reading at the start of a transaction.

Example: if one transaction reads data and another modifies it concurrently, the first will see the old version, and the second — the new one, preventing conflicts and locks.