MVCC (Multi-Version Concurrency Control) is a mechanism used by PostgreSQL to provide concurrent access to the database while ensuring transaction isolation. It allows multiple transactions to read and write data without blocking each other.
Under MVCC, each transaction sees a snapshot of the database at a specific point in time. Whenever a transaction modifies a row in a table, PostgreSQL creates a new copy of the row instead of overwriting the existing row. This new copy is assigned a unique transaction ID, which is used to track changes made to the row.
When a transaction reads a row, it sees the copy that was valid at the time the transaction started, based on its transaction ID. This ensures that the transaction sees a consistent view of the database, without being affected by changes made by other transactions that are still in progress.
To understand how MVCC affects transaction isolation, consider the following scenario:
- Transaction A begins and reads a row with ID 1
- Transaction B begins and updates the row with ID 1
- Transaction A continues and updates the row with ID 1
Under MVCC, transaction A and B each see a snapshot of the database at a specific point in time. When transaction B updates the row with ID 1, it creates a new copy of the row with a unique transaction ID. Transaction A, however, still sees the original version of the row with its own unique transaction ID. When transaction A updates the row, it creates yet another copy with a unique transaction ID.
This means that when the transactions commit, PostgreSQL must reconcile the changes made to the row by each transaction. If the changes do not conflict, they can be merged together. If they conflict, one of the transactions must be rolled back.
MVCC in PostgreSQL helps to ensure that transactions do not block each other unnecessarily, improving performance and scalability. It also provides a high degree of transaction isolation, allowing transactions to operate in a consistent state without being affected by the changes made by other transactions.