What is MVCC in PostgreSQL? What problems can it cause and how to solve them?

MVCC (Multi-Version Concurrency Control) in PostgreSQL is a concurrency control mechanism that allows multiple transactions to read and modify data simultaneously without read locks. Each transaction sees its own "snapshot" of the data at the start, and changes made by other transactions become visible only after they are committed.

Problems that MVCC can cause:

• Accumulation of "dead" row versions — when rows are updated or deleted, old versions remain until VACUUM is run.
• Locks and delays — if a transaction takes a long time to complete, it can hinder the cleanup of old versions.

How to resolve:

• Regularly run VACUUM (preferably autovacuum) to remove outdated versions.
• Monitor long-running transactions and terminate them if necessary.
• Optimize transaction logic to keep transactions short and avoid holding old data snapshots.

Example: if a transaction runs for a long time, autovacuum cannot clean up old row versions, leading to table bloat and performance degradation.