What is MVCC in PostgreSQL and how does it affect the performance of SELECT after a large number of UPDATE/INSERT/DELETE operations?

MVCC (Multiversion Concurrency Control) in PostgreSQL is a mechanism for managing concurrent access to data, allowing transactions to see a consistent state of the database without read locks.

How it works:

• When data is modified (INSERT, UPDATE, DELETE), new versions of rows are created, and old versions remain accessible to other transactions that may read them.
• SELECT only reads the versions of rows that were current at the start of the transaction.

Impact on SELECT performance after many changes:

• Due to the accumulation of old row versions (called "dead" rows), tables and indexes can grow, slowing down scans.
• The VACUUM process is used to clean up old versions, freeing space and improving performance.

If VACUUM is not run regularly, SELECTs may become slower due to the large number of unnecessary row versions.

Summary:

• MVCC provides high concurrency and no read locks.
• But it requires regular maintenance (VACUUM) to maintain performance.

Example: after a series of UPDATE/DELETEs, the number of "dead" rows in a table grows, and SELECT will read more data, slowing down queries.