Interview Q&A / PostgreSQL

1. What is MVCC in PostgreSQL, and why don't readers block writers?

   MVCC (Multi-Version Concurrency Control) means PostgreSQL keeps several versions of a row. An UPDATE writes a new version instead of overwriting the old one, so a reader keeps seeing the version that was valid when its snapshot was taken while the writer works on the new one. Readers never wait for writers, and writers never wait for readers.

   IntermediateIn-depth4 min read
2. How does PostgreSQL implement transaction isolation levels?

   PostgreSQL offers Read Committed (the default), Repeatable Read and Serializable. Dirty reads never happen, even if you ask for Read Uncommitted. Read Committed takes a new snapshot per statement, Repeatable Read keeps one snapshot for the whole transaction, and Serializable adds checks that abort a transaction if the result could not have come from running transactions one at a time.

   AdvancedIn-depth4 min read