Editing PostgreSQL (section)

== Multiversion concurrency control (MVCC) ==
PostgreSQL manages [[concurrency control|concurrency]] through [[multiversion concurrency control]] (MVCC), which gives each transaction a "snapshot" of the database, allowing changes to be made without affecting other transactions. This largely eliminates the need for read locks, and ensures the database maintains [[ACID (computer science)|ACID]] principles. PostgreSQL offers four levels of [[isolation (database systems)|transaction isolation]]: Read Uncommitted, Read Committed, Repeatable Read and Serializable. Because PostgreSQL is immune to dirty reads, requesting a Read Uncommitted transaction isolation level provides read committed instead. PostgreSQL supports full [[serializability]] via the serializable [[snapshot isolation]] (SSI) method.<ref name="ports" /> The PostgreSQL MVCC implementation is prone to performance issues that require tuning when under a heavy write load which updates existing rows.<ref>{{Cite web|url=https://ottertune.com/blog/the-part-of-postgresql-we-hate-the-most|title=The part of PostgreSQL we hate the most |website= OtterTune|type= blog | author1= Bohan Zhang |author2= Andy Pavlo| date=2023}}</ref>