Why is there no read uncommitted in PostgreSQL?

PostgreSQL does not have the Read Uncommitted transaction isolation level because it is effectively equivalent to Read Committed in terms of data visibility. Read Uncommitted allows reading "dirty" data (uncommitted changes), which can lead to unpredictable results and integrity issues.

PostgreSQL uses MVCC (Multi-Version Concurrency Control), which provides a consistent snapshot of data for each transaction. This guarantees that a transaction only sees committed data, excluding dirty reads. Therefore, the Read Uncommitted level is unnecessary and not implemented, with the minimum level being Read Committed, which already excludes dirty reads.