What transaction isolation levels exist in other systems but are not implemented in PostgreSQL?

PostgreSQL implements three standard levels of SQL transaction isolation: Read Committed, Repeatable Read, and Serializable.

However, in some other DBMSs, there is an isolation level called Read Uncommitted, which allows reading uncommitted changes (dirty read). PostgreSQL does not support this level because its architecture and MVCC (multi-version concurrency control) do not allow reading uncommitted data.

There are also specific levels or extensions in other systems, such as Snapshot Isolation, which is similar to Repeatable Read but with some differences in behavior.

Thus, the main difference is the absence in PostgreSQL of the Read Uncommitted level, which exists in some other DBMSs.