On top of read committed and repeatable read, PostgreSQL offers Serializable Snapshot Isolation (SSI) transactions. So, overall, PostgreSQL supports three isolation levels. Note that read uncommitted (which still happens to be the default in some commercial databases) is not supported: if you try to start a read uncommitted transaction, PostgreSQL will silently map to read committed. Let's get back to the serializable isolation level.

The idea behind serializable isolation is simple; if a transaction is known to work correctly when there is only a single user, it will also work in the case of concurrency when this isolation level is chosen. However, users have to be prepared; transactions may fail (by design) and error out. In addition to this, a performance penalty has to be paid.