What is the default isolation level in PostgreSQL?

In PostgreSQL, the default transaction isolation level is Read Committed. This means that each command within a transaction only sees changes that have been committed by other transactions at the moment the command starts. Thus, a transaction does not see uncommitted changes from other transactions but can see changes committed after the transaction itself has started.

This provides a balance between performance and data consistency, preventing dirty reads but allowing non-repeatable reads and phantom reads.