What is the lowest isolation level of transactions implemented in PostgreSQL?

In PostgreSQL, the lowest isolation level is Read Uncommitted. However, in practice, PostgreSQL does not implement this level separately, and when Read Uncommitted transactions are set, it behaves as Read Committed.

The main isolation levels in PostgreSQL:

• Read Committed (default): a transaction only sees data that was committed at the start of the query.
• Repeatable Read: a transaction sees data committed at the start of the transaction, preventing non-repeatable reads.
• Serializable: the strictest level, providing full transaction isolation.

Thus, the minimally implemented level is Read Committed.