What are the isolation levels in PostgreSQL? Is Read Uncommitted available?

PostgreSQL supports the following transaction isolation levels:

• Read Committed (default level) — a transaction sees only the changes that were committed before the current query started. This prevents "dirty" reads but allows non-repeatable reads and phantom reads.

• Repeatable Read — a transaction sees all data as it was at the start of the transaction. This prevents non-repeatable reads, but phantom reads can occur in some cases.

• Serializable — the strictest level, providing full isolation, with transactions executed as if they were sequential.

There is no Read Uncommitted level in PostgreSQL. Although the SQL standard defines this level, PostgreSQL effectively implements Read Uncommitted as Read Committed, meaning it does not allow reading uncommitted data (dirty reads).