Read Uncommitted

PostgreSQL’s Read Uncommitted mode behaves like Read Committed. This is because it is the only sensible way to map the standard isolation levels to PostgreSQL’s multiversion concurrency control architecture.

SELECT query (without a FOR UPDATE/SHARE clause) sees only data committed before the query began. It never sees either uncommitted data or changes committed during query execution by concurrent transactions.
In effect, a SELECT query sees a snapshot of the database as of the instant the query begins to run.