Database Isolation Level

Isolation LevelDirty ReadNonrepeatable ReadPhantom ReadSerialization Anomaly
Read uncommittedAllowed, but not in PGPossiblePossiblePossible
Read committedNot possiblePossiblePossiblePossible
Repeatable readNot possibleNot possibleAllowed, but not in PGPossible
SerializableNot possibleNot possibleNot possibleNot possible

Read Uncommitted

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.