Isolation Level
A transaction-consistency policy that determines which effects of concurrent transactions may become visible and which anomalies are permitted.
An isolation level does not mean that a database literally executes every concurrent transaction in serial order. It defines which observations are allowed across overlapping transactions and which anomaly classes are prevented. Actual behavior depends on the database's locking and MVCC implementation.
Not Merely a Performance Knob
Lower isolation should not be read as simply faster, nor higher isolation as automatically correct. The required level follows from the business invariant and the observations on which that invariant depends. If the application relies on a guarantee the selected level does not provide, write skew or other concurrency anomalies can remain possible.
Related Concepts
Related: Concurrency in Distributed Data Systems, Oracle Database and PL/SQL.