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A begins a transaction; while it is in progress, B attempts to view the changes being made by it before it has been committed.
What you’ve just seen is an example of the isolation property in action.
In order to help database administrators achieve this optimal balance between transaction isolation and security, four different isolation levels are available, ranging from very secure to insecure; these four levels are defined in terms of the three problems described above.
READ UNCOMMITTED: The least secure isolation level, this permits dirty, unrepeatable and phantom reads.
For simple transactions that expose a low risk of data corruption, a lower isolation can speed things up without any significant increase in risk; on the other hand, a high isolation level is necessary for mission-critical applications (like banking) even at the cost of some degradation in performance.
Phantom reads: A phantom read is a variant of an unrepeatable read.The ANSI specification defines three potential problems: Dirty reads: A dirty read is said to occur if a transaction can view the changes made by another transaction before they have been committed.Unrepeatable reads: An unrepeatable read is said to occur if a transaction sees different result sets from the same query while it is in progress.As noted in the first section of this article, isolation implies that the changes made by a transaction become visible only after the transaction has been committed.Thus, as the example above demonstrates, client B cannot see the transaction being executed by client A while it is in progress.