Definition

A phantom read occurs when a transaction re-reads rows matching a condition and finds new rows that were inserted or deleted by another transaction after the initial read.

Core concept

In other words, the set of rows returned changes (some “phantom” row appears or disappears) due to concurrent inserts/deletes by others.

Use cases

Phantom reads are prevented in the Serializable isolation level.

Example

T1 runs SELECT * FROM employees WHERE dept='Sales' and gets 10 rows. T2 inserts a new Sales employee and commits. T1 runs same SELECT again and sees 11 rows. The extra row is a phantom.

References

Standard DB isolation definitions.