Definition

A dirty read is when a transaction reads data written by another uncommitted transaction. If that other transaction rolls back, the reading transaction got invalid data.

Core concept

Dirty reads violate isolation. Isolation levels above READ UNCOMMITTED prevent dirty reads.

Use cases

Rarely acceptable. Sometimes non-critical data analytics might allow it for speed.

Example

T1 does UPDATE accounts SET balance=100 WHERE id=1; but hasn’t committed. T2 reads balance=100 (dirty). If T1 aborts, T2 read was invalid.

References

Standard transaction isolation concepts.