8.6 THE ANOMALY TABLE — the single most useful table in the book

Isolation level	Dirty reads	Read skew	Phantom reads	Lost updates	Write skew
Read uncommitted	✗ Possible	✗ Possible	✗ Possible	✗ Possible	✗ Possible
Read committed	✓ Prevented	✗ Possible	✗ Possible	✗ Possible	✗ Possible
Snapshot isolation	✓ Prevented	✓ Prevented	✓ Prevented	? Depends	✗ Possible
Serializable	✓ Prevented	✓ Prevented	✓ Prevented	✓ Prevented	✓ Prevented

(Dirty writes are omitted because almost all transaction implementations prevent them.)

The six anomalies in one line each:

Anomaly	Definition
Dirty read	One client reads another client's writes before they have been committed
Dirty write	One client overwrites data another client has written but not yet committed
Read skew (nonrepeatable read)	A client sees different parts of the database at different points in time
Phantom read	A transaction reads objects matching a search condition; another client makes a write that affects the results of that search. SI prevents straightforward phantoms; phantoms in the context of write skew require special treatment such as index-range locks
Lost update	Two clients concurrently perform a read-modify-write cycle; one overwrites the other's write without incorporating its changes
Write skew	A transaction reads something, decides based on what it saw, and writes the decision — but by the time the write is made, THE PREMISE OF THE DECISION IS NO LONGER TRUE. Only serializable isolation prevents this