Dirty Read isolation

The Dirty Read isolation (or ANSI Read Uncommitted) level does not place any locks on any rows fetched during a SELECT statement. Dirty Read isolation is appropriate for static tables that are used for queries.

Use Dirty Read isolation with care if update activity occurs at the same time. With Dirty Read, the reader can read a row that has not been committed to the database and might be eliminated or changed during a rollback. For example, consider the following scenario: 
User 1 starts a transaction.
User 1 inserts row A.
User 2 reads row A.
User 1 rolls back row A.

User 2 reads row A, which user 1 rolls back seconds later. In effect, user 2 read a row that was never committed to the database. Uncommitted data that is rolled back can be a problem in applications.

Because the database server does not check or place any locks for queries, Dirty Read isolation offers the best performance of all isolation levels. However, because of potential problems with uncommitted data that is rolled back, use Dirty Read isolation with care.

Because problems with uncommitted data that is rolled back are an issue only with transactions, databases that do not have transaction (and hence do not allow transactions) use Dirty Read as a default isolation level. In fact, Dirty Read is the only isolation level allowed for databases that do not have transaction logging.