SQL Server uses an allocation order scan for a clustered table if a query does not request

any specific order, if the isolation level is Read Uncommitted, or if you are working in a read-

only environment. When SQL Server scans a clustered index, it can also scan in the logical

order of the index by using the index order scan. In each of these cases, the Clustered Index

Scan iterator is used. SQL Server uses the index leaf–level’s linked list to perform an index

order scan. Index order scans are affected negatively by both logical and physical fragmenta-

tion. The following query does not request any specific ordered result; you can see that the

Clustered Index Scan operator’s Ordered property is False, meaning that SQL Server didn’t

have to return the data ordered, as Figure 17-2 shows.