Tables and Indexes: Internal Structure and Access Methods - Pro SQL Server Internals - page 35

There are three different ways that SQL Server can read data from the index. The first one is by an ordered scan. Let's
run the query shown in Listing 2-6, which selects the data from the Customers table ordered by CustomerId column.
Listing 2-6. Ordered scan query
select Name
from dbo.Customers
order by CustomerId
The data on the leaf level of the index is already sorted based on CustomerId column value. As a result, SQL Server
can scan the leaf level of the index from the first to the last page and return the rows in the order they are stored.
SQL Server starts with the root page of the index and reads the first row from there. That row references the
intermediate page with the minimum key value from the table. SQL Server reads that page and repeats the process
until it finds the first page on the leaf level. Then SQL Server starts to read rows one by one, moving through the linked
list of the pages until all rows have been read. Figure 2-8 illustrates this process.
Figure 2-8. Ordered index scan
The execution plan for the query shows the Clustered Index Scan operator with the Ordered property set to true,
as shown in Figure 2-9 .