Index Sort Order
When defining indexes, you should consider whether the data for the index key column should be stored in ascending or descending order. Ascending is the default and maintains compatibility with earlier versions of SQL Server. The syntax of the CREATE INDEX, CREATE TABLE, and ALTER TABLE statements supports the keywords ASC (ascending) and DESC (descending) on individual columns in indexes and constraints.
Specifying the order in which key values are stored in an index is useful when queries referencing the table have ORDER BY clauses that specify different directions for the key column or columns in that index. In these cases, the index can remove the need for a SORT operator in the query plan; therefore, this makes the query more efficient. For example, the buyers in the Adventure Works Cycles purchasing department have to evaluate the quality of products they purchase from vendors. The buyers are most interested in finding products sent by these vendors with a high rejection rate. As shown in the following query, retrieving the data to meet this criteria requires the RejectedQty column in the Purchasing.PurchaseOrderDetail table to be sorted in descending order (large to small) and the ProductID column to be sorted in ascending order (small to large).
USE AdventureWorks; GO SELECT RejectedQty, ((RejectedQty/OrderQty)*100) AS RejectionRate, ProductID, DueDate FROM Purchasing.PurchaseOrderDetail ORDER BY RejectedQty DESC, ProductID ASC;
The following execution plan for this query shows that the query optimizer used a SORT operator to return the result set in the order specified by the ORDER BY clause.
If an index is created with key columns that match those in the ORDER BY clause in the query, the SORT operator can be eliminated in the query plan and the query plan is more efficient.
CREATE NONCLUSTERED INDEX IX_PurchaseOrderDetail_RejectedQty ON Purchasing.PurchaseOrderDetail (RejectedQty DESC, ProductID ASC, DueDate, OrderQty);
After the query is executed again, the following execution plan shows that the SORT operator has been eliminated and the newly created nonclustered index is used.
The Database Engine can move equally efficiently in either direction. An index defined as (RejectedQty DESC, ProductID ASC) can still be used for a query in which the sort direction of the columns in the ORDER BY clause are reversed. For example, a query with the ORDER BY clause ORDER BY RejectedQty ASC, ProductID DESC can use the index.
Sort order can be specified only for key columns. The sys.index_columns catalog view and the INDEXKEY_PROPERTY function report whether an index column is stored in ascending or descending order.