SQL Server 2012 : The Path of the Query (part 4) - Filter by 2 x NC Indexes, Filter by Ordered Composite Index

Query Path 7 — Filter by 2 x NC Indexes

A common indexing dilemma is how to index for multiple where clause criteria. Is it better to create one composite index that includes both key columns? Or do two single key column indexes perform better? Query Paths 7 through 9 evaluate the options.

The following code redefines the indexes: one index keyed on ProductID and one keyed on StartDate.

DROP INDEX Production.WorkOrder.IX_WorkOrder_ProductID;

CREATE INDEX IX_WorkOrder_ProductID
 ON Production.WorkOrder (ProductID);

CREATE INDEX IX_WorkOrder_StartDate
 ON Production.WorkOrder (StartDate);

With these indexes in place, this query filters by both key columns:

SELECT WorkOrderID, StartDate
 FROM Production.WorkOrder
 WHERE ProductID = 757
  AND StartDate = ‘01/04/2006';

To use both indexes, SQL Server uses a merge join to request rows from each index seek and then correlates the data to return the rows that meet both criteria, as shown in Figure 10. This is known as index-intersection. SQL Server makes use of both indexes via separate operations to serve the query.

Figure 10 Filtering by two indexes adds a merge join into the mix.

Examining the performance stat in Table 1, multiple indexes have a query optimizer cost of .12 and use four logical reads.

For infrequent queries, Query Path 7, with its multiple indexes, is more than adequate and much better than no index at all. However, for those few queries that run constantly, the next query path is a better solution for multiple criteria.

Query Path 8 — Filter by Ordered Composite Index

For the raw performance, the fastest solution to the multiple-where-clause-criteria problem is a single composite index as demonstrated in Query Path 8.

Creating a composite index with ProductID and StartDate as key columns sets up the test:

DROP INDEX Production.WorkOrder.IX_WorkOrder_ProductID
DROP INDEX Production.WorkOrder.IX_WorkOrder_StartDate

CREATE INDEX IX_WorkOrder_ProductID
 ON Production.WorkOrder (ProductID, StartDate);

Rerunning the same query,

SELECT WorkOrderID, StartDate
 FROM Production.WorkOrder
 WHERE ProductID = 757
  AND StartDate = ‘2006-01-04'; 

The query execution plan, as shown in Figure , is a simple single index seek operation, and it performs wonderfully.

Figure 11 Filtering two criteria using a composite index performs like greased lighting.