4. Covering Your Queries
Now that you have your
clustered indexes created, think about what will happen if you want to
retrieve all of the orders in a given time frame. Let's run the
following query. Figure 5 shows the execution plan.
SELECT soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM apWriter.SalesOrderHeader soh
WHERE soh.OrderDate > '2003-12-31'
From looking at the execution plan in Figure 5,
you can see that the query optimizer decided to do a clustered index
scan to retrieve the data. That's the same as scanning the entire
table. The query optimizer decided that it was faster to scan the
entire table than to use any of the indexes defined on the table. Of
course, the only index that we've defined so far is not useful for the
query we've just executed.
We can improve things by helping the query optimizer out. Execute the code example in Listing 10-4 to create a nonclustered index on the OrderDate column. Then the listing executes the query that was ran previously. Listing 2 adds an additional query to the listing to prove another point. You should see the same execution plan as shown in Figure 6.
Example 2. SQL Script to Create Nonclustered Index and Query the Data for a Given Date Range
CREATE NONCLUSTERED INDEX ix_OrderDate ON apWriter.SalesOrderHeader(OrderDate)
SELECT soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM apWriter.SalesOrderHeader soh
WHERE soh.OrderDate > '2003-12-31'
SELECT OrderDate,soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM AdventureWorks2008.apWriter.SalesOrderHeader soh
WHERE soh.OrderDate = '2004-01-01'
|
After running the first
query, you should have noticed that the query optimizer still uses the
clustered index to retrieve the data in the date range and the
nonclustered index for an equal query. Why is that? To help answer that
question, let's examine the execution plan of the second query in Listing 3.
The query optimizer determines that it is faster to use the
nonclustered index to find the specific order date and then do a lookup
for the additional information required to fulfill the query results.
Remember, the nonclustered index only contains the nonclustered index
key and a row locator for the remainder of the data. The queries are
looking for the subtotal, the tax amount, and the total due for an
order. So when the query is looking for specific indexed data, like the
second query, the query optimizer can quickly find that information
using the nonclustered index.
On the other hand, the first
query is looking for a range of data. The query optimizer decided that
with the number of records that will be retrieved, it's faster to scan
the clustered index for the date range since no lookup is required. To
prove the cost difference between the two indexes, the query in Listing 10-5 forces the query optimizer to use the nonclustered index on the order date for the first query. Figure 10-25 shows the resulting execution plan.
Example 3. SQL Script That Forces SQL Server to Use the Nonclustered Index for the Date Range Scan
SELECT soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM apWriter.SalesOrderHeader soh with(index(ix_OrderDate))
WHERE soh.OrderDate > '2003-12-31'
SELECT soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM apWriter.SalesOrderHeader soh
WHERE soh.OrderDate > '2003-12-31'
|
As you can see, the use of the
nonclustered index included the bookmark lookups, and that plan is more
costly than just scanning the clustered index. One thing you can do to
increase the performance impact of the nonclustered index is to cover
the query completely. Listing 4 shows an example of a nonclustered index created on OrderDate that includes the subtotal, the tax amount, and the total due columns.
To show the cost
difference between using each index, the code forces the use of each of
the different indexes that we've created. That way, you can see the
difference between using a clustered index, the nonclustered index on
order date, and the nonclustered index on order date with the include
columns. Figure 8
shows the resulting execution plans. Clearly, you can see that the
nonclustered index with include columns covering the query provides the
most efficient method for accessing the data.
Example 4. SQL Script That Creates a Nonclustered Index and Uses the Different Indexes That Exist on the Table
CREATE NONCLUSTERED INDEX ix_OrderDatewInclude
ON apWriter.SalesOrderHeader(OrderDate) INCLUDE (SubTotal, TaxAmt, TotalDue)
SELECT soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM apWriter.SalesOrderHeader soh WITH (index(ix_OrderDate))
WHERE soh.OrderDate > '2003-12-31'
SELECT soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM apWriter.SalesOrderHeader soh WITH(index(ix_OrderDatewInclude))
WHERE soh.OrderDate > '2003-12-31'
SELECT soh.SalesOrderID,soh.SubTotal, soh.TaxAmt, soh.TotalDue
FROM apWriter.SalesOrderHeader soh WITH(index(ix_SalesOrderId))
WHERE soh.OrderDate > '2003-12-31'
|
