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Lesson 2: Creating Indexes
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-- Rebuild the table's clustered index. ALTER INDEX ALL ON Test.OtherTable REBUILD;
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-- Create table containing the rows used to perform the inserts. CREATE TABLE Test.OtherTable ( PKCol INT IDENTITY(100000,4) NOT NULL PRIMARY KEY ,OtherCol INT NOT NULL );
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INSERT Test.OtherTable (OtherCol) SELECT Col1 FROM Test.IncludedColumnsTest WHERE Col1 BETWEEN 1 AND 10000;
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The following is the first test, without any nonclustered indexes defined on the table. The execution plan for this INSERT statement is shown in Figure 6-8:
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INSERT Test.IndexInsertTest (Col1) SELECT PKCol FROM Test.OtherTable;
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FiGURe 6-8 The actual execution plan from SSMS of the INSERT statement used in the first test
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The estimated query cost for the INSERT statement in this test was 0.88, and SQL Server touched 32,085 pages in the Test.IndexInsertTest table while performing the INSERTs. The following is the second test, with one nonclustered index defined on the table. The execution plan for this INSERT statement is shown in Figure 6-9:
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-- 1. Drop and re-create the Test.IndexInsertTest table. -- 2. Add one non-clustered index. CREATE NONCLUSTERED INDEX NCIdx1 ON Test.IndexInsertTest (Col1);
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-- 3. Execute the insert statement. INSERT Test.IndexInsertTest (Col1) SELECT PKCol FROM Test.OtherTable;
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Techniques to Improve Query Performance
FiGURe 6-9 The actual execution plan from SSMS of the INSERT statement used in the second test
The estimated query cost for the INSERT statement in this test was 1.58, and SQL Server touched 64,902 pages in the Test.IndexInsertTest table while performing the INSERTs. This is roughly twice the cost and twice the number of pages compared with Test 1. The following is the third test, with five nonclustered indexes defined on the table. The execution plan for this INSERT statement is shown in Figure 6-10:
-- 1. Drop and recreate the Test.IndexInsertTest table. -- 2. Add five non-clustered indexes. CREATE NONCLUSTERED INDEX NCIdx1 ON Test.IndexInsertTest (Col1); CREATE NONCLUSTERED INDEX NCIdx2 ON Test.IndexInsertTest (Col1); CREATE NONCLUSTERED INDEX NCIdx3 ON Test.IndexInsertTest (Col1); CREATE NONCLUSTERED INDEX NCIdx4 ON Test.IndexInsertTest (Col1); CREATE NONCLUSTERED INDEX NCIdx5 ON Test.IndexInsertTest (Col1);
-- 3. Execute the insert statement. INSERT Test.IndexInsertTest (Col1) SELECT PKCol FROM Test.OtherTable;
This time, the estimated query cost for the INSERT statement was 5.04 and SQL Server handled a staggering 196,170 pages in the Test.IndexInsertTest table while performing the INSERTs. As you can see, the cost for performing the INSERTs is roughly doubled with each new nonclustered index. However, in this case, each nonclustered index is roughly the same width as the table itself. For typical tables, the nonclustered indexes are narrower than the table and do not hurt performance (percentage-wise) to the same degree as in this test. Because the ratio between read and write operations varies greatly between systems (and even tables), it is typically a good idea to create indexes to optimize read performance and then test the effect that the created indexes have on write performance. So long as the write
Lesson 2: Creating Indexes
FiGURe 6-10 The actual execution plan from SSMS of the INSERT statement used in the third test
performance is acceptable (and you have enough disk space to manage the created indexes), you can keep the created indexes. It is typically also advisable to run such a test every so often to verify that the read versus write ratio for the table hasn t changed. You should also note that both the UPDATE and DELETE statements benefit from certain indexes to locate the rows in the table that they need to update or delete.
Techniques to Improve Query Performance
Using Computed Columns
A computed column is generally derived from other columns in the same table and can reference both system- and user-defined functions in its definition. To be able to create an index on a computed column, it must adhere to a few requirements, which you can find in SQL Server Books Online under the topic Creating Indexes on Computed Columns, at http://msdn.microsoft.com/en-us/library/ms189292.aspx. By defining a computed column and indexing it, it is possible to make queries that would typically require an index or table scan to instead use a seek operation. Consider the following query for sales orders in the AdventureWorks database. The query s execution plan is shown in Figure 6-11:
USE AdventureWorks;
-- First create an index on the OrderDate column -- to support this query. CREATE NONCLUSTERED INDEX OrderDateIndex ON Sales.SalesOrderHeader (OrderDate); GO
SET STATISTICS IO ON;
SELECT COUNT(*) FROM Sales.SalesOrderHeader WHERE MONTH(OrderDate) = 5;
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