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Covering indexes usually slow down modifications
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Listing 4
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Comparing covering and noncovering index performance
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SELECT [DueDate],[OrderQty] FROM [AdventureWorks].[Production].[WorkOrder] WHERE [DueDate] BETWEEN '20040630' AND '20040830' SELECT [DueDate],[OrderQty] FROM [AdventureWorks_Copy].[Production].[WorkOrder] WHERE [DueDate] BETWEEN '20040630' AND '20040830'
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As you have seen, a covered query runs faster, because It may be satisfied by a simpler plan (this is true for many but not all queries). It needs to perform fewer reads. It may need to acquire less locks (this is true for many but not all queries).
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Some rules of thumb about indexes aren t true for covering indexes
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In general, blanket statements are rarely true in database programming. In particular, some rules of thumb are true for noncovering indexes, but not true for covering indexes. For instance, the rule an index can only be used if the condition in the WHERE clause is index friendly doesn t apply to covering indexes although the following condition isn t index friendly:
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SELECT DISTINCT [LastName],[FirstName] FROM [Person].[Contact] WHERE [LastName] LIKE '%quist'
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A covering index on (LastName, FirstName) can still be used to satisfy this query, although it ll require an index scan, not an index seek, and as such will take longer and more page reads than a query that can seek. You don t even have to use the leftmost column of your covering index in your WHERE clause a covering index on (FirstName, LastName) can also be used to satisfy this select just as well. The rule that you need to have the most selective column first in your index isn t true for covering indexes, either. For instance, even though ContactID is more selective than FirstName, the following query runs faster against an index on (FirstName, ContactID) than against an index on (ContactID, FirstName):
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SELECT DISTINCT [ContactID] FROM [Person].[Contact] WHERE [FirstName] = N'Jane'
Covering indexes usually slow down modifications
Covering indexes usually slow down modifications. This is a no-brainer. All indexes usually slow down modifications, including covering indexes. The one word I feel that I need to explain is usually I added it to the title of this section automatically, because blanket statements are rarely true in database programming. In the example in listing 5, a covering index speeds up a modification.
546 Listing 5
Speeding up your queries with index covering
Update statement performance improved with a covering index
BEGIN TRANSACTION UPDATE [AdventureWorks].[Production].[WorkOrder] SET [OrderQty] = [OrderQty]+1 WHERE [DueDate] = '20040630' AND [OrderQty] = 12345678 UPDATE [AdventureWorks_Copy].[Production].[WorkOrder] SET [OrderQty] = [OrderQty]+1 WHERE [DueDate] = '20040630' AND [OrderQty] = 12345678 ROLLBACK
As you ve seen, in some rare cases a covering index may speed up an update, if it makes it easier to locate the rows being modified.
One index should cover many queries
Suppose that you have a table with a clustered index and two nonclustered indexes. Inserting one row into such a table means that at least three pages need to be modified or added (maybe more than three if there s a page split). The more nonclustered indexes you add on a table, the more pages a single-row insert must modify or insert. Similar reasoning is true for multi-row modifications, including updates and deletes their real execution costs increase as the number of indexes grows. This is why it s important to keep the number of indexes low for tables that are frequently modified. For example, consider two separate indexes on the [Person].[Contact] table, each covering a different SELECT statement as described in listing 6.
Listing 6 Creating two covering indexes
CREATE NONCLUSTERED INDEX [IX_Contact_Covering1] ON [Person].[Contact] ( [LastName], [FirstName], [EmailAddress] ) GO -- covers the following query: SELECT [LastName], [FirstName], [EmailAddress] FROM [Person].[Contact] WHERE [LastName] LIKE 'Lars%' GO CREATE NONCLUSTERED INDEX [IX_Contact_Covering2] ON [Person].[Contact] ( [LastName], [FirstName], [Phone] ) GO -- covers the following query: SELECT [LastName], [FirstName], [EmailAddress] FROM [Person].[Contact] WHERE [LastName] LIKE 'Lars%' GO
Summary
You might instead consider creating only one index, as follows:
CREATE NONCLUSTERED INDEX [IX_Contact_Covering] ON [Person].[Contact] ( [LastName], [FirstName], [Phone], [EmailAddress] )
This index will cover both selects and will slow down modifications less severely than the two previous indexes would.
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