1. Filtered Indexes in SQL Server 2008 | TechRepublic
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Filtered Indexes in SQL Server
2008
By Tim Chapman
December 22, 2008, 8:44 AM PST
Takeaway: Filtered indexes are a neat new feature in SQL Server 2008 that allows you to
define indexes on subsets of data. In today’s article, database architect Tim Chapman shows how
you can take advantage of this useful new feature.
A filtered index is a non-clustered index created on a well-defined subset of data in a SQL Server
table object. By “well-defined”, I am talking about those sets of data that are used exclusively to
satisfy query criteria. For example, if you have a field in a table that contains predominately NULL
values, you may benefit from creating a filtered index that only contains those values that are NOT
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NULL. Note that you cannot define a clustered index with a filter.
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Filtered indexes can provider performance gains in those scenarios where a majority of queries on www.mapr.com/Free-download
a table filter on a specific subset of data. These indexes are likely going to be much smaller than Google Docs For Business
an index on the entire field, so there is less index storage involved. Also, filtered indexes are Create & Upload Images, Tables, Equations,
likely going to take less work to maintain. Because the filtered index will be smaller, data Drawings, Links & More!
manipulation operations will affect smaller portions of the index, making these operations less www.google.com/apps
costly in terms of database I/O.
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Creating a filtered index
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Let’s take a look at how to create a filtered index, and how we can see some performance benefits www.llpdynamics.ro
from its use. First, run the following script to create the SalesHistory table and populate it.
IF OBJECT_ID(’SalesHistory’, ‘U’) IS NOT NULL Keep Up with TechRepublic
DROP TABLE SalesHistory
GO
CREATE TABLE [dbo].[SalesHistory]
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(
[SaleID] [int] IDENTITY(1,1),
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[Product] [varchar](10) NULL,
Follow us however you choose!
[SaleDate] [datetime] NULL,
http://www.techrepublic.com/blog/datacenter/filtered-indexes-in-sql-server-2008/490[08/29/2012 3:48:09 PM]

2. Filtered Indexes in SQL Server 2008 | TechRepublic
[SalePrice] [money] NULL,
CONSTRAINT PK_SalesHistory_SaleID PRIMARY KEY CLUSTERED (SaleID ASC)
)
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GO
SET NOCOUNT ON
BEGIN TRANSACTION
DECLARE @i INT PHOTO GALLERY (1 of 15)
Curiosity's autonomous
SET @i = 1 'seven minutes of...
WHILE (@i =5000) More Galleries »
BEGIN
INSERT INTO [SalesHistory](Product, SaleDate, SalePrice)
VALUES (’Computer’, DATEADD(ww, @i, ‘3/11/1919′),
DATEPART(ms, GETDATE()) + (@i + 57)) VIDEO (1 of 13)
Cracking Open: HTC Titan II
INSERT INTO [SalesHistory](Product, SaleDate, SalePrice)
More Videos »
VALUES(’BigScreen’, DATEADD(ww, @i, ‘3/11/1927′),
DATEPART(ms, GETDATE()) + (@i + 13)) Hot Questions View All
INSERT INTO [SalesHistory](Product, SaleDate, SalePrice)
3 SSL redirection
VALUES(’PoolTable’, DATEADD(ww, @i, ‘3/11/1908′),
DATEPART(ms, GETDATE()) + (@i + 29)) Switching from a Job to a career in
3
the IT field: Need an IT pro's
SET @i = @i + 1 advice
END
2 windows 7 won't shutdown and
COMMIT TRANSACTION keeps switching on
GO can anyone suggest if any such
2
software exist with similar
With some records in my SalesHistory table, I am going to update the SaleDate to NULL for 6 out
functionality?
of every 7 records in the table. This will give me a pretty sparse distribution of values in the
SaleDate field. After the update I will then create a normal nonclustered index on the SaleDate
field. Ask a Question
UPDATE SalesHistory
SET SaleDate = NULL Hot Discussions View All
WHERE (SaleID % 7) 0 Should developers be sued for
221
security holes?
GO
CREATE INDEX idx_SalesHistory_SaleDate 80 The sitting duck that is open
source
ON SalesHistory(SaleDate)
27 Five fast Windows desktop search
Run the following command to run IO statistics on. This allows you to view IO values for each
utilities
TSQL command ran.
SET STATISTICS IO ON 30 Is the death knell sounding for
traditional antivirus?
The following query returns all rows from the SalesHistory table where the SaleDate contains a
http://www.techrepublic.com/blog/datacenter/filtered-indexes-in-sql-server-2008/490[08/29/2012 3:48:09 PM]

3. Filtered Indexes in SQL Server 2008 | TechRepublic
value. This query uses the idx_SalesHistory_SaleDate index we created earlier, and uses an
Index Seek operation to return 2142 rows. This query requires 8 logical reads from the database Start a Discussion
to return the necessary rows.
Note that the queries used in this article only return the SaleDate field in the resultset. The reason
for this excluding or including more fields in the SELECT list will alter the execution plan. So, for Blog Archive
the purposes of this article, I will only return the field for which I am setting criteria.
August 2012 December 2011
SELECT SaleDate July 2012 November 2011
FROM SalesHistory June 2012 October 2011
May 2012 September 2011
WHERE SaleDate IS NOT NULL
April 2012 August 2011
I can view index related data for my idx_SalesHistory_SaleDate index through querying some March 2012 July 2011
system views. February 2012 June 2011
January 2012
SELECT i.name, p.rows, i.filter_definition
FROM
sys.partitions p
JOIN sys.indexes i ON p.object_id = i.object_id AND p.index_id = i.index_id
WHERE
OBJECT_NAME(i.object_id) = ‘SalesHistory’
AND i.name = ‘idx_SalesHistory_SaleDate’
The following query is similar to the query specified above, but filters those rows where the
SaleDate does contain a NULL value. This query also does an index seek on the index I created
above, but this time requires 31 logical reads from the database as the query returns 12858
records.
SELECT SaleDate
FROM SalesHistory
WHERE SaleDate IS NULL
Now that I’ve looked a little bit how nonclustered indexes work, I’ll take a look at how you can user
fliteres when defining the nonclustered index to index only subsets of data. First, I’ll need to drop
the index I created above.
DROP INDEX SalesHistory.idx_SalesHistory_SaleDate
In the following script I create a filtered nonclustered index on the SaleDate field. This index will
contain data pointers for ONLY those records for which the SaleDate IS NOT NULL. This means
that for any records where the SaleDate IS NULL, the index will not be considered at all.
CREATE INDEX idx_SalesHistory_SaleDate
ON SalesHistory(SaleDate)
WHERE SaleDate IS NOT NULL
In the following query, an index scan of the idx_SalesHistory_SalePrice is used with 7 logical
database reads. So, even though an index scan was performed, the operation took less logical
reads due to the filtered index.
SELECT SaleDate
FROM SalesHistory
WHERE SaleDate IS NOT NULL
http://www.techrepublic.com/blog/datacenter/filtered-indexes-in-sql-server-2008/490[08/29/2012 3:48:09 PM]

4. Filtered Indexes in SQL Server 2008 | TechRepublic
I can query the same system table query as before to view the number of records contained in the
index. The previous index contained all 15,000 records from the table, whereas the current index
contains only records where the SaleDate IS NOT NULL (2142 records in this case).
SELECT i.name, p.rows, i.filter_definition
FROM
sys.partitions p
JOIN sys.indexes i ON p.object_id = i.object_id AND p.index_id = i.index_id
WHERE
OBJECT_NAME(i.object_id) = ‘SalesHistory’
AND i.name = ‘idx_SalesHistory_SaleDate’
In the following query, I look for those records where the SaleDate IS NULL. Remember that the
index I defined earlier only contains those records where the SaleDate IS NOT NULL, so it will not
be considered for this query. In fact, a clustered index scan is used to find the records where the
SaleDate IS NULL, resulting in 79 logical database reads.
SELECT SaleDate
FROM SalesHistory
WHERE SaleDate IS NULL
Conclusion
The new filtered index feature in SQL Server 2008 is a very useful new feature. It allows you to
create indexes on only subsets of frequently used data. However, these types of indexes should
be used with care. In almost all circumstances, a normal non-clustered index will be the more
useful index to use rather than a filtered on. Only after you are comfortable with the data usage
patterns of your database should you consider creating filtered indexes, otherwise you may cause
yourself
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About Tim Chapman
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