Analyze and Fix Index Fragmentation in SQL Server 2008

Analyze and Fix Index Fragmentation in SQL Server 2008

It is very common that over time SQL Server tables and indexes tend to become fragmented. The fragmentation generally happens when data within the underlying tables on which an index exists is modified. The data modification basically can be an insert, update or a delete operation. The indexes over time become ineffective because they get fragmented. In this article you will see an example of how an index gets fragmented and the steps which database administrator needs to take to fix index fragmentations.

Example to Analyze and Fix Index Fragmentation in SQL Server 2008

Follow the below mentioned steps to see how an index fragmentation occurs on a table which has indexes defined on it. And finally you will see the steps which you need to take to fix index fragmentation issues.

Create AnalyzeFragmentation Database

First let us create a new database named AnalyzeFragmentation for this example. Database can be created by executing the below mentioned TSQL Query.

Use master
GO
IF EXISTS (SELECT name FROM sys.databases WHERE name = N'AnalyzeFragmentation')
DROP DATABASE [AnalyzeFragmentation]
GO
CREATE DATABASE AnalyzeFragmentation
GO

Create FindAndFixFragmentation Table in AnalyzeFragmentation Database

The next step will be to create a new table named FindAndFixFragmentation within the AnalyzeFragmentation database.


USE AnalyzeFragmentation
GO
IF OBJECT_ID (N'dbo.FindAndFixFragmentation', N'U') IS NOT NULL
DROP TABLE dbo.FindAndFixFragmentation;
GO

— Create FindAndFixFragmentation Table–
CREATE TABLE [dbo].[FindAndFixFragmentation]
(

[AddressID] [int] NOT NULL,
[AddressLine1] [nvarchar](60) NOT NULL,
[City] [nvarchar](30) NOT NULL,
[PostalCode] [nvarchar](15) NOT NULL,
[ModifiedDate] [datetime] NOT NULL,
[RowGUID] [UNIQUEIDENTIFIER] NOT NULL

)
ON [PRIMARY]

GO

Populate the FindAndFixFragmentation Table using the below TSQL code

The next step will be to populate the FindAndFixFragmentation table which you have created earlier by executing the below mentioned TSQL code. For this example we will be using the data which is available in Person.Address table available in AdventureWorks database.


USE AnalyzeFragmentation
GO

— Populate FindAndFixFragmentation table with data from AdventureWorks.Person.Address —

INSERT INTO FindAndFixFragmentation
SELECT
AddressID,
AddressLine1,
City,
PostalCode,
ModifiedDate,
RowGUID
FROM AdventureWorks.Person.Address
GO

Create a Clustered Index on FindAndFixFragmentation Table using the below TSQL code

The next step will be to create a clustered index named CL_FindAndFixFragmentation_Index on FindAndFixFragmentation table using the below mentioned TSQL code.

-- Drop the index if it is already existing--
IF EXISTS (SELECT * FROM sys.indexes WHERE object_id = OBJECT_ID(N'[dbo].[FindAndFixFragmentation]') AND name = N'CL_FindAndFixFragmentation_Index')

DROP INDEX [CL_FindAndFixFragmentation_Index] ON [dbo].[FindAndFixFragmentation]
GO
— Create Clustered Index on FindAndFixFragmentation(RowGUID) —
CREATE CLUSTERED INDEX [CL_FindAndFixFragmentation_Index] ON [dbo].[FindAndFixFragmentation]
(
[RowGUID] ASC
)
WITH (FILLFACTOR = 90) ON [PRIMARY]
GO

You can see that we are creating a clustered index on FindAndFixFragmentation table with a Fill Factor 90. The fill factor option is basically provided for fine tuning index data storage and to improve performance. Whenever an index is created or it is rebuilt, the fill factor value basically determines the percentage of space on each leaf level page that needs to be filled with data. Based on the fill factor value a percentage of free space is allocated on every single page. By default the fill factor value is 0 or 100 which means there will be no free space allocated on each leaf level page. The value for fill factor is defined in percentages and this can be any value in between 1 to 100. In this example the fill factor value provide is 90 which mean on every single page there will be a 10 percentage of free space left to accommodate future growth.

Query to Find Existing Fragmentation on FindAndFixFragmentation Table

Next step will be to execute the below mentioned TSQL query to know the existing fragmentation on FindAndFixFragmentation table. The important values which need to be noted by the database administrators are AvgPageFragmentation and PageCounts. The value for AvgPageFragmentation is 0.341296928327645, which means there is a very little fragmentation existing on the table at this point of time. However the value for PageCounts is 293, which mean the data is stored in that many data pages on SQL Server. This query will be executing many a times in this article.


-- Find index fragmentation --
SELECT
DB_NAME(DATABASE_ID) AS [DatabaseName],
OBJECT_NAME(OBJECT_ID) AS TableName,
SI.NAME AS IndexName,
INDEX_TYPE_DESC AS IndexType,
AVG_FRAGMENTATION_IN_PERCENT AS AvgPageFragmentation,
PAGE_COUNT AS PageCounts
FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL , NULL, N'LIMITED') DPS
INNER JOIN sysindexes SI
ON DPS.OBJECT_ID = SI.ID AND DPS.INDEX_ID = SI.INDID
GO

Perform Update Operation on FindAndFixFragmentation Table

Next step will be to perform updates on FindAndFixFragmentation table by executing the below mentioned TSQL code. This query will modify all the data for RowGUID column on which we have created clustered index with fill factor as 90.

— Update all the rows within to FindAndFixFragmentation table create index fragmentation —

USE AnalyzeFragmentation
GO
UPDATE FindAndFixFragmentation
SET RowGUID =NEWID()
GO

Execute the query to find existing fragmentation on FindAndFixFragmentation table as shown in the below snippet.

Now you can see that the value for AvgPageFragmentation has changed from 0.341296928327645 to 99.0049751243781, which means index is completely fragmentation. At the same time the value for PageCounts has changed from 293 to 603, which mean more number of data pages are required to store the content. Now the question which comes to your mind is how this can be fixed.

There are two methods to fix index fragmentation issues in SQL Server 2005 and higher versions. The two methods are Reorganize or Rebuild Index. The Reorganize Index is an online operation, however Rebuild Index is not an online operation until you have specified the option ONLINE=ON while performing the Rebuild. Next step will be to perform first REORGANIZE Index option and then finally perform we will perform the REBUILD and see which options is the best.

Perform Reorgainize Index Operation on Clustered Index of FindAndFixFragmentation Table

First let us perform REORGANIZE Index operation on the clustered index, and then execute the query as shown in the snippet to find the fragmentation on FindAndFixFragmentation table.


-- Reorganize [CL_FindAndFixFragmentation_Index] index on FindAndFixFragmentation --

ALTER INDEX [CL_FindAndFixFragmentation_Index] ON FindAndFixFragmentation

REORGANIZE;

GO

Once we have performed the REORGANIZE Index operation you can see that the value for AvgPageFragmentation has changed from 99.0049751243781 to 5.70469798657718, which means index fragmentation is much better that how it was earlier. And at the same time the value for PageCounts has also come down from 603 to 298, this is considerable improvement.

Perform Rebuild Index Operation on Clustered Index of FindAndFixFragmentation Table

Now let us perform REBUILD Index operation on the clustered index, when you are using the Rebuild index operation it basically drops and recreates the index. The important thing what we need to see is does this results in reducing the index fragmentation further down from 5.70469798657718. Once you have performed the Rebuild operation execute the query as shown in the snippet to check the fragmentation on FindAndFixFragmentation table.


-- Rebuild [CL_FindAndFixFragmentation_Index] index on FindAndFixFragmentation --

ALTER INDEX [CL_FindAndFixFragmentation_Index] ON FindAndFixFragmentation
REBUILD WITH (FILLFACTOR = 90, ONLINE=ON)
GO

You can see that the value for AvgPageFragmentation is back to 0.341296928327645, which means the fragmentation is same as it was when we began this exercise. And at the same time the value for PageCounts is back to 293. This proves that using REBUILD Index operation is better than REORGANIZE Index operation.

Reorganize Index

Reorganize Index uses minimal system resources and it is performed online. The biggest advantage is it does not require locks for long time therefore it does not block updates or other user queries. If the index fragmentation ranges in between 5% to 30% then it is better to perform Reorganize Index.

Rebuild Index

Rebuild Index basically drops and recreates the index; this is by far the best approach. If the index fragmentation is greater than 30% then the best strategy will be to use Rebuild Index instead of Reorganize Index.

Conclusion

Database Administrators should always make sure that fragmentation of indexes is handled on time. If the indexes are fragmented then the query response will not only be very slow; the data storage will also require more disk space. In this article you have seen an example where the clustered index gets fragmented over time and the steps which you need to perform to resolve index fragmentation issues.

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