Reorganize and Rebuild Indexes

Article
06/13/2017

This topic describes how to reorganize or rebuild a fragmented index in SQL Server 2014 by using SQL Server Management Studio or Transact-SQL. The SQL Server Database Engine automatically maintains indexes whenever insert, update, or delete operations are made to the underlying data. Over time these modifications can cause the information in the index to become scattered in the database (fragmented). Fragmentation exists when indexes have pages in which the logical ordering, based on the key value, does not match the physical ordering inside the data file. Heavily fragmented indexes can degrade query performance and cause your application to respond slowly.

You can remedy index fragmentation by reorganizing or rebuilding an index. For partitioned indexes built on a partition scheme, you can use either of these methods on a complete index or a single partition of an index. Rebuilding an index drops and re-creates the index. This removes fragmentation, reclaims disk space by compacting the pages based on the specified or existing fill factor setting, and reorders the index rows in contiguous pages. When ALL is specified, all indexes on the table are dropped and rebuilt in a single transaction. Reorganizing an index uses minimal system resources. It defragments the leaf level of clustered and nonclustered indexes on tables and views by physically reordering the leaf-level pages to match the logical, left to right, order of the leaf nodes. Reorganizing also compacts the index pages. Compaction is based on the existing fill factor value.

In This Topic

Before you begin:

Detecting Fragmentation

Limitations and Restrictions

Security
To check the fragmentation of an index, using:

SQL Server Management Studio

Transact-SQL
To reorganize or rebuild an index, using:

SQL Server Management Studio

Transact-SQL

Before You Begin

Detecting Fragmentation

The first step in deciding which defragmentation method to use is to analyze the index to determine the degree of fragmentation. By using the system function sys.dm_db_index_physical_stats, you can detect fragmentation in a specific index, all indexes on a table or indexed view, all indexes in a database, or all indexes in all databases. For partitioned indexes, sys.dm_db_index_physical_stats also provides fragmentation information for each partition.

The result set returned by the sys.dm_db_index_physical_stats function includes the following columns.

Column	Description
avg_fragmentation_in_percent	The percent of logical fragmentation (out-of-order pages in the index).
fragment_count	The number of fragments (physically consecutive leaf pages) in the index.
avg_fragment_size_in_pages	Average number of pages in one fragment in an index.

After the degree of fragmentation is known, use the following table to determine the best method to correct the fragmentation.

avg_fragmentation_in_percent value	Corrective statement
> 5% and < = 30%	ALTER INDEX REORGANIZE
> 30%	ALTER INDEX REBUILD WITH (ONLINE = ON) ¹

¹ Rebuilding an index can be executed online or offline. Reorganizing an index is always executed online. To achieve availability similar to the reorganize option, you should rebuild indexes online.

Tip

These values provide a rough guideline for determining the point at which you should switch between ALTER INDEX REORGANIZE and ALTER INDEX REBUILD. However, the actual values may vary from case to case. It is important that you experiment to determine the best threshold for your environment. For example, if a given index is used mainly for scan operations, removing fragmentation can improve performance of these operations. The performance benefit is less noticeable for indexes that are used primarily for seek operations. Similarly, removing fragmentation in a heap (a table with no clustered index) is especially useful for nonclustered index scan operations, but has little effect in lookup operations.

Very low levels of fragmentation (less than 5 percent) should typically not be addressed by either of these commands, because the benefit from removing such a small amount of fragmentation is almost always vastly outweighed by the cost of reorganizing or rebuilding the index.

Note

Rebuilding or reorganizing small indexes often does not reduce fragmentation. The pages of small indexes are sometimes stored on mixed extents. Mixed extents are shared by up to eight objects, so the fragmentation in a small index might not be reduced after reorganizing or rebuilding it.

Index defragmentation considerations

Under certain conditions, rebuilding a clustered index will automatically rebuild any nonclustered index that reference the clustering key, if the physical or logical identifiers contained in the nonclustered index records needs to change.

Scenarios that force all nonclustered indexes to be automatically rebuilt on a table:

Creating a clustered index on a table
Removing a clustered index, causing the table to be stored as a heap
Changing the clustering key to include or exclude columns

Scenarios that do not require all nonclustered indexes to be automatically rebuilt on a table:

Rebuilding a unique clustered index
Rebuilding a non-unique clustered index
Changing the index schema, such as applying a partitioning scheme to a clustered index or moving the clustered index to a different filegroup

Limitations and Restrictions

Indexes with more than 128 extents are rebuilt in two separate phases: logical and physical. In the logical phase, the existing allocation units used by the index are marked for deallocation, the data rows are copied and sorted, then moved to new allocation units created to store the rebuilt index. In the physical phase, the allocation units previously marked for deallocation are physically dropped in short transactions that happen in the background, and do not require many locks. For more information about extents, refer to the Pages and Extents Architecture Guide.

The ALTER INDEX REORGANIZE statement requires the data file containing the index to have space available, because the operation can only allocate temporary work pages on the same file, not another file within the filegroup. So although the filegroup might have free pages available, the user can still encounter error 1105: Could not allocate space for object '###' in database '###' because the '###' filegroup is full. Create disk space by deleting unneeded files, dropping objects in the filegroup, adding additional files to the filegroup, or setting autogrowth on for existing files in the filegroup.

Creating and rebuilding non-aligned indexes on a table with more than 1,000 partitions is possible, but is not recommended. Doing so may cause degraded performance or excessive memory consumption during these operations.

An index cannot be reorganized or rebuilt if the filegroup in which it is located is offline or set to read-only. When the keyword ALL is specified and one or more indexes are in an offline or read-only filegroup, the statement fails.

Security

Permissions

Requires ALTER permission on the table or view. User must be a member of the sysadmin fixed server role or the db_ddladmin and db_owner fixed database roles.

Using SQL Server Management Studio

To check the fragmentation of an index

In Object Explorer, Expand the database that contains the table on which you want to check an index's fragmentation.
Expand the Tables folder.
Expand the table on which you want to check an index's fragmentation.
Expand the Indexes folder.
Right-click the index of which you want to check the fragmentation and select Properties.
Under Select a page, select Fragmentation.

The following information is available on the Fragmentation page:

Page fullness
Indicates average fullness of the index pages, as a percentage. 100% means the index pages are completely full. 50% means that, on average, each index page is half full.

Total fragmentation
The logical fragmentation percentage. This indicates the number of pages in an index that are not stored in order.

Average row size
The average size of a leaf level row.

Depth
The number of levels in the index, including the leaf level.

Forwarded records
The number of records in a heap that have forward pointers to another data location. (This state occurs during an update, when there is not enough room to store the new row in the original location.)

Ghost rows
The number of rows that are marked as deleted but not yet removed. These rows will be removed by a clean-up thread, when the server is not busy. This value does not include rows that are being retained due to an outstanding snapshot isolation transaction.

Index type
The type of index. Possible values are Clustered index, Nonclustered index, and Primary XML. Tables can also be stored as a heap (without indexes), but then this Index Properties page cannot be opened.

Leaf-level rows
The number of leaf level rows.

Maximum row size
The maximum leaf-level row size.

Minimum row size
The minimum leaf-level row size.

Pages
The total number of data pages.

Partition ID
The partition ID of the b-tree containing the index.

Version ghost rows
The number of ghost records that are being retained due to an outstanding snapshot isolation transaction.

Using Transact-SQL

To check the fragmentation of an index

In Object Explorer, connect to an instance of Database Engine.
On the Standard bar, click New Query.

Copy and paste the following example into the query window and click Execute.

USE AdventureWorks2012;  
GO  
-- Find the average fragmentation percentage of all indexes  
-- in the HumanResources.Employee table.   
SELECT a.index_id, name, avg_fragmentation_in_percent  
FROM sys.dm_db_index_physical_stats (DB_ID(N'AdventureWorks2012'), OBJECT_ID(N'HumanResources.Employee'), NULL, NULL, NULL) AS a  
    JOIN sys.indexes AS b ON a.object_id = b.object_id AND a.index_id = b.index_id;   
GO

The statement above might return a result set similar to the following.

index_id    name                                                  avg_fragmentation_in_percent  
----------- ----------------------------------------------------- ----------------------------  
1           PK_Employee_BusinessEntityID                          0  
2           IX_Employee_OrganizationalNode                        0  
3           IX_Employee_OrganizationalLevel_OrganizationalNode    0  
5           AK_Employee_LoginID                                   66.6666666666667  
6           AK_Employee_NationalIDNumber                          50  
7           AK_Employee_rowguid                                   0  

(6 row(s) affected)

For more information, see sys.dm_db_index_physical_stats (Transact-SQL).

Using SQL Server Management Studio

To reorganize or rebuild an index

In Object Explorer, Expand the database that contains the table on which you want to reorganize an index.
Expand the Tables folder.
Expand the table on which you want to reorganize an index.
Expand the Indexes folder.
Right-click the index you want to reorganize and select Reorganize.
In the Reorganize Indexes dialog box, verify that the correct index is in the Indexes to be reorganized grid and click OK.
Select the Compact large object column data check box to specify that all pages that contain large object (LOB) data are also compacted.
Click OK.

To reorganize all indexes in a table

In Object Explorer, Expand the database that contains the table on which you want to reorganize the indexes.
Expand the Tables folder.
Expand the table on which you want to reorganize the indexes.
Right-click the Indexes folder and select Reorganize All.
In the Reorganize Indexes dialog box, verify that the correct indexes are in the Indexes to be reorganized. To remove an index from the Indexes to be reorganized grid, select the index and then press the Delete key.
Select the Compact large object column data check box to specify that all pages that contain large object (LOB) data are also compacted.
Click OK.

To rebuild an index

In Object Explorer, Expand the database that contains the table on which you want to reorganize an index.
Expand the Tables folder.
Expand the table on which you want to reorganize an index.
Expand the Indexes folder.
Right-click the index you want to reorganize and select Reorganize.
In the Rebuild Indexes dialog box, verify that the correct index is in the Indexes to be rebuilt grid and click OK.
Select the Compact large object column data check box to specify that all pages that contain large object (LOB) data are also compacted.
Click OK.

Using Transact-SQL

To reorganize a defragmented index

In Object Explorer, connect to an instance of Database Engine.
On the Standard bar, click New Query.

Copy and paste the following example into the query window and click Execute.

USE AdventureWorks2012;   
GO  
-- Reorganize the IX_Employee_OrganizationalLevel_OrganizationalNode index on the HumanResources.Employee table.   

ALTER INDEX IX_Employee_OrganizationalLevel_OrganizationalNode ON HumanResources.Employee  
REORGANIZE ;   
GO

To reorganize all indexes in a table

In Object Explorer, connect to an instance of Database Engine.
On the Standard bar, click New Query.

Copy and paste the following example into the query window and click Execute.

USE AdventureWorks2012;   
GO  
-- Reorganize all indexes on the HumanResources.Employee table.  
ALTER INDEX ALL ON HumanResources.Employee  
REORGANIZE ;   
GO

To rebuild a defragmented index

In Object Explorer, connect to an instance of Database Engine.
On the Standard bar, click New Query.
Copy and paste the following example into the query window and click Execute. The example rebuilds a single index on the Employee table.
```
USE AdventureWorks2012;
GO
ALTER INDEX PK_Employee_BusinessEntityID ON HumanResources.Employee
REBUILD;
GO
```

To rebuild all indexes in a table

In Object Explorer, connect to an instance of Database Engine.
On the Standard bar, click New Query.
Copy and paste the following example into the query The example specifies the keyword ALL. This rebuilds all indexes associated with the table. Three options are specified.
```
USE AdventureWorks2012;
GO
ALTER INDEX ALL ON Production.Product
REBUILD WITH (FILLFACTOR = 80, SORT_IN_TEMPDB = ON,
              STATISTICS_NORECOMPUTE = ON);
GO
```

For more information, see ALTER INDEX (Transact-SQL).

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Reorganize and Rebuild Indexes

Before You Begin

Detecting Fragmentation

Index defragmentation considerations

Limitations and Restrictions

Security

Permissions

Using SQL Server Management Studio

To check the fragmentation of an index

Using Transact-SQL

To check the fragmentation of an index

Using SQL Server Management Studio

To reorganize or rebuild an index

To reorganize all indexes in a table

To rebuild an index

Using Transact-SQL

To reorganize a defragmented index

To reorganize all indexes in a table

To rebuild a defragmented index

To rebuild all indexes in a table

See Also

Additional resources