This topic describes how to reorganize or rebuild a fragmented index in SQL Server 2012 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.
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)*
* 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.
alter index <index name> on <tablename> Rebuild/reorganize
