Thursday, November 28, 2013

Index Fragmentation

Fragmentation:- Fragmentation can be defined as condition where data is stored in a non continuous manner. In can be defined into two types

1. Internal Fragmentation

2. External Fragmentation

Internal Fragmentation:- In this fragmentation, there exists a space between the different records within a page. This is caused due to the Insert, delete or Update process and due to this Index takes more space than it needs to and it result in more read operation during scanning. It can also be caused due to the low value of fill factor of the page which determine how much % of the page should be used for storing the records. 

External Fragmentation:- In this fragmentation, the extents of the table is not physically stored continuously on the disk which causes the jump from one extent to another extent which takes longer time. 

Both the fragmentation can be resolved by Rebuilding or Reorganization of the indexes of the tables. 


Rebuild and  Reorganization of Indexes:- SQL Server has the ability of maintaining the indexes whenever we makes changes (update, Insert, Delete) in the tables. Over a period of time, the may causes the fragmentation on the table in which  the logical ordering based on the key value pairs does not match with the physical ordering inside the data files. This causes the degradation of the performance of the SQL Query.To solve this problem of fragmentation, we use rebuilding or reorganization of the indexes.

In case of Rebuilding, it drop the particular index and again recreate it.It removes fragmentation, reclaims the disk space by compacting the pages based on the specified or existing fill factor setting, and again reorders the index rows in those contiguous pages. We can rebuild all the indexes of the table within a single transaction by specifying the ALL with it.

ALTER INDEX [Indexname] ON [tablename] REBUILD


In case of Reorganization, it defragments the leaf level nodes of indexes by physically reordering the leaf-level pages to match the logical, left to right, order of the leaf nodes.  It uses minimal system resources and also compact the index pages

ALTER INDEX [Indexname] ON [tablename] REORGANIZE


Rebuild should be used when the fragmentation index is greater than 30% and reorganization option should be used when the fragmentation index is between 5% and 30%. Rebuilding of an Index can be done online or offline. But to achieve the availability of the index, rebuilding should be done online. Reorganization can be done online.

Find fragmentation of the database for all the indexes

SELECT ps.database_id, ps.OBJECT_ID,
FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL) AS ps
AND ps.index_id = b.index_id
WHERE ps.database_id = DB_ID()

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