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SQL Server FAQ - What Is Index Fragmentation

By: FYIcenter.com

(Continued from previous topic...)

What Is Index Fragmentation?

Index fragmentation is a phenomena where index contents are no longer stored continuously in the storage. When index contents become scattered in the storage, fragmented, performance on index will degrade.

If you want to see the fragmentation level of an index, you can use the system function called sys.dm_db_index_physical_stats() in the following format:

SELECT * FROM sys.dm_db_index_physical_stats(
   database_id, table_id, index_id, DEFAULT, DEFAULT

The tutorial exercise below shows you how to view the fragmentation level of all indexes on table "fyi_links_indexed":

USE FyiCenterData

SELECT COUNT(*) FROM fyi_links_indexed;

SELECT i.index_id, i.name, s.avg_fragmentation_in_percent
   FROM sys.dm_db_index_physical_stats (
      DEFAULT, DEFAULT, DEFAULT) s, sys.indexes i
   WHERE s.object_id = i.object_id
      AND s.index_id = i.index_id;
index_id name             avg_fragmentation_in_percent
0        NULL             0.574712643678161
2        fyi_links_url    0
3        fyi_links_counts 0

Since there is no clustered index (index_id=0) on this table, the first row in the return is just a place holder.

The other two rows show that "fyi_links_url" and "fyi_links_counts" have zero fragmentation, since they were created not long ago.

(Continued on next topic...)

  1. What Are Indexes?
  2. How To Create an Index on an Existing Table?
  3. How To View Existing Indexes on an Given Table using SP_HELP?
  4. How To View Existing Indexes on an Given Table using sys.indexes?
  5. How To Drop Existing Indexes?
  6. Is the PRIMARY KEY Column of a Table an Index?
  7. Does the UNIQUE Constraint Create an Index?
  8. What Is the Difference Between Clustered and Non-Clustered Indexes?
  9. How To Create a Clustered Index?
  10. How To Create an Index for Multiple Columns?
  11. How To Create a Large Table with Random Data for Index Testing?
  12. How To Measure Performance of INSERT Statements?
  13. Does Index Slows Down INSERT Statements?
  14. Does Index Speed Up SELECT Statements?
  15. What Happens If You Add a New Index to Large Table?
  16. What Is the Impact on Other User Sessions When Creating Indexes?
  17. What Is Index Fragmentation?
  18. What Causes Index Fragmentation?
  19. How To Defragment Table Indexes?
  20. How To Defragment Indexes with ALTER INDEX ... REORGANIZE?
  21. How To Rebuild Indexes with ALTER INDEX ... REBUILD?
  22. How To Rebuild All Indexes on a Single Table?
  23. How To Recreate an Existing Index?

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