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SQL Server FAQ - What Causes Index Fragmentation

By: FYIcenter.com

(Continued from previous topic...)

What Causes Index Fragmentation?

Index fragmentation is usually caused by deleting of existing rows or updating existing values of the indexed column. Inserting new rows should not cause any index fragmentation.

This tutorial exercise shows you how update statements of 50000 rows on the table "fyi_link_indexed" with 100000 rows caused the index fragmented 84%:

USE FyiCenterData;
GO

SELECT COUNT(*) FROM fyi_links_indexed;
GO
100000

SELECT i.index_id, i.name, s.avg_fragmentation_in_percent
   FROM sys.dm_db_index_physical_stats (
      DB_ID(N'FyiCenterData'), 
      OBJECT_ID(N'fyi_links_indexed'),
      DEFAULT, DEFAULT, DEFAULT) s, sys.indexes i
   WHERE s.object_id = i.object_id
      AND s.index_id = i.index_id;
GO
0        NULL             0.574712643678161
2        fyi_links_url    0
3        fyi_links_counts 0

UPDATE fyi_links_indexed SET url = REVERSE(url)
   WHERE id <=50000;
GO
(50000 row(s) affected)

SELECT i.index_id, i.name, s.avg_fragmentation_in_percent
   FROM sys.dm_db_index_physical_stats (
      DB_ID(N'FyiCenterData'), 
      OBJECT_ID(N'fyi_links_indexed'),
      DEFAULT, DEFAULT, DEFAULT) s, sys.indexes i
   WHERE s.object_id = i.object_id
      AND s.index_id = i.index_id;
GO
0        NULL             0.574712643678161
2        fyi_links_url    84.053862508859
3        fyi_links_counts 0.448430493273543

Note that only the index on "url" fragmented. Index on "counts" did not affected, because the update statement only touched the "url" column.

(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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