⟩ 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 "ggl_link_indexed" with 100000 rows caused the index fragmented 84%:
USE GlobalGuideLineDatabase;
GO
SELECT COUNT(*) FROM ggl_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'GlobalGuideLineDatabase'),
OBJECT_ID(N'ggl_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 ggl_links_url 0
3 ggl_links_counts 0
UPDATE ggl_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'GlobalGuideLineDatabase'),
OBJECT_ID(N'ggl_links_indexed'),
DEFAULT, DEFAULT, DEFAULT) s, sys.indexes i
WHERE s.object_id = i.object_id
AND s.index_id = i.index_id; GO
