What Is Index Fragmentation in SQL Server?

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
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
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.

⇒ What Causes Index Fragmentation in SQL Server

⇐ CREATE INDEX - Impact on Other User Sessions in SQL Server

⇑ Understanding and Managing Indexes in SQL Server

⇑⇑ SQL Server Database Tutorials