SQL Server index Fragmentation is a common source of database performance degradation. Fragmentation occurs when there is a lot of empty space on a data page (internal fragmentation) or when the logical order of pages in the index doesn’t match the physical order of pages in the data file (external fragmentation).

Fragmentation-related performance issues are most often observed when executing queries that perform index scans. Queries that perform index seeks may not be affected by high index fragmentation.