When a table index is fragmented to a certain percentage, you need to defragment the index to maintain its performance level. There are 3 ways to defragment:

1. "ALTER INDEX index_name ON table_name REORGANIZE" - Defragmenting the specified index performed in online mode. No locks applied on affected table. Used for indexes with a small fragmentation percentage.

2. "ALTER INDEX index_name ON table_name REBUILD" - Defragmenting the specified index performed in offline mode by default. It can be performed in online mode. Used for indexes with a large fragmentation percentage.

3. "CREATE INDEX ... WITH (DROP_EXISTING = ON)" - Re-creating the specified index. The definition of the index can be changed.