In order to improve your applications and your databases, they will need to change over time. The structure of the database changes, the structure of the tables change, the data in the tables change, the application changes, the queries against the data change. Indexes that once aided performance now sit unused while new indexes are added.
When index needs change and you want to test how removing an index will affect performance, you have two options – you can disable or drop the index. Which is better?
Disabling an Index
To disable in index, issue an ALTER INDEX command.
ALTER INDEX IX_IndexName ON Schema.TableName DISABLE
What happens when you do this? The metadata about the index is retained, as are the usage statistics in sys.dm_db_index_usage_stats. The index is not usable by the query optimizer, however. If you have an index hint in a query specifying the index, the query will fail. Also, foreign key constraints are disabled.
Disabling a nonclustered index will delete the index pages – the space is freed in the database.
Disabling a clustered index has additional effects. The data in the table still exists, but will be inaccessible for anything other than a drop or rebuild operation. All related nonclustered indexes and views are also unavailable. Foreign key constraints that reference the table are disabled. Queries against the table will fail.
If you want to re-enable the index, you need to rebuild it – there is no ALTER INDEX…ENABLE command.
ALTER INDEX IX_IndexName ON Schema.TableName REBUILD
When you rebuild an index, the usage stats will be reset in sys.dm_db_index_usage_stats.
Dropping an Index
To drop a clustered or nonclustered index, issue a DROP INDEX command.
DROP INDEX IndexName ON Schema.TableName
When you do this, the metadata, statistics, and index pages are removed. If you drop a clustered index, the table will become a heap.
Once an index has been dropped, it can’t be rebuilt – it must be created again. Thus, if you are going to drop an existing index, make sure you script the CREATE statement also. If it is determined that you need it again in the future, it will be much easier to open the script than try to remember how you had it set up previously!
Do you want to maintain statistics?
The biggest difference between disabling and dropping an index is whether the metadata and statistics are persisted. If disabled, they are. If dropped, they are not. Make sure you carefully weigh your options before performing either action, and always have a way to recreate the index available.