For some reason, which I have never understood, SQL Server allows you to create duplicate indexes on the same object (table or view). You can create as many non-clustered indexes as you like with the exact index keys and included columns as well as the exact index properties. The only difference between the indexes would be the index ID and the index name. This is a very undesirable situation, because there is clearly no benefit from having the same index more than once, but on the other hand there is quite a lot of overhead that each index incurs. The overhead includes the storage consumed by the index, the on-going maintenance during DML statements, the periodical rebuild and/or reorganize operations, and the additional complexity that the optimizer has to deal with when evaluating possible access methods to the relevant table or view.
I don’t fully agree with Guy’s definition of redundancy, but it’s more a quibble than anything else—if I have an index on (A,B,C) and an index on (A,B), it might seem redundant, but there are cases when it isn’t. For example, suppose C is a large
NVARCHAR column such that we barely fit (A,B,C) into the window for an index (1700 bytes in SQL Server 2016, 900 in prior versions), but A and B are
INT types. If we have a lot of cases where we need (A,B) but not C, that second index is definitely not redundant.
Regardless, click through for Guy’s argument and a script to help you find potentially redundant indexes.