I used DMV sys.dm_db_missing_index_details and sys.dm_db_missing_index_group_stats to create some indexes in my client's database. He reported me that these indexes caused serious performance degradation in queries and had to drop them. He talked about 3 seconds vs. 3 minutes in performance degradation. I know that extra indexes can cause overhead in OLTP and operations that changes data but I can't figure out how an index that helps some queries can slow down dramatically others. I haven't reviewed this issue yet, but I would like to check some information before. Do you have some useful information about this?