Using sys.dm_db_missing_index_group_stats
?returns summary information about groups of missing indexes, excluding spatial indexes. An example follows:
SELECT TOP 5 * FROM sys.dm_db_missing_index_group_stats ORDER BY avg_total_user_cost * avg_user_impact * (user_seeks + user_scans)
The above query determines which 5 missing indexes would produce the highest anticipated cumulative improvement.