Performance Tuning with SQL Server Dynamic Management Views

Chapter 5: Indexing Strategy and Maintenance
Also, be warned that the order in which these DMOs list missing columns does not accurately
suggest the correct column order for an index key. Furthermore, in our experience,
these DMOs are often over-enthusiastic in suggesting INCLUDE columns. They are best
used to find the biggest "holes" in an indexing strategy, not as a fine-tuning tool.
In addition, Microsoft Books Online lists the following specific limitations:
• cannot gather statistics for more than 500 missing index groups
• return less accurate cost information for queries involving only inequality predicates
• reports only include columns for some queries, so index key columns must be
manually selected
• return only raw information about columns on which indexes might be missing
• can return different costs for the same missing index group that appears multiple times
in XML Showplans.
Finding the most beneficial missing indexes
So how do you go about putting these DMOs to good use? Our goal is clearly to obtain
a list of missing indexes, with the most useful ones listed at the top. The SQL Server
Query Optimization Team at Microsoft proposed the following formula for calculating
the overall benefit of a suggested index, based on the columns in the _group_stats
DMV, and it has been widely adopted:
(user_seeks + user_scans) * avg_total_user_cost * (avg_user_impact * 0.01)
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