Performance Tuning with SQL Server Dynamic Management Views

Chapter 5: Indexing Strategy and Maintenance
• sys.dm_db_missing_index_groups – a DMV that provides details of missing
indexes in a specific group; this is the intermediate join table between sys.dm_db_
missing_index_details and sys.dm_db_missing_index_group_stats.
Napoleon Bonaparte stated that a good sketch is better than a long speech. I promise
that this is my one and only quote from a 19th century French dictator in this book,
but the adage is quite appropriate in this case. Figure 5.4 shows the many-to-many
relationship between missing_index_details and index_group_stats, via
missing_index_groups.
The first thing to note is that there is no index_id in any of the missing index DMOs.
This is because the returned results are recommendations for indexes which have yet
to be created, and are therefore non-materialized. The unique identifier for the records
in these DMVs is the index_handle column, which is unique across the entire SQL
Server instance.
The data stored by each of these DMOs is reset on a server restart. This is why it is so
important to preserve this cumulative data and keep your instances in a constantly
running state; you need to make sure, when you use this data, that the stored statistics
are fully representative of your normal query workload. One service restart, and your
accrued history (and the ability to generate meaningful results for this and other
DMV-based queries) is, pardon the pun, history.
Furthermore, the data stored in these DMOs is also volatile and based on active queries.
By implementing a single new index on a given table or view, the results of the DMO
query for that object may no longer be valid.
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