Key Performance and Monitoring DMVs and DMFs in .NET

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Key Performance and Monitoring DMVs and DMFs
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DMVs and DMFs are divided into dozens of categories that encompass various features, subsystems, and statistical categories. Categorization of the views and functions is achieved by using a standardized naming convention in which the first part of the
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15
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Monitoring and Troubleshooting SQL Server Performance
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name, or prefix, indicates the category for a DMV or DMF. Table 15-1 lists the prefixes for each category and the general purpose of the DMVs or DMFs in each category.
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Table 15-1 DMV and DMF Prefixes
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Prefix dm_db_* dm_exec_* dm_io_* dm_os_*
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General purpose Provides general database statistics such as space and index utilization. Provides query statistics. Provides I/O statistics. Provides hardware-level information.
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Database Statistics
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You can use one DMV and two DMFs to gather basic index usage information within a database. The sys.dm_db_index_usage_stats DMV contains core statistics about each index within a database. Use this view when you need to find the number of seeks, scans, lookups, or updates that have occurred with an index.
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BEST PRACTICES
Using sys.dm_db_index_usage_stats
The sys.dm_db_index_usage_stats DMV is a good place to start to find any indexes that the query optimizer is not using. If the system has been running for awhile, and an index does not have any seeks, scans, or lookups registered for it, it is a strong possibility that the index is not being used to satisfy any queries. Or an index might show activity but is no longer being used. You can determine the last time an index was used by examining the last_user_seek, last_user_scan and last_user_lookup columns.
Of much more interest on a day-to-day basis, however, are the sys.dm_db_index_ operational_stats and sys.dm_db_index_physical_stats DMFs. The index_operational_stats function takes four parameters: database_id, object_id, index_id, and partition_id. This function displays all the current I/O statistics related to locking, latching, and access. You use this function to find out how heavily a particular index is being used in terms of modifications as well as scans or lookups. You would also reference the output of this function when you are concerned about locking or latching contention in the database.
Lesson 4: Using Dynamic Management Views and Functions
The index_physical_stats function takes five parameters: database_id, object_id, index_id, partition_id, and mode. This function returns row size and fragmentation information. In previous versions of SQL Server, DBCC SHOWCONTIG was used to return this type of data. The final set of views and functions essentially provide a real-time index analysis. The views beginning with sys.dm_db_missing_index_* track indexes that could be created against your database. When queries are executed that cause the table to be scanned, and SQL Server determines that it could have taken advantage of an index to satisfy the query, it logs entries in sys.dm_db_missing_index_details, sys.dm_db_missing_ index_group_stats, and sys.dm_db_missing_index_groups. The group stats view contains counters for the number of times a particular index could be used as well as the seeks, scans, and some basic costing values. The index details view contains information about the table that should have an index created on it as well as the column for that index. The index groups view provides an aggregation functionality. By combining these three views together, you can proactively analyze new indexes while a system is operating without requiring workload traces to be generated for analysis in DTA. Although these views are not a replacement for DTA, which also considers indexed views and partitions and provides a more exhaustive analysis of indexes, they can be a very effective initial level of analysis.
BEST PRACTICES
Calculating the value of proposed indexes
The most difficult decision to make is which of the indexes proposed by the sys.dm_db_missing_index* views can provide the most benefit. Applying some basic calculations, you can derive a numerical comparison based on SELECT activity only for each of the proposed indexes. The following example shows the code you can use to apply the calculations:
SELECT * FROM (SELECT user_seeks * avg_total_user_cost * (avg_user_impact * 0.01) AS index_advantage, migs.* FROM sys.dm_db_missing_index_group_stats migs) AS migs_adv INNER JOIN sys.dm_db_missing_index_groups AS mig ON migs_adv.group_handle = mig.index_group_handle INNER JOIN sys.dm_db_missing_index_details AS mid ON mig.index_handle = mid.index_handle ORDER BY migs_adv.index_advantage
On operational systems, values above 5,000 indicate indexes that should be evaluated for creation. When the value passes 10,000, you generally have an index that can provide a significant performance improvement for read operations. This algorithm accounts only for read activity, so you will always want to consider the impact of maintenance operations as well.
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