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The structure of sys.dm_db_index_usage_stats
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You ll notice that the DMV lists the number of seeks, scans, lookups, and updates that users and the system perform, including when the latest of each type was done. The DMV is reset when the SQL Server service starts, but that s just a warning to people who might have thought that data remained there from long ago. An index that isn t used won t have an entry in this view. If no seeks, scans, lookups, or updates have been performed on an index, this view simply won t list the index. Incidentally, bear in mind that to get the name of the index, you may want to join this view to sys.indexes. You can also find out which indexes are used by looking at the execution plans that are being used by the queries issued against the database. This is even more useful, as the impact of an index can be easily evaluated by its impact on individual queries. If you consider the performance gain on an individual query, and examine how often this query is executed, you have a fantastic metric for the impact of an index. Query plans make it clear which indexes are being used, as an icon is shown for each index scan or seek. SQL Server 2008 Management Studio has significantly improved the readability of execution plans by displaying only the table name and index name, rather than using the three-part naming convention for the table and the index name. Figure 2 is a screen shot of an execution plan from SSMS 2005. Compare it with figure 3, which is a screen shot of an execution plan from SSMS 2008.
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When is an unused index not an unused index
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Figure 2 Execution plan in SSMS 2005
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As you can see in figure 3, the query plan is not different; it s just displayed in a more readable way. This is the main reason I opted to use SQL Server 2008 Management Studio throughout this chapter. Although you could use a SQL trace to collect the query plan for every query that runs, this is not practical. The impact on performance of this type of trace can be significant, and processing the plans can also be painful. Using the sys.dm_db_index_ usage_stats DMV to evaluate index usage is far easier. Querying the DMV every so often (particularly prior to any maintenance windows that might involve a service restart) can allow the information to be collected and analyzed, giving a strong indication of which indexes are used, and which are needlessly clogging up the system. After the unused indexes have been identified, it is simple to drop them and free up the space in the data files.
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Figure 3 Execution plan in SSMS 2008
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Let s consider a couple of different queries, looking at which indexes are being used and which aren t. We ll look at the queries themselves, and also look at the execution plans and the metadata stored within sys.indexes and sys.dm_db_index_usage_ stats. To avoid distractions I ll use my local server, which has just had its service restarted, thereby resetting sys.dm_db_index_usage_stats. I will also use a fresh copy of the AdventureWorks sample database on SQL Server 2005. You can find AdventureWorks on the CodePlex website, with all the other samples that Microsoft makes available. It s also found on the installation DVD for SQL Server 2005 Developer Edition. I ll use the Production.Product and Production.ProductSubcategory tables. The Production.Product table has a field called ProductSubcategoryID, which acts as a foreign key to the Production.ProductSubcategory table. The ProductSubcategoryID field in Production.Product allows NULLs, to cater for those products which are not allocated to a SubCategory. Let s consider the following query:
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SELECT DISTINCT color FROM Production.Product;
This gives us ten rows, each containing a different color that is used in the Product table. The word DISTINCT ensures that no two rows are identical. If we consider its execution plan, shown in figure 4, we can clearly see how the DISTINCT operator is applied through the use of a distinct sort. We don t have an index that starts with the Color field, so the system has no better way of finding different colors than to scan through the whole table (on the Clustered Index) and then perform a distinct sort. But what if we also needed the ProductNumber of each one Let s run the query, and look at the execution plan, shown in figure 5.
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