Before analyzing a tracing session, the Index Consultant asks you for the type of recommendations you want:
Recommend clustered indexes If this option is selected, the Index Consultant analyzes the effect of clustered and unclustered indexes.
Properly selected clustered indexes can provide significant performance improvements over unclustered indexes for some workloads, but you must reorganize the table (using the REORGANIZE TABLE statement) for them to be effective. In addition, the analysis takes longer if the effects of clustered indexes are considered.
Keep existing secondary indexes The Index Consultant can perform its analysis by either maintaining the existing set of secondary indexes in the database, or by ignoring the existing secondary indexes. A secondary index is an index that is not a unique constraint or a primary or foreign key. Indexes that are present to enforce referential integrity constraints are always considered when selecting access plans.
The analysis includes the following steps:
Generate candidate indexes For each tracing session, the Index Consultant generates a set of candidate indexes. Creating a real index on a large table can be a time consuming operation, so the Index Consultant creates its candidates as virtual indexes. A virtual index cannot be used to actually execute queries, but the optimizer can use virtual indexes to estimate the cost of execution plans as if such an index were available. Virtual indexes allow the Index Consultant to perform a "what-if" analysis without the expense of creating and managing real indexes. Virtual indexes have a limit of four columns.
Testing the benefits and costs of candidate indexes The Index Consultant asks the optimizer to estimate the cost of executing the queries in the tracing database, with and without different combinations of candidate indexes.
Generating recommendations The Index Consultant assembles the results of the query costs and sorts the indexes by the total benefit they provide. It provides a SQL script, which you can run to implement the recommendations or which you can save for your own review and analysis.
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