Chapter 6: Understanding Statistics

The SQL Server Query Optimizer is cost-based, so the quality of the execution plans it generates is directly related to the accuracy of its cost estimations. In the same way, the estimated cost of a plan is based on the algorithms or operators used, as well as their cardinality estimations. For this reason, to correctly estimate the cost of an execution plan, the Query Optimizer needs to estimate, as precisely as possible, the number of records returned by a given query.

During query optimization, SQL Server explores many candidate plans, estimates their relative costs, and selects the most efficient one. As such, incorrect cardinality and cost estimation may cause the Query Optimizer to choose inefficient ...

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