Microsoft SQL Server 2014 Unleashed

Row Estimation and Index Selection

When the query analysis phase of optimization is complete and all SARGs, OR clauses, and join clauses have been identified, the next step is to determine the selectivity of the expressions (that is, the estimated number of matching rows) and to determine the cost of finding the rows. Selectivity is the measure of how selective a predicate is. The Cardinality Estimator (CE) is the component that determines the selectivity of WHERE, JOIN, and HAVING clauses.

The cost of a query is measured primarily in terms of logical and physical I/O, with the goal of generating a query plan that results in the lowest estimated I/O and processing cost. Primarily, the Query Optimizer attempts to identify whether an index exists ...