Step 2: 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 (estimated number of matching rows) and to determine the index cost to find the rows. The index costs are measured in terms of logical I/Os. If multiple indexes can be considered, the costs of each are compared with each other and also against the cost of a table scan to determine the least expensive access path.
Indexes are typically considered useful for an expression if the first column in the index is used in the expression and the search argument in the expression provides a means to limit the search. If no useful indexes are found ...
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