"Software in a relational database that tries to determine how to process the query by assigning estimated costs to various table lookup alternatives." (Ralph Kimball & Margy Ross, "The Data Warehouse Toolkit" 2nd Ed., 2002)
"A cost-based optimizer evaluates the cost in terms of the amount of computing resources used for execution plans. The optimizer analyzes the possible plans and then chooses for execution the access plan with the lowest estimated cost." (Microsoft Corporation, "Microsoft SQL Server 7.0 Data Warehouse Training Kit", 2000)
"An Oracle optimizer methodology that relies on the characteristics of the tables being queried to determine the method used to run the query. A cost is calculated for estimated CPU, I/O, and sorting for the possible execution paths. The path with the lowest overall cost is used to perform the query." (Bob Bryla, "Oracle Database Foundations", 2004)
"A query optimizer technique that uses an algorithm based on statistics about the objects being accessed, that is, number of rows, indexes available, indexes sparsity, and so on." (Carlos Coronel et al, "Database Systems: Design, Implementation, and Management" 9th Ed., 2011)
"The legacy name for the optimizer. In earlier releases, the cost-based optimizer was an alternative to the rule-based optimizer (RBO)." (Oracle)
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