What Is a Query Optimizer?
For any given SQL statement, the source tables can be accessed in many ways to build the desired resultset. The query optimizer analyzes all the possible ways that the resultset can be built and chooses the most appropriate method. This method is called the query execution plan. SQL Server uses a cost-based optimizer. The optimizer assigns a cost to every possible execution plan in terms of CPU resource usage and disk I/O. The optimizer then chooses the execution plan with the least associated cost.
Thus, the primary goal of the query optimizer is to find the cheapest execution to minimize the total time to process the query. Because I/O is the most significant factor in query processing time, the optimizer analyzes ...
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