Listen Software's How To: SQL Tuning--Optimizer Hints
July 8, 2002
Optimizer Steps: Each step of the execution plan returns a set of rows that are used either by the next step or in the last step.
The optimizer calculates the cost based on the estimated computer resources, including but not limited to I/O, CPU time, and memory.
The goal of the cost-based approach is the best throughput, or minimal resource usage necessary to process all rows accessed by the statement.
The optimizer generates a set of execution plans based on the possible join orders, join operations, and available access paths. The optimizer estimates the costs of each plan and chooses the one with the lowest cost.
Indexes improve the performance of queries that select a small percentage of rows from the table (2 to 4 percent).
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