Cost-Based Query Optimizer
The optimizer enumerates alternative plans (access paths, join algorithms, join orders), assigns each a cost from an IO-and-CPU model fed by table statistics, and executes the cheapest. It optimizes the cost estimate, not runtime, so wrong statistics or a mismatched cost model produce confidently bad plans.
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The optimizer enumerates alternative plans (access paths, join algorithms, join orders), assigns each a cost from an IO-and-CPU model fed by table statistics, and executes the cheapest. It optimizes the cost estimate, not runtime, so wrong statistics or a mismatched cost model produce confidently bad plans.
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