Optimizer Components

 The optimizer contains three main components: the transformer, estimator, and plan generator.

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The following diagram depicts the components:

Figure 7-2 Optimizer Components

Description of "Figure 7-2 Optimizer Components"

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The input to the optimizer is a parsed query. The optimizer performs the following operations:

1. The optimizer receives the parsed query and generates a set of potential plans for the SQL statement based on available access paths and hints.
   

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2. The optimizer estimates the cost of each plan based on statistics in the data dictionary. The cost is an estimated value proportional to the expected resource use needed to execute the statement with a particular plan.

3. The optimizer compares the costs of plans and chooses the lowest-cost plan, known as the query plan, to pass to the row source generator.

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See also

• "SQL Parsing"

• "SQL Row Source Generation"