public Interface VTICosting

VTICosting is the interface that the query optimizer uses to cost Table Functions. The methods on this interface provide the optimizer with the following information:

• The estimated number of rows returned by the Table Function in a single instantiation.
• The estimated cost to instantiate and iterate through the Table Function.
• Whether or not the Table Function can be instantiated multiple times within a single query execution.

The optimizer places a Table Function in the join order after making some assumptions:

• Cost - The optimizer hard-codes a guess about how expensive it is to materialize a Table Function.
• Count - The optimizer also hard-codes a guess about how many rows a Table Function returns.
• Repeatability - The optimizer assumes that the same results come back each time you invoke a Table Function.

The class which contains your Table Function can override these assumptions and improve the join order as follows:

• Implement - The class must implement VTICosting.
• Construct - The class must contain a public, no-arg constructor.

The methods in this interface take a VTIEnvironment argument. This is a state variable created by the optimizer. The methods in this interface can use this state variable to pass information to one another and learn other details of the operating environment.