## Support for Table Functions.

Derby lets you declare functions which return ResultSets. You can then use these function results as tables in your queries. This, in turn, lets you do the following:

- Migrate - Bulk-load data from an external database. The external data source could be any vendor's database.
- Integrate - Transform live data from an existing legacy server and load the data into Derby. This lets users build new Derby-powered apps against subsets of legacy data, keep the data current, but limit the burden which the new apps place on the legacy server.
- Snapshot - Copy a subset of server data to a laptop before travelling.
- Federate - Join data from multiple external data sources. The external sources could be other relational databases or they could be non-relational data feeds.

Here is an example of how to declare and invoke a Table Function:

>     CREATE FUNCTION externalEmployees
>     ()
>     RETURNS TABLE
>     (
>       employeeId    INT,
>       lastName       VARCHAR( 50 ),
>       firstName      VARCHAR( 50 ),
>       birthday         DATE
>     )
>     LANGUAGE JAVA
>     PARAMETER STYLE DERBY_JDBC_RESULT_SET
>     NO SQL
>     EXTERNAL NAME 'com.acme.hrSchema.EmployeesTable.read'
>     ;

>     INSERT INTO employees
>       SELECT s.*
>         FROM TABLE (externalEmployees() ) s;

The Derby optimizer makes some assumptions about these Table Functions:

- 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.

Based on these assumptions, the optimizer decides where to place the Table Function in the join order. Using the interfaces in this package, you may override the optimizer's guesses and force the optimizer to choose a better join order.

- VTICosting - This interface exposes methods which let you override the optimizer's guesses.
- VTIEnvironment - This is a state variable, created by the optimizer and passed to the methods in VTICosting. VTICosting methods use this state variable to communicate with one another and learn more about the operating environment.