Module org.apache.derby.engine

# Package org.apache.derby.vti

## 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.

- Interface Summary
  Interface | Description
  --- | ---
  AwareVTI | Interface describing a table function which can be given information about the context in which it runs.
  RestrictedVTI | Interface for Table Functions which can be told which columns need to be fetched plus simple bounds on those columns.
  VTICosting | VTICosting is the interface that the query optimizer uses to cost Table Functions.
  VTIEnvironment | VTIEnvironment is the state variable created by the optimizer to help it place a Table Function in the join order.

- Class Summary
  Class | Description
  --- | ---
  ForeignTableVTI | This class contains a table function which can be used to bulk-import data from a foreign database.
  ForwardingVTI | This class contains a table function which forwards its behavior to another ResultSet wrapped inside it.
  Restriction | An expression to be pushed into a Table Function so that the Table Function can short-circuit its processing and return fewer rows.
  Restriction.AND | An AND of two Restrictions
  Restriction.ColumnQualifier | A simple comparison of a column to a constant value.
  Restriction.OR | An OR of two Restrictions
  StringColumnVTI | This is an abstract table function which assumes that all columns are strings and which coerces the strings to reasonable values for various getXXX() methods.
  VTIContext | Context parameter which is passed to an AwareVTI.
  VTITemplate | An abstract implementation of ResultSet that is useful when writing table functions, read-only VTIs (virtual table interface), and the ResultSets returned by executeQuery in read-write VTI classes.
  VTITemplate.ColumnDescriptor | A struct class which is useful for describing columns and parameters.