Description: Once you have measured your table function's cost, you can write the VTICosting methods.
Keywords: Functions, VTICosting example

# Example VTICosting implementation

Once you have measured your table function's cost, you can write the VTICosting methods.

Optimizer fine-tuning can be added to the EmployeeTable table function shown before in Example Derby-style table function:

    package com.acme.hrSchema;

    import java.io.Serializable;
    import java.sql.*;

    import org.apache.derby.vti.VTICosting;
    import org.apache.derby.vti.VTIEnvironment;

    /**
     * Tuned table function.
     */
    public class TunedEmployeeTable extends EmployeeTable implements VTICosting
    {
        public TunedEmployeeTable() {}

        public double getEstimatedRowCount( VTIEnvironment optimizerState ) throws SQLException
        {
            return getRowCount( optimizerState );
        }

        public double getEstimatedCostPerInstantiation( VTIEnvironment optimizerState ) throws SQLException
        {
            double      I = 100.0;  // optimizer imprecision
            double      P = 10.0;   // cost per row in milliseconds
            double      E = 0.0;    // cost of instantiating the external ResultSet
            double      N = getRowCount( optimizerState );

            return I * ( ( P * N ) + E );
        }

        public boolean supportsMultipleInstantiations( VTIEnvironment optimizerState ) throws SQLException
        {
            return true;
        }

        //////////////////////////////////////////////////////////////////////////////

        private double  getRowCount( VTIEnvironment optimizerState )
            throws SQLException
        {
            String            ROW_COUNT_KEY = "rowCountKey";
            Double          estimatedRowCount = (Double) getSharedState( optimizerState, ROW_COUNT_KEY );

            if ( estimatedRowCount == null )
            {
                Connection                  conn = getConnection();
                PreparedStatement       ps = conn.prepareStatement( "select count(*) from hrSchema.EmployeeTable" );
                ResultSet                      rs = ps.executeQuery();

                rs.next();
                estimatedRowCount = new Double( rs.getDouble( 1 ) );

                setSharedState( optimizerState, ROW_COUNT_KEY, estimatedRowCount );

                rs.close();
                ps.close();
                conn.close();
            }

            return estimatedRowCount.doubleValue();
        }

        private Serializable  getSharedState( VTIEnvironment optimizerState, String key )
        { return (Serializable) optimizerState.getSharedState( key ); }
        private void    setSharedState( VTIEnvironment optimizerState, String key, Serializable value )
        { optimizerState.setSharedState( key, value ); }
    }

Parent topic: Optimizer support for Derby-style table functions