Java example: overloading your UDx

The following example code demonstrates creating a user-defined scalar function (UDSF) that adds two or three integers together. The Add2or3ints class is prepared to handle two or three arguments. It checks the number of arguments that have been passed to it, and adds all two or three of them together. The processBlock() method checks whether it has been called with less than 2 or more than 3 arguments. In theory, this should never happen, since Vertica only calls the UDSF if the user's function call matches a signature on one of the factory classes you create for your function. In practice, it is a good idea to perform this sanity checking, in case your (or someone else's) factory class reports that your function class accepts a set of arguments that it actually does not.

// You need to specify the full package when creating functions based on
// the classes in your library.
package com.mycompany.multiparamexample;
// Import the entire Vertica SDK
import com.vertica.sdk.*;
// This ScalarFunction accepts two or three integer arguments. It tests
// the number of input columns to determine whether to read two or three
// arguments as input.
public class Add2or3ints extends ScalarFunction
{
    @Override
    public void processBlock(ServerInterface srvInterface,
                             BlockReader argReader,
                             BlockWriter resWriter)
                throws UdfException, DestroyInvocation
    {
        // See how many arguments were passed in
        int numCols = argReader.getNumCols();

        // Return an error if less than two or more than 3 aerguments
        // were given. This error only occurs if a Factory class that
        // accepts the wrong number of arguments instantiates this
        // class.
        if (numCols < 2 || numCols > 3) {
            throw new UdfException(0,
                "Must supply 2 or 3 integer arguments");
        }

        // Process all of the rows of input.
        do {
            // Get the first two integer arguments from the BlockReader
            long a = argReader.getLong(0);
            long b = argReader.getLong(1);

            // Assume no third argument.
            long c = 0;

            // Get third argument value if it exists
            if (numCols == 3) {
                c = argReader.getLong(2);
            }

            // Process the arguments and come up with a result. For this
            // example, just add the three arguments together.
            long result = a+b+c;

            // Write the integer output value.
            resWriter.setLong(result);

            // Advance the output BlocKWriter to the next row.
            resWriter.next();

            // Continue processing input rows until there are no more.
        } while (argReader.next());
    }
}

The main difference between the Add2ints class and the Add2or3ints class is the inclusion of a section that gets the number of arguments by calling BlockReader.getNumCols(). This class also tests the number of columns it received from Vertica to ensure it is in the range it is prepared to handle. This test will only fail if you create a ScalarFunctionFactory whose getPrototype() method defines a signature that accepts less than two or more than three arguments. This is not really necessary in this simple example, but for a more complicated class it is a good idea to test the number of columns and data types that Vertica passed your function class.

Within the do loop, Add2or3ints uses a default value of zero if Vertica sent it two input columns. Otherwise, it retrieves the third value and adds that to the other two. Your own class needs to use default values for missing input columns or alter its processing in some other way to handle the variable columns.

You must define your function class in its own source file, rather than as an inner class of one of your factory classes since Java does not allow the instantiation of an inner class from outside its containing class. You factory class has to be available for instantiation by multiple factory classes.

Once you have created a function class or classes, you create a factory class for each signature you want your function class to handle. These factory classes can call individual function classes, or they can all call the same class that is prepared to accept multiple sets of arguments.

The following example's createScalarFunction() method instantiates a member of the Add2or3ints class.

// You will need to specify the full package when creating functions based on
// the classes in your library.
package com.mycompany.multiparamexample;
// Import the entire Vertica SDK
import com.vertica.sdk.*;
public class Add2intsFactory extends ScalarFunctionFactory
{
    @Override
    public void getPrototype(ServerInterface srvInterface,
                             ColumnTypes argTypes,
                             ColumnTypes returnType)
    {
        // Accept two integers as input
        argTypes.addInt();
        argTypes.addInt();
        // writes one integer as output
        returnType.addInt();
    }
    @Override
    public ScalarFunction createScalarFunction(ServerInterface srvInterface)
    {
        // Instantiate the class that can handle either 2 or 3 integers.
        return new Add2or3ints();
    }
}

The following ScalarFunctionFactory subclass accepts three integers as input. It, too, instantiates a member of the Add2or3ints class to process the function call:

// You will need to specify the full package when creating functions based on
// the classes in your library.
package com.mycompany.multiparamexample;
// Import the entire Vertica SDK
import com.vertica.sdk.*;
public class Add3intsFactory extends ScalarFunctionFactory
{
    @Override
    public void getPrototype(ServerInterface srvInterface,
                             ColumnTypes argTypes,
                             ColumnTypes returnType)
    {
        // Accepts three integers as input
        argTypes.addInt();
        argTypes.addInt();
        argTypes.addInt();
        // Returns a single integer
        returnType.addInt();
    }
    @Override
    public ScalarFunction createScalarFunction(ServerInterface srvInterface)
    {
        // Instantiates the Add2or3ints ScalarFunction class, which is able to
        // handle eitehr 2 or 3 integers as arguments.
        return new Add2or3ints();
    }
}

The factory classes and the function class or classes they call must be packaged into the same JAR file (see Compiling and packaging a Java library for details). If a host in the database cluster has the JDK installed on it, you could use the following commands to compile and package the example:

$ cd pathToJavaProject$ javac -classpath /opt/vertica/bin/VerticaSDK.jar \
> com/mycompany/multiparamexample/*.java
$ jar -cvf Add2or3intslib.jar com/vertica/sdk/BuildInfo.class \
> com/mycompany/multiparamexample/*.class
added manifest
adding: com/vertica/sdk/BuildInfo.class(in = 1202) (out= 689)(deflated 42%)
adding: com/mycompany/multiparamexample/Add2intsFactory.class(in = 677) (out= 366)(deflated 45%)
adding: com/mycompany/multiparamexample/Add2or3ints.class(in = 919) (out= 601)(deflated 34%)
adding: com/mycompany/multiparamexample/Add3intsFactory.class(in = 685) (out= 369)(deflated 46%)

Once you have packaged your overloaded UDx, you deploy it the same way as you do a regular UDx, except you use multiple CREATE FUNCTION statements to define the function, once for each factory class.

=> CREATE LIBRARY add2or3intslib as '/home/dbadmin/Add2or3intslib.jar'
-> language 'Java';
CREATE LIBRARY
=> CREATE FUNCTION add2or3ints as LANGUAGE 'Java' NAME 'com.mycompany.multiparamexample.Add2intsFactory' LIBRARY add2or3intslib;
CREATE FUNCTION
=> CREATE FUNCTION add2or3ints as LANGUAGE 'Java' NAME 'com.mycompany.multiparamexample.Add3intsFactory' LIBRARY add2or3intslib;
CREATE FUNCTION

You call the overloaded function the same way you call any other function.

=> SELECT add2or3ints(2,3);
 add2or3ints
-------------
           5
(1 row)
=> SELECT add2or3ints(2,3,4);
 add2or3ints
-------------
           9
(1 row)
=> SELECT add2or3ints(2,3,4,5);
ERROR 3457:  Function add2or3ints(int, int, int, int) does not exist, or permission is denied for add2or3ints(int, int, int, int)
HINT:  No function matches the given name and argument types. You may need to add explicit type casts

The last error was generated by Vertica, not the UDx code. It returns an error if it cannot find a factory class whose signature matches the function call's signature.

Creating an overloaded UDx is useful if you want your function to accept a limited set of potential arguments. If you want to create a more flexible function, you can create a polymorphic function.