Getting parameter values in UDxs

Your UDx uses the parameter values it declared in its factory class (see Defining the parameters your UDx accepts) in its function class's processing method (for example, processBlock() or processPartition()). It gets its parameter values from a ParamReader object, which is available from the ServerInterface object that is passed to your processing method. Reading parameters from this object is similar to reading argument values from BlockReader or PartitionReader objects: you call a data-type-specific function with the name of the parameter to retrieve its value. For example, in C++:

// Get the parameter reader from the ServerInterface to see if there are supplied parameters.
ParamReader paramReader = srvInterface.getParamReader();
// Get the value of an int parameter named constant.
const vint constant = paramReader.getIntRef("constant");

Using parameters in the factory class

In addition to using parameters in your UDx function class, you can also access the parameters in the factory class. You may want to access the parameters to let the user control the input or output values of your function in some way. For example, your UDx can have a parameter that lets the user choose to have your UDx return a single- or double-precision value. The process of accessing parameters in the factory class is the same as accessing it in the function class: get a ParamReader object from the ServerInterface's getParamReader() method, them read the parameter values.

Testing whether the user supplied parameter values

Unlike its handling of arguments, Vertica does not immediately return an error if a user's function call does not include a value for a parameter defined by your UDx's factory class. This means that your function can attempt to read a parameter value that the user did not supply. If it does so, by default Vertica returns a non-existent parameter warning to the user, and the query containing the function call continues.

If you want your parameter to be optional, you can test whether the user supplied a value for the parameter before attempting to access its value. Your function determines if a value exists for a particular parameter by calling the ParamReader's containsParameter() method with the parameter's name. If this call returns true, your function can safely retrieve the value. If this call returns false, your UDx can use a default value or change its processing in some other way to compensate for not having the parameter value. As long as your UDx does not try to access the non-existent parameter value, Vertica does not generate an error or warning about missing parameters.

See C++ example: defining parameters for an example.