1. An analytical data processing system comprising: a processor; a non-transitory computer readable medium, comprising machine readable instructions executable by the processor, the instructions comprising: a query compiler to identify a call to a user defined function, including an input expression that defines input values for the user defined function and a parameter expression, comprising a key and a value pair where key is the name of the parameter field metadata and the value is a constant or an expression that evaluates to a constant value at the time of invoking the user, for the user defined function, within a query; and a processing engine to execute the user defined function to provide a function output according to the input expression and the parameter expression.