UDF callback functions and calling patterns

Calling patterns are steps the functions perform as results are gathered.

• UDF and UDAF callback functions
  The set of callback functions supplied by the engine through the a_v3_extfn_scalar_context structure and used within the user's UDF functions include:
• Scalar UDF calling pattern
  eExpected calling pattern for supplied function pointers for a scalar UDF calling pattern.
• Aggregate UDF calling patterns
  The calling patterns for the user-supplied aggregate UDF functions are more complex and varied than the scalar calling patterns.