The specific name of the external UDF to be used to convert source_data_type to target_data_type. The specific_function_name must specify a unique function.
You cannot invoke an SQL UDF from a CREATE CAST or REPLACE CAST request.
If you do not specify a set of data types for a function name, then the specified function must be the only external UDF with that name in the database.
The specified function must satisfy the following conditions:
It can have only one parameter.
Its parameter data type must be the same as source_data_type.
Its result data type must be the same as target_data_type.
It must be defined to be deterministic.
Its function name must identify an external UDF in the SYSUDTLIB database.