Currently, all of the arithmetic operators, such as add/sub/mult/div, are implemented as old-style UDFs and java reflection is used to determine the return type TypeInfos/ObjectInspectors, based on the return type of the evaluate() method chosen for the expression. This works fine for types that don't have type params.
Hive decimal type participates in these operations just like int or double. Different from double or int, however, decimal has precision and scale, which cannot be determined by just looking at the return type (decimal) of the UDF evaluate() method, even though the operands have certain precision/scale. With the default of "decimal" without precision/scale, then (10, 0) will be the type params. This is certainly not desirable.
To solve this problem, all of the arithmetic operators would need to be implemented as GenericUDFs, which allow returning ObjectInspector during the initialize() method. The object inspectors returned can carry type params, from which the "exact" return type can be determined.
It's worth mentioning that, for user UDF implemented in non-generic way, if the return type of the chosen evaluate() method is decimal, the return type actually has (10,0) as precision/scale, which might not be desirable. This needs to be documented.
This JIRA will cover minus, plus, divide, multiply, mod, and pmod, to limit the scope of review. The remaining ones will be covered under