Sometimes users want to round to a precision that is data-driven (e.g. using a lookup table). They can't currently do this with decimal. I think we could support this by just using the input decimal type as the output type when the second argument is non-constant.
Motivated by a user trying to do something like this; http://community.cloudera.com/t5/Interactive-Short-cycle-SQL/Impala-round-function-does-not-return-expected-result/m-p/80200#M4906