@Override
    public RexNode visitInputRef(RexInputRef inputRef) {
      return inputRef;
    }
Breaks the predicate into 2 pieces. The first piece is the expressions that
only contain partition columns and can be used for Partition Pruning; the
second piece is the predicates that are left.

@param cluster
@param hiveTable
@param predicate
@return a Pair of expressions, each of which maybe null. The 1st predicate
        is expressions that only contain partition columns; the 2nd
        predicate contains the remaining predicates.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/PartitionPrune.java