public ConstantPropagateOption getConstantPropagateOption() {
    return constantPropagateOption;
  }
Get propagated constant map from parents.

Traverse all parents of current operator, if there is propagated constant (determined by
assignment expression like column=constant value), resolve the column using RowResolver and add
it to current constant map.

@param op
       operator getting the propagated constants.
@return map of ColumnInfo to ExprNodeDesc. The values of that map must be either
        ExprNodeConstantDesc or ExprNodeNullDesc.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ConstantPropagateProcCtx.java