private static ExprNodeDesc findParentExpr(ExprNodeColumnDesc col, Operator<?> op) {
    ExprNodeDesc parentExpr = col;
    Map<String, ExprNodeDesc> mapping = op.getColumnExprMap();
    if (mapping != null) {
      parentExpr = mapping.get(col.getColumn());
      if (parentExpr == null && op instanceof ReduceSinkOperator) {
        return col;
      }
    }
    return parentExpr;
  }
Checks whether the keys of a child operator are a prefix of the keys of a
parent operator.
@param childKeys keys of the child operator
@param parentKeys keys of the parent operator
@param childOp child operator
@param parentOp parent operator
@return true if the keys are a prefix, false otherwise
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/plan/ExprNodeDescUtils.java