public List<FieldNode> getPrunedColList(Operator<? extends OperatorDesc> op) {
    return prunedColLists.get(op);
  }
A mapping from operators to nested column paths being used in them.
Note: paths are of format "s.a.b" which represents field "b" of
  struct "a" is being used, while "a" itself is a field of struct "s".
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ColumnPrunerProcCtx.java