@Override
    public void close() {
      for (Runnable runnable : undoSteps) {
        runnable.run();
      }
    }
Temporarily truncates the operator tree to avoid full cloning of the tree.

It does the following simplifications:
<ul>
 <li> hides UNION operator parents which are unrelated to the current set of roots
 <li> hides childs from RS and FS operators
<ul>
ql/src/java/org/apache/hadoop/hive/ql/parse/GenTezUtils.java