@Override
  public void setConstituentInputIsReady(Input input) {
    synchronized (this) {
      readyInputs.put(input, Boolean.TRUE);
    }
    if (readyInputs.size() == getInputs().size()) {
      informInputReady();
    }
  }
TezMergedLogicalInput is an adapter to make union input look like
a single input in tez.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/tools/TezMergedLogicalInput.java