public SafetyPile addSafetyPile() {
    checkExpansion();
    safetyPiles[size] = INITIAL_PILE_POSITION;
    return new SafetyPile(size++);
  }
Get the safe index.

@return the index of the first element that cannot be evicted. in other words, elements whose
    index are <b>less than</b> the return value can be evicted.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/transformation/dag/memory/SafetyLine.java