public Dependency getDependency(DataContainer dc, FieldSchema col) {
    return index.get(new DependencyKey(dc, col));
  }
Gets the dependency for a table, column tuple.
@param dc The data container of the column whose dependency is being inspected.
@param col The column whose dependency is being inspected.
@return Dependency for that particular table, column tuple.
        null if no dependency is found.
ql/src/java/org/apache/hadoop/hive/ql/hooks/LineageInfo.java