private static void addEdge(Map<String, Vertex> vertexCache, List<Edge> edges,
      Set<BaseColumnInfo> srcCols, Vertex target, String expr, Edge.Type type) {
    Set<Vertex> targets = new LinkedHashSet<Vertex>();
    targets.add(target);
    addEdge(vertexCache, edges, srcCols, targets, expr, type);
  }
Based on the final select operator, find out all the target columns.
For each target column, find out its sources based on the dependency index.
ql/src/java/org/apache/hadoop/hive/ql/hooks/LineageLogger.java