private static void addEdge(Map<String, Vertex> vertexCache, List<Edge> edges,
      Set<BaseColumnInfo> srcCols, Set<Vertex> targets, String expr, Edge.Type type) {
    Set<Vertex> sources = createSourceVertices(vertexCache, srcCols);
    Edge edge = findSimilarEdgeBySources(edges, sources, expr, type);
    if (edge == null) {
      edges.add(new Edge(sources, targets, expr, type));
    } else {
      edge.targets.addAll(targets);
    }
  }
Find an edge from all edges that has the same source vertices.
If found, add the more targets to this edge's target vertex list.
Otherwise, create a new edge and add to edge list.
ql/src/java/org/apache/hadoop/hive/ql/hooks/LineageLogger.java