Flink SQL 子图复用逻辑分析

子图复用优化是为了找到SQL执行计划中重复的节点，将其复用，避免这部分重复计算的逻辑。先回顾SQL执行的主要流程 parser -> validate -> logical optimize -> physical optimize -> translateToExecNode。
而子图复用的逻辑就是在这个阶段进行的

private[flink] def translateToExecNodeGraph(
    optimizedRelNodes: Seq[RelNode],
    isCompiled: Boolean): ExecNodeGraph = {
    val nonPhysicalRel = optimizedRelNodes.filterNot(_.isInstanceOf[FlinkPhysicalRel])
    if (nonPhysicalRel.nonEmpty) {
      throw new TableException(
        "The expected optimized plan is FlinkPhysicalRel plan, " +
        s"actual plan is ${nonPhysicalRel.head.getClass.getSimpleName} plan.")
    }

    require(optimizedRelNodes.forall(_.isInstanceOf[FlinkPhysicalRel]))
    // Rewrite same rel object to different rel objects
    // in order to get the correct dag (dag reuse is based on object not digest)
    val shuttle = new SameRelObjectShuttle()
    val relsWithoutSameObj = optimizedRelNodes.map(_.accept(shuttle))
    // reuse subplan
    val reusedPlan = SubplanReuser.reuseDuplicatedSubplan(relsWithoutSameObj, tableConfig)
    // convert FlinkPhysicalRel DAG to ExecNodeGraph
    val generator = new ExecNodeGraphGenerator()
    val execGraph = generator.generate(reusedPlan.map(_.asInstanceOf[FlinkPhysicalRel]), isCompiled)

    // process the graph
    val context = new ProcessorContext(this)
    val processors = getExecNodeGraphProcessors
    processors.foldLeft(execGraph)((graph, processor) => processor.process(graph, context))
  }

可以看到这里首先会校验relNodes都是FlinkPhysicalRel 物理执行计划的节点

require(optimizedRelNodes.forall(_.isInstanceOf[FlinkPhysicalRel]))

SameRelObjectShuttle

/**
* Rewrite same rel object to different rel objects.
*
* <p>e.g.
* {{{
*      Join                       Join
*     /    \                     /    \
* Filter1 Filter2     =>     Filter1 Filter2
*     \   /                     |      |
*      Scan                  Scan1    Scan2
* }}}
* After rewrote, Scan1 and Scan2 are different object but have same digest.
*/
class SameRelObjectShuttle extends DefaultRelShuttle {
  private val visitedNodes = Sets.newIdentityHashSet[RelNode]()

  override def visit(node: RelNode): RelNode = {
    val visited = !visitedNodes.add(node)
    var change = false
    val newInputs = node.getInputs.map {
      input =>
      val newInput = input.accept(this)
      change = change || (input ne newInput)
      newInput
    }
    if (change || visited) {
      node.copy(node.getTraitSet, newInputs)
    } else {
      node
    }
  }
}

然后进行rel节点重写，RelShuttle的作用就是提供visit的模式根据实现的逻辑来替换树中的某些节点。可以看到这个实现中会将 同一个objec(注意这里保存visitedNodes使用的是identity hash set) 第二次访问时 copy成一个新的对象，但是有相同的digest，这一步的目的是什么呢？
我们往下面看在后续生成ExecNode时, 会创建一个IdentityHashMap 来保存访问过的Rels，所以意思就是真正生成ExecNode时，是和Rels对象一一对应的。

private final Map<FlinkPhysicalRel, ExecNode<?>> visitedRels = new IdentityHashMap();
private ExecNode<?> generate(FlinkPhysicalRel rel, boolean isCompiled) {
    ExecNode<?> execNode = visitedRels.get(rel);
    if (execNode != null) {
        return execNode;
    }

    if (rel instanceof CommonIntermediateTableScan) {
        throw new TableException("Intermediate RelNode can't be converted to ExecNode.");
    }

    List<ExecNode<?>> inputNodes = new ArrayList<>();
    for (RelNode input : rel.getInputs()) {
        inputNodes.add(generate((FlinkPhysicalRel) input, isCompiled));
    }

    execNode = rel.translateToExecNode(isCompiled);
    // connects the input nodes
    List<ExecEdge> inputEdges = new ArrayList<>(inputNodes.size());
    for (ExecNode<?> inputNode : inputNodes) {
        inputEdges.add(ExecEdge.builder().source(inputNode).target(execNode).build());
    }
    execNode.setInputEdges(inputEdges);

    visitedRels.put(rel, execNode);
    return execNode;
}

看到这里上面将同一个object 拆成两个的目的就更不可理解了，因为本来是一个object的话在这里天然就复用了，但是拆成2个反而就不能复用了。
这里的目的是先将相同的object被重复引用的节点拆开，然后再根据digest相同以及内部规则来决定是否复用。这样就可以有Flink引擎来控制哪些节点是可以合并的。

SubplanReuseContext

在context中通过ReusableSubplanVisitor构造两组映射关系

// mapping a relNode to its digest
private val mapRelToDigest = Maps.newIdentityHashMap[RelNode, String]()
// mapping the digest to RelNodes
private val mapDigestToReusableNodes = new util.HashMap[String, util.List[RelNode]]()

中间的逻辑比较简单就是遍历整棵树，查找是否存在可reusable的节点，怎么判断可reusable呢？

• 同一digest下，挂了多个RelNode节点，那么这一组RelNode是同一语义的，是可以复用的候选
• 节点没有disable reusable

/** Returns true if the given node is reusable disabled */
private def isNodeReusableDisabled(node: RelNode): Boolean = {
  node match {
    // TableSourceScan node can not be reused if reuse TableSource disabled
    case _: FlinkLogicalLegacyTableSourceScan | _: CommonPhysicalLegacyTableSourceScan |
    _: FlinkLogicalTableSourceScan | _: CommonPhysicalTableSourceScan =>
    !tableSourceReuseEnabled
    // Exchange node can not be reused if its input is reusable disabled
    case e: Exchange => isNodeReusableDisabled(e.getInput)
    // TableFunctionScan and sink can not be reused
    case _: TableFunctionScan | _: LegacySink | _: Sink => true
    case _ => false
  }
}

例如TableFunctionScan就不能被Reuse(这个原因还没理解)，或者exchange只有input被reuse时，该节点才能复用

SubplanReuseShuttle

在以上的visit执行完之后以及知道哪些节点是可以复用的了，最后通过一个Shuttle来将可复用的节点进行替换

class SubplanReuseShuttle(context: SubplanReuseContext) extends DefaultRelShuttle {
  private val mapDigestToNewNode = new util.HashMap[String, RelNode]()

  override def visit(rel: RelNode): RelNode = {
    val canReuseOtherNode = context.reuseOtherNode(rel)
    val digest = context.getRelDigest(rel)
    if (canReuseOtherNode) {
      val newNode = mapDigestToNewNode.get(digest)
      if (newNode == null) {
        throw new TableException("This should not happen")
      }
      newNode
    } else {
      val newNode = visitInputs(rel)
      mapDigestToNewNode.put(digest, newNode)
      newNode
    }
  }
}

实现的方式就是记录每个digest对应的newNode，当可以复用时，那么直接返回该复用digest对应的RelNode(替换了原先的digest相同，对象不同的RelNode)，这样整棵树中可复用的节点又重新合并了。