1、背景

由于多表join场景下和旅行商问题场景非常相似，故可以用DP来解决。

目前Spark CBO中的JoinReorders只会用DP处理默认深度<=12的join场景，而在[SPARK-27714]中提出了可以用类似于Postgre SQL的GEQO来实现深度较深场景下的JoinReorder（非本文重点，会在后续文章中分析实现）。

2、Code

CostBasedJoinReorder入口，只匹配inner join，然后reorder

reorder中树的深度在(2,`spark.sql.cbo.joinReorder.dp.threshold`]范围内，才会进入dp，否则直接返回(说好的决策树呢？？)

dp核心思想参考了论文http://www.inf.ed.ac.uk/teaching/courses/adbs/AccessPath.pdf

在multi-way join中，由低level到高level逐步推算并保证Cost最小。

def search(
      conf: SQLConf,
      items: Seq[LogicalPlan],
      conditions: Set[Expression],
      output: Seq[Attribute]): LogicalPlan = {

    val startTime = System.nanoTime()
    // Level i maintains all found plans for i + 1 items.
    // Create the initial plans: each plan is a single item with zero cost.
    val itemIndex = items.zipWithIndex
    val foundPlans = mutable.Buffer[JoinPlanMap](itemIndex.map {
      case (item, id) => Set(id) -> JoinPlan(Set(id), item, Set.empty, Cost(0, 0))
    }.toMap)

    // Build filters from the join graph to be used by the search algorithm.
    val filters = JoinReorderDPFilters.buildJoinGraphInfo(conf, items, conditions, itemIndex)

    // Build plans for next levels until the last level has only one plan. This plan contains
    // all items that can be joined, so there's no need to continue.
    val topOutputSet = AttributeSet(output)
    while (foundPlans.size < items.length) {
      // Build plans for the next level.
      foundPlans += searchLevel(foundPlans, conf, conditions, topOutputSet, filters)
    }

    val durationInMs = (System.nanoTime() - startTime) / (1000 * 1000)
    logDebug(s"Join reordering finished. Duration: $durationInMs ms, number of items: " +
      s"${items.length}, number of plans in memo: ${foundPlans.map(_.size).sum}")

    // The last level must have one and only one plan, because all items are joinable.
    assert(foundPlans.size == items.length && foundPlans.last.size == 1)
    foundPlans.last.head._2.plan match {
      case p @ Project(projectList, j: Join) if projectList != output =>
        assert(topOutputSet == p.outputSet)
        // Keep the same order of final output attributes.
        p.copy(projectList = output)
      case finalPlan =>
        finalPlan
    }
  }

注释中有一个例子：

继续看其核心方法searchLevel->buildJoin

star-join优化：

if star-join filter is enabled, allow the following combinations:
      1) (oneJoinPlan U otherJoinPlan) is a subset of star-join
      2) star-join is a subset of (oneJoinPlan U otherJoinPlan)
      3) (oneJoinPlan U otherJoinPlan) is a subset of non star-join

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