上一章生成的Physical Plan 还需要经过prepareForExecution这一步，做执行前的一些准备工作，代码如下：
/ executedPlan should not be used to initialize any SparkPlan. It should be
// only used for execution.
lazy val executedPlan: SparkPlan = prepareForExecution(sparkPlan)
/**
* Prepares a planned [[SparkPlan]] for execution by inserting shuffle operations and internal
* row format conversions as needed.
*/
protected def prepareForExecution(plan: SparkPlan): SparkPlan = {
preparations.foldLeft(plan) { case (sp, rule) => rule.apply(sp) }
}
看到上面的 foldLeft 是不是很熟悉，大家应该明白和前面是一个套路了，就是将规则遍历应用到plan。
直接看preparations，注释也写的很清楚：
/** A sequence of rules that will be applied in order to the physical plan before execution. */
protected def preparations: Seq[Rule[SparkPlan]] = Seq(
python.ExtractPythonUDFs,
PlanSubqueries(sparkSession),
EnsureRequirements(sparkSession.sessionState.conf),
CollapseCodegenStages(sparkSession.sessionState.conf),
ReuseExchange(sparkSession.sessionState.conf),
ReuseSubquery(sparkSession.sessionState.conf))
rule并不是很多，其中EnsureRequirements是很重要的，下面着重讲一下：
apply 入口：
def apply(plan: SparkPlan): SparkPlan = plan.transformUp {
// TODO: remove this after we create a physical operator for `RepartitionByExpression`.
case operator @ ShuffleExchangeExec(upper: HashPartitioning, child, _) =>
child.outputPartitioning match {
case lower: HashPartitioning if upper.semanticEquals(lower) => child
case _ => operator
}
case operator: SparkPlan =>
// 执行 ensureDistributionAndOrdering
ensureDistributionAndOrdering(reorderJoinPredicates(operator))
}
private def ensureDistributionAndOrdering(operator: SparkPlan): SparkPlan = {
val requiredChildDistributions: Seq[Distribution] = operator.requiredChildDistribution
val requiredChildOrderings: Seq[Seq[SortOrder]] = operator.requiredChildOrdering
var children: Seq[SparkPlan] = operator.children
assert(requiredChildDistributions.length == children.length)
assert(requiredChildOrderings.length == children.length)
// Ensure that the operator's children satisfy their output distribution requirements.
// children的实际输出分布(其实就是partitioning)满足要求的输出分布
children = children.zip(requiredChildDistributions).map {
//满足，直接返回
case (child, distribution) if child.outputPartitioning.satisfies(distribution) =>
child
// 广播单独处理
case (child, BroadcastDistribution(mode)) =>
BroadcastExchangeExec(mode, child)
case (child, distribution) =>
val numPartitions = distribution.requiredNumPartitions
.getOrElse(defaultNumPreShufflePartitions)
// 做一次shuffle(可以认为是重新分区,改变分布)，满足需求。
ShuffleExchangeExec(distribution.createPartitioning(numPartitions), child)
}
// Get the indexes of children which have specified distribution requirements and need to have
// same number of partitions.
// 过滤有指定分布需求的children
val childrenIndexes = requiredChildDistributions.zipWithIndex.filter {
case (UnspecifiedDistribution, _) => false
case (_: BroadcastDistribution, _) => false
case _ => true
}.map(_._2)
// children 的 partition 数量
val childrenNumPartitions =
childrenIndexes.map(children(_).outputPartitioning.numPartitions).toSet
if (childrenNumPartitions.size > 1) {
// Get the number of partitions which is explicitly required by the distributions.
// children 的分布需要的partition数量
val requiredNumPartitions = {
val numPartitionsSet = childrenIndexes.flatMap {
index => requiredChildDistributions(index).requiredNumPartitions
}.toSet
assert(numPartitionsSet.size <= 1,
s"$operator have incompatible requirements of the number of partitions for its children")
numPartitionsSet.headOption
}
// partition 数量目标
val targetNumPartitions = requiredNumPartitions.getOrElse(childrenNumPartitions.max)
// 指定分布的children的partition数量全部统一为 targetNumPartitions
children = children.zip(requiredChildDistributions).zipWithIndex.map {
case ((child, distribution), index) if childrenIndexes.contains(index) =>
if (child.outputPartitioning.numPartitions == targetNumPartitions) {
// 符合目标，直接返回
child
} else {
// 不符合目标，shuffle
val defaultPartitioning = distribution.createPartitioning(targetNumPartitions)
child match {
// If child is an exchange, we replace it with a new one having defaultPartitioning.
case ShuffleExchangeExec(_, c, _) => ShuffleExchangeExec(defaultPartitioning, c)
case _ => ShuffleExchangeExec(defaultPartitioning, child)
}
}
case ((child, _), _) => child
}
}
// 添加ExchangeCoordinator，这个单独讲
// Now, we need to add ExchangeCoordinator if necessary.
// Actually, it is not a good idea to add ExchangeCoordinators while we are adding Exchanges.
// However, with the way that we plan the query, we do not have a place where we have a
// global picture of all shuffle dependencies of a post-shuffle stage. So, we add coordinator
// at here for now.
// Once we finish https://issues.apache.org/jira/browse/SPARK-10665,
// we can first add Exchanges and then add coordinator once we have a DAG of query fragments.
children = withExchangeCoordinator(children, requiredChildDistributions)
// 如果有sort的需求，则加上SortExec
// Now that we've performed any necessary shuffles, add sorts to guarantee output orderings:
children = children.zip(requiredChildOrderings).map { case (child, requiredOrdering) =>
// If child.outputOrdering already satisfies the requiredOrdering, we do not need to sort.
if (SortOrder.orderingSatisfies(child.outputOrdering, requiredOrdering)) {
child
} else {
SortExec(requiredOrdering, global = false, child = child)
}
}
operator.withNewChildren(children)
}
withExchangeCoordinator的逻辑如下,他用来调节多个spark plan的数据分布：
/**
* Adds [[ExchangeCoordinator]] to [[ShuffleExchangeExec]]s if adaptive query execution is enabled
* and partitioning schemes of these [[ShuffleExchangeExec]]s support [[ExchangeCoordinator]].
*/
private def withExchangeCoordinator(
children: Seq[SparkPlan],
requiredChildDistributions: Seq[Distribution]): Seq[SparkPlan] = {
// 判断是否需要添加 ExchangeCoordinator
val supportsCoordinator =
if (children.exists(_.isInstanceOf[ShuffleExchangeExec])) {
// Right now, ExchangeCoordinator only support HashPartitionings.
children.forall {
// 条件1 children中有ShuffleExchangeExec且分区为HashPartitioning
case e @ ShuffleExchangeExec(hash: HashPartitioning, _, _) => true
case child =>
child.outputPartitioning match {
case hash: HashPartitioning => true
case collection: PartitioningCollection =>
collection.partitionings.forall(_.isInstanceOf[HashPartitioning])
case _ => false
}
}
} else {
// In this case, although we do not have Exchange operators, we may still need to
// shuffle data when we have more than one children because data generated by
// these children may not be partitioned in the same way.
// Please see the comment in withCoordinator for more details.
// 条件2 分布为ClusteredDistribution 或 HashClusteredDistribution
val supportsDistribution = requiredChildDistributions.forall { dist =>
dist.isInstanceOf[ClusteredDistribution] || dist.isInstanceOf[HashClusteredDistribution]
}
children.length > 1 && supportsDistribution
}
val withCoordinator =
// adaptiveExecutionEnabled 且 符合条件1或2
if (adaptiveExecutionEnabled && supportsCoordinator) {
val coordinator =
new ExchangeCoordinator(
children.length,
targetPostShuffleInputSize,
minNumPostShufflePartitions)
children.zip(requiredChildDistributions).map {
case (e: ShuffleExchangeExec, _) =>
// 条件1， 直接添加 coordinator
// This child is an Exchange, we need to add the coordinator.
e.copy(coordinator = Some(coordinator))
case (child, distribution) =>
// If this child is not an Exchange, we need to add an Exchange for now.
// Ideally, we can try to avoid this Exchange. However, when we reach here,
// there are at least two children operators (because if there is a single child
// and we can avoid Exchange, supportsCoordinator will be false and we
// will not reach here.). Although we can make two children have the same number of
// post-shuffle partitions. Their numbers of pre-shuffle partitions may be different.
// For example, let's say we have the following plan
// Join
// / \
// Agg Exchange
// / \
// Exchange t2
// /
// t1
// In this case, because a post-shuffle partition can include multiple pre-shuffle
// partitions, a HashPartitioning will not be strictly partitioned by the hashcodes
// after shuffle. So, even we can use the child Exchange operator of the Join to
// have a number of post-shuffle partitions that matches the number of partitions of
// Agg, we cannot say these two children are partitioned in the same way.
// Here is another case
// Join
// / \
// Agg1 Agg2
// / \
// Exchange1 Exchange2
// / \
// t1 t2
// In this case, two Aggs shuffle data with the same column of the join condition.
// After we use ExchangeCoordinator, these two Aggs may not be partitioned in the same
// way. Let's say that Agg1 and Agg2 both have 5 pre-shuffle partitions and 2
// post-shuffle partitions. It is possible that Agg1 fetches those pre-shuffle
// partitions by using a partitionStartIndices [0, 3]. However, Agg2 may fetch its
// pre-shuffle partitions by using another partitionStartIndices [0, 4].
// So, Agg1 and Agg2 are actually not co-partitioned.
//
// It will be great to introduce a new Partitioning to represent the post-shuffle
// partitions when one post-shuffle partition includes multiple pre-shuffle partitions.
// 条件2，这个child的children虽然分区数目一样，但是不一定是同一种分区方式，所以加上coordinator
val targetPartitioning = distribution.createPartitioning(defaultNumPreShufflePartitions)
assert(targetPartitioning.isInstanceOf[HashPartitioning])
ShuffleExchangeExec(targetPartitioning, child, Some(coordinator))
}
} else {
// If we do not need ExchangeCoordinator, the original children are returned.
children
}
withCoordinator
}
如下所示，adaptiveExecutionEnabled 默认是false的，所以ExchangeCoordinator默认是关闭的，等下一章我们会详细讲解它的执行逻辑，在这里大家先有个印象就好。
`val ADAPTIVE_EXECUTION_ENABLED = buildConf("spark.sql.adaptive.enabled")`
.doc("When true, enable adaptive query execution.")
.booleanConf
`.createWithDefault(false)`
总结一下，Preparations 会比较 children的实际输出分布和需求输出分布的不同(比较partitioning)，然后添加ShuffleExchangeExec、ExchangeCoordinator、SortExec做转化处理，使其匹配。
下面给一个例子，代码如下：
val df = spark.read.json("examples/src/main/resources/test.json")
df.createOrReplaceTempView("A")
spark.sql("SELECT A.B FROM A ORDER BY A.B").collectAsList()
执行到prepare时断点调试一下：
屏幕快照 2018-08-15 下午3.40.56
requireChildDistribution 是 OrderedDistribution, 但 child的outputPartitioning是null，所以会触发添加ShuffleExchangeExec的逻辑。
OrderedDistribution代码如下，会创建RangePartitioning：
case class OrderedDistribution(ordering: Seq[SortOrder]) extends Distribution {
require(
ordering != Nil,
"The ordering expressions of an OrderedDistribution should not be Nil. " +
"An AllTuples should be used to represent a distribution that only has " +
"a single partition.")
override def requiredNumPartitions: Option[Int] = None
override def createPartitioning(numPartitions: Int): Partitioning = {
RangePartitioning(ordering, numPartitions)
}
}
执行前后结果对比，和预期相符合：
执行前：
Sort [B#6 ASC NULLS FIRST], true, 0
+- FileScan json [B#6] Batched: false, Format: JSON, Location: InMemoryFileIndex[file:/Users/neal/github/spark2.3/spark/examples/src/main/resources/test.json], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<B:string>
执行后：
*(2) Sort [B#6 ASC NULLS FIRST], true, 0
+- Exchange rangepartitioning(B#6 ASC NULLS FIRST, 200)
+- *(1) FileScan json [B#6] Batched: false, Format: JSON, Location: InMemoryFileIndex[file:/Users/neal/github/spark2.3/spark/examples/src/main/resources/test.json], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<B:string>
