Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/17240#discussion_r105823209
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/CostBasedJoinReorder.scala
---
@@ -122,46 +119,48 @@ case class CostBasedJoinReorder(conf: CatalystConf)
extends Rule[LogicalPlan] wi
* level 3: p({A, B, C, D})
* where p({A, B, C, D}) is the final output plan.
*
- * For cost evaluation, since physical costs for operators are not
available currently, we use
- * cardinalities and sizes to compute costs.
+ * To evaluate cost for a given plan, we calculate the sum of
cardinalities for all intermediate
+ * joins in the plan.
*/
object JoinReorderDP extends PredicateHelper {
def search(
conf: CatalystConf,
items: Seq[LogicalPlan],
- conditions: Set[Expression],
- topOutput: AttributeSet): Option[LogicalPlan] = {
+ conditions: Set[Expression]): Option[LogicalPlan] = {
// 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 itemIndex = items.zipWithIndex.map(_.swap).toMap
val foundPlans = mutable.Buffer[JoinPlanMap](itemIndex.map {
- case (item, id) => Set(id) -> JoinPlan(Set(id), item, Set(), Cost(0,
0))
- }.toMap)
+ case (id, item) => Set(id) -> JoinPlan(Set(id), item, cost = 0)
+ })
- for (lev <- 1 until items.length) {
+ while (foundPlans.size < items.length && foundPlans.last.size > 1) {
// Build plans for the next level.
- foundPlans += searchLevel(foundPlans, conf, conditions, topOutput)
+ foundPlans += searchLevel(foundPlans, conf, conditions)
}
- val plansLastLevel = foundPlans(items.length - 1)
- if (plansLastLevel.isEmpty) {
- // Failed to find a plan, fall back to the original plan
- None
- } else {
- // There must be only one plan at the last level, which contains all
items.
- assert(plansLastLevel.size == 1 && plansLastLevel.head._1.size ==
items.length)
- Some(plansLastLevel.head._2.plan)
+ // Find the best plan
+ assert(foundPlans.last.size <= 1)
+ val bestJoinPlan = foundPlans.last.headOption
--- End diff --
and what if the last level has more than one entries? shall we pick the
best among them?
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