ulysses-you commented on a change in pull request #30368:
URL: https://github.com/apache/spark/pull/30368#discussion_r524034982
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File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
##########
@@ -1452,11 +1452,27 @@ object PushPredicateThroughJoin extends
Rule[LogicalPlan] with PredicateHelper {
}
/**
- * Combines two adjacent [[Limit]] operators into one, merging the
- * expressions into one single expression.
+ * 1. Eliminate [[Limit]] operators if it's child max row <= limit.
+ * 2. Combines two adjacent [[Limit]] operators into one, merging the
+ * expressions into one single expression.
*/
-object CombineLimits extends Rule[LogicalPlan] {
- def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+object EliminateLimits extends Rule[LogicalPlan] {
+ private def canEliminate(limitExpr: Expression, child: LogicalPlan): Boolean
= {
+ // We skip such case that Sort is after Limit since
+ // SparkStrategies will convert them to TakeOrderedAndProjectExec
+ val skipEliminate = child match {
+ case Sort(_, true, _) => true
Review comment:
It's a good question. I make a simple benchmark for this case.
```
runBenchmark("Sort and Limit") {
val N = 10000
val benchmark = new Benchmark("benchmark sort and limit", N)
benchmark.addCase("TakeOrderedAndProject", 3) { _ =>
spark.range(N).toDF("c").sort("c").take(20000)
}
benchmark.addCase("Sort And Limit", 3) { _ =>
withSQLConf("spark.sql.execution.topKSortFallbackThreshold" -> "-1")
{
spark.range(N).toDF("c").sort("c").take(20000)
}
}
benchmark.addCase("Sort", 3) { _ =>
spark.range(N).toDF("c").sort("c").collect()
}
benchmark.run()
}
```
And the result is:
```
================================================================================================
Sort and Limit
================================================================================================
Java HotSpot(TM) 64-Bit Server VM 1.8.0_191-b12 on Mac OS X 10.15.6
Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
benchmark sort and limit: Best Time(ms) Avg Time(ms)
Stdev(ms) Rate(M/s) Per Row(ns) Relative
------------------------------------------------------------------------------------------------------------------------
TakeOrderedAndProject 181 210
29 0.1 18148.6 1.0X
Sort And Limit 90 109
16 0.1 8996.1 2.0X
Sort 57 63
9 0.2 5678.5 3.2X
```
Seems `TakeOrderedAndProject` have the redundant loop in this case.
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