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[SPARK-38832][SQL] Remove unnecessary distinct in aggregate expression by distinctKeys #36117

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[SPARK-38832][SQL] Remove unnecessary distinct in aggregate expression by distinctKeys #36117

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7ebbafa
Remove unnecessary distinct in aggregate expression by distinctKeys
ulysses-you Apr 8, 2022
2ffc5ee
address comment
ulysses-you Apr 11, 2022
dc67cfd
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ulysses-you Apr 12, 2022
c16e995
address comment
ulysses-you Apr 12, 2022
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22 changes: 17 additions & 5 deletions sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -411,14 +411,26 @@ abstract class Optimizer(catalogManager: CatalogManager)
}

/**
* Remove useless DISTINCT for MAX and MIN.
* Remove useless DISTINCT:
* 1. For some aggregate expression, e.g.: MAX and MIN.
* 2. If the distinct semantics is guaranteed by child.
*
* This rule should be applied before RewriteDistinctAggregates.
*/
object EliminateDistinct extends Rule[LogicalPlan] {
override def apply(plan: LogicalPlan): LogicalPlan = plan.transformAllExpressionsWithPruning(
_.containsPattern(AGGREGATE_EXPRESSION)) {
case ae: AggregateExpression if ae.isDistinct && isDuplicateAgnostic(ae.aggregateFunction) =>
ae.copy(isDistinct = false)
override def apply(plan: LogicalPlan): LogicalPlan = plan.transformWithPruning(
_.containsPattern(AGGREGATE)) {
case agg: Aggregate =>
agg.transformExpressionsWithPruning(_.containsPattern(AGGREGATE_EXPRESSION)) {
case ae: AggregateExpression if ae.isDistinct &&
isDuplicateAgnostic(ae.aggregateFunction) =>
ae.copy(isDistinct = false)

case ae: AggregateExpression if ae.isDistinct &&
agg.child.distinctKeys.exists(
_.subsetOf(ExpressionSet(ae.aggregateFunction.children.filterNot(_.foldable)))) =>
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@sigmod sigmod Apr 12, 2022 •
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Is it correct?

If input plan to this rule is:

SELECT a, count(distinct c) FROM (
   SELECT distinct a, b, c 
   FROM t
)
GROUP BY a

Will the added case branch rewrite the plan to

SELECT a, count(c) FROM (
   SELECT distinct a, b, c 
   FROM t
)
GROUP BY a

agg.child.distinctKeys is {a, b, c}
ExpressionSet(ae.aggregateFunction.children.filterNot(_.foldable)) is {c}.

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@ulysses-you ulysses-you Apr 12, 2022

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the distinctKeys of distinct a, b, c is ExpressionSet(a, b, c) not ExpressionSet(a), ExpressionSet(b), ExpressionSet(c)

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@sigmod sigmod Apr 12, 2022

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You're right. I forgot distinctKeys is a set of sets.

How about:

agg.child.distinctKeys.exists(
           key =>  !key.isEmpty() && 
                         key.subsetOf(ExpressionSet(ae.aggregateFunction.children.filterNot(_.foldable))))

Alternatively, we can do a require here to make sure that we never return an empty key:
https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlanDistinctKeys.scala#L32

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@ulysses-you ulysses-you Apr 12, 2022

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make sense, I add a require at LogicalPlanDistinctKeys

ae.copy(isDistinct = false)
}
}

def isDuplicateAgnostic(af: AggregateFunction): Boolean = af match {
Expand Down
18 changes: 18 additions & 0 deletions ...alyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/EliminateDistinctSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,7 @@ class EliminateDistinctSuite extends PlanTest {
}

val testRelation = LocalRelation($"a".int)
val testRelation2 = LocalRelation($"a".int, $"b".string)

Seq(
Max(_),
Expand Down Expand Up @@ -71,4 +72,21 @@ class EliminateDistinctSuite extends PlanTest {
comparePlans(Optimize.execute(query), answer)
}
}

test("SPARK-38832: Remove unnecessary distinct in aggregate expression by distinctKeys") {
val q1 = testRelation2.groupBy($"a")($"a")
.rebalance().groupBy()(countDistinct($"a") as "x", sumDistinct($"a") as "y").analyze
val r1 = testRelation2.groupBy($"a")($"a")
.rebalance().groupBy()(count($"a") as "x", sum($"a") as "y").analyze
comparePlans(Optimize.execute(q1), r1)

// not a subset of distinct attr
val q2 = testRelation2.groupBy($"a", $"b")($"a", $"b")
.rebalance().groupBy()(countDistinct($"a") as "x", sumDistinct($"a") as "y").analyze
comparePlans(Optimize.execute(q2), q2)

// child distinct key is empty
val q3 = testRelation2.groupBy($"a")(countDistinct($"a") as "x").analyze
comparePlans(Optimize.execute(q3), q3)
}
}