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[SPARK-27351][SQL] Wrong outputRows estimation after AggregateEstimation with only null value column #24286

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[SPARK-27351][SQL] Wrong outputRows estimation after AggregateEstimation with only null value column #24286

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SPARK-27351 Wrong outputRows estimation after AggregateEstimation wit…
Apr 3, 2019
7c7685c
merge upstream/master
Apr 11, 2019
aa9859a
check null value count for only null value column
pengbo Apr 11, 2019
6a9d35f
refine code to make columnStat.distinctCount.get called only once
pengbo Apr 14, 2019
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12 changes: 10 additions & 2 deletions ...ala/org/apache/spark/sql/catalyst/plans/logical/statsEstimation/AggregateEstimation.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -39,8 +39,16 @@ object AggregateEstimation {
// Multiply distinct counts of group-by columns. This is an upper bound, which assumes
// the data contains all combinations of distinct values of group-by columns.
var outputRows: BigInt = agg.groupingExpressions.foldLeft(BigInt(1))(
(res, expr) => res *
childStats.attributeStats(expr.asInstanceOf[Attribute]).distinctCount.get)
(res, expr) => {
val columnStat = childStats.attributeStats(expr.asInstanceOf[Attribute])
val distinctValue: BigInt = if (columnStat.distinctCount.get == 0 &&

@srowen srowen Apr 14, 2019

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I'm slightly concerned about performance here. Can you save the value of distinctCount.get so it isn't accessed twice, or is it not actually recomputed twice as-is?

@pengbo pengbo Apr 14, 2019 •
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Done.
"childStats.distinctCount.get" is to get the value from the class variable/Option. The cost seems to be negligible, but it's better to save it by one variable.

columnStat.nullCount.get > 0) {
1
} else {
columnStat.distinctCount.get
}
res * distinctValue
})

outputRows = if (agg.groupingExpressions.isEmpty) {
// If there's no group-by columns, the output is a single row containing values of aggregate
Expand Down
12 changes: 11 additions & 1 deletion ...c/test/scala/org/apache/spark/sql/catalyst/statsEstimation/AggregateEstimationSuite.scala
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Expand Up @@ -38,7 +38,9 @@ class AggregateEstimationSuite extends StatsEstimationTestBase with PlanTest {
attr("key22") -> ColumnStat(distinctCount = Some(2), min = Some(10), max = Some(20),
nullCount = Some(0), avgLen = Some(4), maxLen = Some(4)),
attr("key31") -> ColumnStat(distinctCount = Some(0), min = None, max = None,
nullCount = Some(0), avgLen = Some(4), maxLen = Some(4))
nullCount = Some(0), avgLen = Some(4), maxLen = Some(4)),
attr("key32") -> ColumnStat(distinctCount = Some(0), min = None, max = None,
nullCount = Some(4), avgLen = Some(4), maxLen = Some(4))
))

private val nameToAttr: Map[String, Attribute] = columnInfo.map(kv => kv._1.name -> kv._1)
Expand Down Expand Up @@ -116,6 +118,14 @@ class AggregateEstimationSuite extends StatsEstimationTestBase with PlanTest {
expectedOutputRowCount = 0)
}

test("group-by column with only null value") {
checkAggStats(
tableColumns = Seq("key22", "key32"),
tableRowCount = 6,
groupByColumns = Seq("key22", "key32"),
expectedOutputRowCount = nameToColInfo("key22")._2.distinctCount.get)
}

test("non-cbo estimation") {
val attributes = Seq("key12").map(nameToAttr)
val child = StatsTestPlan(
Expand Down