[SPARK-27351][SQL] Wrong outputRows estimation after AggregateEstimation wit…

## What changes were proposed in this pull request?
The upper bound of group-by columns row number is to multiply distinct counts of group-by columns. However, column with only null value will cause the output row number to be 0 which is incorrect.
Ex:
col1 (distinct: 2, rowCount 2)
col2 (distinct: 0, rowCount 2)
=> group by col1, col2
Actual: output rows: 0
Expected: output rows: 2

## How was this patch tested?
According unit test has been added, plus manual test has been done in our tpcds benchmark environement.

Closes apache#24286 from pengbo/master.

Lead-authored-by: pengbo <[email protected]>
Co-authored-by: mingbo_pb <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: 2 people

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/statsEstimation/AggregateEstimation.scala

@@ -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 distinctCount = columnStat.distinctCount.get
+          val distinctValue: BigInt = if (distinctCount == 0 && columnStat.nullCount.get > 0) {
+            1
+          } else {
+            distinctCount
+          }
+          res * distinctValue
+        })
 
       outputRows = if (agg.groupingExpressions.isEmpty) {
         // If there's no group-by columns, the output is a single row containing values of aggregate

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/statsEstimation/AggregateEstimationSuite.scala

@@ -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)
@@ -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(