Fix NullType

sql-plugin/src/main/scala/org/apache/spark/sql/rapids/aggregate/GpuHyperLogLogPlusPlus.scala

@@ -21,7 +21,7 @@ import scala.collection.immutable.Seq
 import ai.rapids.cudf
 import ai.rapids.cudf.{DType, GroupByAggregation, ReductionAggregation}
 import com.nvidia.spark.rapids._
-import com.nvidia.spark.rapids.Arm.withResourceIfAllowed
+import com.nvidia.spark.rapids.Arm.{withResource, withResourceIfAllowed}
 import com.nvidia.spark.rapids.RapidsPluginImplicits.ReallyAGpuExpression
 import com.nvidia.spark.rapids.jni.HyperLogLogPlusPlusHostUDF
 import com.nvidia.spark.rapids.shims.ShimExpression
@@ -34,11 +34,24 @@ import org.apache.spark.sql.vectorized.ColumnarBatch
 case class CudfHLLPP(override val dataType: DataType,
     precision: Int) extends CudfAggregate {
   override lazy val reductionAggregate: cudf.ColumnVector => cudf.Scalar =
-    (input: cudf.ColumnVector) => input.reduce(
-      ReductionAggregation.hostUDF(
-        HyperLogLogPlusPlusHostUDF.createHLLPPHostUDF(
-          HyperLogLogPlusPlusHostUDF.AggregationType.Reduction, precision)),
-      DType.STRUCT)
+    (input: cudf.ColumnVector) => {
+      if (input.getNullCount == input.getRowCount) {
+        // For NullType column or all values are null,
+        // return a struct scalar: struct(0L, 0L, ..., 0L)
+        val numCols = (1 << precision) / 10 + 1
+        withResource(cudf.ColumnVector.fromLongs(0L)) { cv =>
+          // Underlying uses deep-copy, so we can reuse this `cv` and fill multiple times.
+          val cvs: Array[cudf.ColumnView] = Array.fill(numCols)(cv)
+          cudf.Scalar.structFromColumnViews(cvs: _*)
+        }
+      } else {
+        input.reduce(
+          ReductionAggregation.hostUDF(
+            HyperLogLogPlusPlusHostUDF.createHLLPPHostUDF(
+              HyperLogLogPlusPlusHostUDF.AggregationType.Reduction, precision)),
+          DType.STRUCT)
+      }
+    }
   override lazy val groupByAggregate: GroupByAggregation =
     GroupByAggregation.hostUDF(
       HyperLogLogPlusPlusHostUDF.createHLLPPHostUDF(