Fix HyperUniquesAggregatorFactory.estimateCardinality null handling to respect output type

processing/src/main/java/org/apache/druid/query/aggregation/hyperloglog/HyperUniquesAggregatorFactory.java

@@ -58,17 +58,13 @@ public class HyperUniquesAggregatorFactory extends AggregatorFactory
 {
   public static Object estimateCardinality(@Nullable Object object, boolean round)
   {
-    if (object == null) {
-      return 0;
-    }
-
     final HyperLogLogCollector collector = (HyperLogLogCollector) object;
 
-    // Avoid ternary, it causes estimateCardinalityRound to be cast to double.
+    // Avoid ternary for round check as it causes estimateCardinalityRound to be cast to double.
     if (round) {
-      return collector.estimateCardinalityRound();
+      return collector == null ? 0L : collector.estimateCardinalityRound();
     } else {
-      return collector.estimateCardinality();
+      return collector == null ? 0d : collector.estimateCardinality();
     }
   }
 

processing/src/test/java/org/apache/druid/query/SchemaEvolutionTest.java

@@ -241,7 +241,7 @@ public void testHyperUniqueEvolutionTimeseries(boolean doVectorize)
 
     // index1 has no "uniques" column
     Assert.assertEquals(
-        timeseriesResult(ImmutableMap.of("uniques", 0)),
+        timeseriesResult(ImmutableMap.of("uniques", 0d)),
         runQuery(query, factory, ImmutableList.of(index1))
     );
 

processing/src/test/java/org/apache/druid/query/aggregation/hyperloglog/HyperUniquesAggregatorFactoryTest.java

@@ -30,6 +30,7 @@
 import org.junit.Assert;
 import org.junit.Test;
 
+import java.nio.ByteBuffer;
 import java.util.Comparator;
 import java.util.Random;
 
@@ -173,6 +174,30 @@ public void testCompareToShouldBehaveConsistentlyWithEstimatedCardinalitiesEvenI
     }
   }
 
+  @Test
+  public void testEstimateCardinalityForZeroCardinality()
+  {
+    HyperLogLogCollector emptyHyperLogLogCollector = HyperUniquesBufferAggregator.doGet(
+        ByteBuffer.allocate(HyperLogLogCollector.getLatestNumBytesForDenseStorage()),
+        0
+    );
+
+    Assert.assertEquals(0L, HyperUniquesAggregatorFactory.estimateCardinality(null, true));
+    Assert.assertEquals(0d, HyperUniquesAggregatorFactory.estimateCardinality(null, false));
+
+    Assert.assertEquals(0L, HyperUniquesAggregatorFactory.estimateCardinality(emptyHyperLogLogCollector, true));
+    Assert.assertEquals(0d, HyperUniquesAggregatorFactory.estimateCardinality(emptyHyperLogLogCollector, false));
+
+    Assert.assertEquals(
+        HyperUniquesAggregatorFactory.estimateCardinality(emptyHyperLogLogCollector, true).getClass(),
+        HyperUniquesAggregatorFactory.estimateCardinality(null, true).getClass()
+    );
+    Assert.assertEquals(
+        HyperUniquesAggregatorFactory.estimateCardinality(emptyHyperLogLogCollector, false).getClass(),
+        HyperUniquesAggregatorFactory.estimateCardinality(null, false).getClass()
+    );
+  }
+
   @Test
   public void testSerde() throws Exception
   {

processing/src/test/java/org/apache/druid/query/timeseries/TimeseriesQueryRunnerTest.java

@@ -2568,7 +2568,7 @@ public void testTimeseriesWithTimestampResultFieldContextForArrayResponse()
           );
           Assert.assertEquals(
               0.0D,
-              NullHandling.sqlCompatible() ? (Double) result[4] : (Integer) result[4],
+              (Double) result[4],
               0.02
           );
           Assert.assertEquals(
@@ -2581,7 +2581,7 @@ public void testTimeseriesWithTimestampResultFieldContextForArrayResponse()
               result[3]
           );
           Assert.assertEquals(
-              (Integer) result[4],
+              (Double) result[4],
               0.0,
               0.02
           );

processing/src/test/java/org/apache/druid/query/topn/TopNQueryRunnerTest.java

@@ -647,15 +647,15 @@ public void testTopNOverMissingUniques()
                 Arrays.<Map<String, Object>>asList(
                     ImmutableMap.<String, Object>builder()
                         .put("market", "spot")
-                        .put("uniques", 0)
+                        .put("uniques", 0d)
                         .build(),
                     ImmutableMap.<String, Object>builder()
                         .put("market", "total_market")
-                        .put("uniques", 0)
+                        .put("uniques", 0d)
                         .build(),
                     ImmutableMap.<String, Object>builder()
                         .put("market", "upfront")
-                        .put("uniques", 0)
+                        .put("uniques", 0d)
                         .build()
                 )
             )