Fix NPE when subquery returns empty result

processing/src/main/java/io/druid/segment/incremental/IncrementalIndexStorageAdapter.java

@@ -126,8 +126,13 @@ public class IncrementalIndexStorageAdapter implements StorageAdapter
   @Override
   public Iterable<Cursor> makeCursors(final Filter filter, final Interval interval, final QueryGranularity gran)
   {
+    if (index.isEmpty()) {
+      return ImmutableList.of();
+    }
+
     Interval actualIntervalTmp = interval;
 
+
     final Interval dataInterval = new Interval(getMinTime().getMillis(), gran.next(getMaxTime().getMillis()));
     if (!actualIntervalTmp.overlaps(dataInterval)) {
       return ImmutableList.of();

processing/src/test/java/io/druid/query/QueryRunnerTestHelper.java

@@ -103,6 +103,9 @@ public class QueryRunnerTestHelper
   public static final QuerySegmentSpec fullOnInterval = new MultipleIntervalSegmentSpec(
       Arrays.asList(new Interval("1970-01-01T00:00:00.000Z/2020-01-01T00:00:00.000Z"))
   );
+  public static final QuerySegmentSpec emptyInterval = new MultipleIntervalSegmentSpec(
+      Arrays.asList(new Interval("2020-04-02T00:00:00.000Z/P1D"))
+  );
 
   @SuppressWarnings("unchecked")
   public static Collection<?> makeQueryRunners(

processing/src/test/java/io/druid/query/groupby/GroupByQueryRunnerTest.java

@@ -58,7 +58,9 @@ import org.joda.time.DateTime;
 import org.joda.time.DateTimeZone;
 import org.joda.time.Interval;
 import org.joda.time.Period;
+import org.junit.Assert;
 import org.junit.Before;
+import org.junit.Ignore;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.Parameterized;
@@ -910,6 +912,39 @@ public class GroupByQueryRunnerTest
     TestHelper.assertExpectedObjects(expectedResults, results, "");
   }
 
+  @Test
+  public void testEmptySubquery()
+  {
+    GroupByQuery subquery = GroupByQuery
+        .builder()
+        .setDataSource(QueryRunnerTestHelper.dataSource)
+        .setQuerySegmentSpec(QueryRunnerTestHelper.emptyInterval)
+        .setDimensions(Lists.<DimensionSpec>newArrayList(new DefaultDimensionSpec("quality", "alias")))
+        .setAggregatorSpecs(
+            Arrays.<AggregatorFactory>asList(
+                QueryRunnerTestHelper.rowsCount,
+                new LongSumAggregatorFactory("idx", "index")
+            )
+        )
+        .setGranularity(QueryRunnerTestHelper.dayGran)
+        .build();
+
+    GroupByQuery query = GroupByQuery
+        .builder()
+        .setDataSource(subquery)
+        .setQuerySegmentSpec(QueryRunnerTestHelper.firstToThird)
+        .setAggregatorSpecs(
+            Arrays.<AggregatorFactory>asList(
+                new MaxAggregatorFactory("idx", "idx")
+            )
+        )
+        .setGranularity(QueryRunnerTestHelper.dayGran)
+        .build();
+
+    Iterable<Row> results = runQuery(query);
+    Assert.assertFalse(results.iterator().hasNext());
+  }
+
   private Iterable<Row> runQuery(GroupByQuery query)
   {
     QueryToolChest<Row, GroupByQuery> toolChest = factory.getToolchest();