diff --git a/indexing-service/src/main/java/org/apache/druid/indexing/input/DruidInputSource.java b/indexing-service/src/main/java/org/apache/druid/indexing/input/DruidInputSource.java
index 4388c705fb4f..f10e40b17f46 100644
--- a/indexing-service/src/main/java/org/apache/druid/indexing/input/DruidInputSource.java
+++ b/indexing-service/src/main/java/org/apache/druid/indexing/input/DruidInputSource.java
@@ -65,6 +65,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
@@ -78,6 +79,31 @@ public class DruidInputSource extends AbstractInputSource implements SplittableI
 {
   private static final Logger LOG = new Logger(DruidInputSource.class);
 
+  /**
+   * A Comparator that orders {@link WindowedSegmentId} mainly by segmentId (which is important), and then by intervals
+   * (which is arbitrary, and only here for totality of ordering).
+   */
+  private static final Comparator<WindowedSegmentId> WINDOWED_SEGMENT_ID_COMPARATOR =
+      Comparator.comparing(WindowedSegmentId::getSegmentId)
+                .thenComparing(windowedSegmentId -> windowedSegmentId.getIntervals().size())
+                .thenComparing(
+                    (WindowedSegmentId a, WindowedSegmentId b) -> {
+                      // Same segmentId, same intervals list size. Compare each interval.
+                      int cmp = 0;
+
+                      for (int i = 0; i < a.getIntervals().size(); i++) {
+                        cmp = Comparators.intervalsByStartThenEnd()
+                                         .compare(a.getIntervals().get(i), b.getIntervals().get(i));
+
+                        if (cmp != 0) {
+                          return cmp;
+                        }
+                      }
+
+                      return cmp;
+                    }
+                );
+
   private final String dataSource;
   // Exactly one of interval and segmentIds should be non-null. Typically 'interval' is specified directly
   // by the user creating this firehose and 'segmentIds' is used for sub-tasks if it is split for parallel
@@ -313,6 +339,8 @@ public static Iterator<InputSplit<List<WindowedSegmentId>>> createSplits(
     //noinspection ConstantConditions
     return Iterators.transform(
         convertedSplitHintSpec.split(
+            // segmentIdToSize is sorted by segment ID; useful for grouping up segments from the same time chunk into
+            // the same input split.
             segmentIdToSize.keySet().iterator(),
             segmentId -> new InputFileAttribute(
                 Preconditions.checkNotNull(segmentIdToSize.get(segmentId), "segment size for [%s]", segmentId)
@@ -322,7 +350,10 @@ public static Iterator<InputSplit<List<WindowedSegmentId>>> createSplits(
     );
   }
 
-  private static Map<WindowedSegmentId, Long> createWindowedSegmentIdFromTimeline(
+  /**
+   * Returns a map of {@link WindowedSegmentId} to size, sorted by {@link WindowedSegmentId#getSegmentId()}.
+   */
+  private static SortedMap<WindowedSegmentId, Long> createWindowedSegmentIdFromTimeline(
       List<TimelineObjectHolder<String, DataSegment>> timelineHolders
   )
   {
@@ -335,9 +366,9 @@ private static Map<WindowedSegmentId, Long> createWindowedSegmentIdFromTimeline(
         ).addInterval(holder.getInterval());
       }
     }
-    // It is important to create this map after windowedSegmentIds is completely filled
-    // because WindowedSegmentId can be updated.
-    Map<WindowedSegmentId, Long> segmentSizeMap = new HashMap<>();
+    // It is important to create this map after windowedSegmentIds is completely filled, because WindowedSegmentIds
+    // can be updated while being constructed. (Intervals are added.)
+    SortedMap<WindowedSegmentId, Long> segmentSizeMap = new TreeMap<>(WINDOWED_SEGMENT_ID_COMPARATOR);
     windowedSegmentIds.forEach((segment, segmentId) -> segmentSizeMap.put(segmentId, segment.getSize()));
     return segmentSizeMap;
   }