jihoonson commented on a change in pull request #5913: Move Caching Cluster 
Client to java streams and allow parallel intermediate merges
URL: https://github.com/apache/incubator-druid/pull/5913#discussion_r219652313
 
 

 ##########
 File path: server/src/main/java/io/druid/client/CachingClusteredClient.java
 ##########
 @@ -162,34 +184,75 @@ public CachingClusteredClient(
     return new SpecificQueryRunnable<>(queryPlus, 
responseContext).run(timelineConverter);
   }
 
-  @Override
-  public <T> QueryRunner<T> getQueryRunnerForSegments(final Query<T> query, 
final Iterable<SegmentDescriptor> specs)
+  private <T> QueryRunner<T> runAndMergeWithTimelineChange(
+      final Query<T> query,
+      final UnaryOperator<TimelineLookup<String, ServerSelector>> 
timelineConverter
+  )
   {
-    return new QueryRunner<T>()
-    {
-      @Override
-      public Sequence<T> run(final QueryPlus<T> queryPlus, final Map<String, 
Object> responseContext)
-      {
-        return CachingClusteredClient.this.run(
+    final OptionalLong mergeBatch = 
QueryContexts.getIntermediateMergeBatchThreshold(query);
+
+    if (mergeBatch.isPresent()) {
+      final QueryRunnerFactory<T, Query<T>> queryRunnerFactory = 
conglomerate.findFactory(query);
+      final QueryToolChest<T, Query<T>> toolChest = 
queryRunnerFactory.getToolchest();
+      return (queryPlus, responseContext) -> {
+        final Stream<? extends Sequence<T>> sequences = run(
             queryPlus,
             responseContext,
-            timeline -> {
-              final VersionedIntervalTimeline<String, ServerSelector> 
timeline2 =
-                  new VersionedIntervalTimeline<>(Ordering.natural());
-              for (SegmentDescriptor spec : specs) {
-                final PartitionHolder<ServerSelector> entry = 
timeline.findEntry(spec.getInterval(), spec.getVersion());
-                if (entry != null) {
-                  final PartitionChunk<ServerSelector> chunk = 
entry.getChunk(spec.getPartitionNumber());
-                  if (chunk != null) {
-                    timeline2.add(spec.getInterval(), spec.getVersion(), 
chunk);
-                  }
-                }
-              }
-              return timeline2;
-            }
+            timelineConverter
+        );
+        return MergeWorkTask.parallelMerge(
+            sequences.parallel(),
+            sequenceStream ->
+                new FluentQueryRunnerBuilder<>(toolChest)
+                    .create(
+                        queryRunnerFactory.mergeRunners(
 
 Review comment:
   In addition to @gianm's comment, I wonder the same goal can be achieved by 
using the processing threads of brokers instead of adding a new ForkJoinPool. 
The broker already has its processing thread pool, but it's not being used. I 
think this might be better because we can avoid adding a new configuration for 
ForkJoinPool which looks similar to that for the existing processing thread 
pool.

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