while GroupBy merging use unsorted facts in IncrementalIndex wherever possible

processing/src/main/java/io/druid/query/groupby/GroupByQueryHelper.java

@@ -43,6 +43,7 @@ import java.util.concurrent.ConcurrentLinkedQueue;
 public class GroupByQueryHelper
 {
   private static final String CTX_KEY_MAX_RESULTS = "maxResults";
+  public final static String CTX_KEY_SORT_RESULTS = "sortResults";
 
   public static <T> Pair<IncrementalIndex, Accumulator<IncrementalIndex, T>> createIndexAccumulatorPair(
       final GroupByQuery query,
@@ -81,7 +82,9 @@ public class GroupByQueryHelper
     );
     final IncrementalIndex index;
 
-    if (query.getContextBoolean("useOffheap", false)) {
+    final boolean sortResults = query.getContextValue(CTX_KEY_SORT_RESULTS, true);
+
+    if (query.getContextValue("useOffheap", false)) {
       index = new OffheapIncrementalIndex(
           // use granularity truncated min timestamp
           // since incoming truncated timestamps may precede timeStart
@@ -90,7 +93,7 @@ public class GroupByQueryHelper
           aggs.toArray(new AggregatorFactory[aggs.size()]),
           false,
           true,
-          true,
+          sortResults,
           Math.min(query.getContextValue(CTX_KEY_MAX_RESULTS, config.getMaxResults()), config.getMaxResults()),
           bufferPool
       );
@@ -103,7 +106,7 @@ public class GroupByQueryHelper
           aggs.toArray(new AggregatorFactory[aggs.size()]),
           false,
           true,
-          true,
+          sortResults,
           Math.min(query.getContextValue(CTX_KEY_MAX_RESULTS, config.getMaxResults()), config.getMaxResults())
       );
     }

processing/src/main/java/io/druid/query/groupby/GroupByQueryQueryToolChest.java

@@ -161,7 +161,18 @@ public class GroupByQueryQueryToolChest extends QueryToolChest<Row, GroupByQuery
         throw new UnsupportedOperationException("Subqueries must be of type 'group by'");
       }
 
-      final Sequence<Row> subqueryResult = mergeGroupByResults(subquery, runner, context);
+      final Sequence<Row> subqueryResult = mergeGroupByResults(
+          subquery.withOverriddenContext(
+              ImmutableMap.<String, Object>of(
+                  //setting sort to false avoids unnecessary sorting while merging results. we only need to sort
+                  //in the end when returning results to user.
+                  GroupByQueryHelper.CTX_KEY_SORT_RESULTS,
+                  false
+              )
+          ),
+          runner,
+          context
+      );
       final Set<AggregatorFactory> aggs = Sets.newHashSet();
 
       // Nested group-bys work by first running the inner query and then materializing the results in an incremental
@@ -200,7 +211,14 @@ public class GroupByQueryQueryToolChest extends QueryToolChest<Row, GroupByQuery
           .setLimitSpec(query.getLimitSpec().merge(subquery.getLimitSpec()))
           .build();
 
-      final IncrementalIndex innerQueryResultIndex = makeIncrementalIndex(innerQuery, subqueryResult);
+      final IncrementalIndex innerQueryResultIndex = makeIncrementalIndex(
+          innerQuery.withOverriddenContext(
+              ImmutableMap.<String, Object>of(
+                  GroupByQueryHelper.CTX_KEY_SORT_RESULTS, true
+              )
+          ),
+          subqueryResult
+      );
 
       //Outer query might have multiple intervals, but they are expected to be non-overlapping and sorted which
       //is ensured by QuerySegmentSpec.
@@ -253,7 +271,10 @@ public class GroupByQueryQueryToolChest extends QueryToolChest<Row, GroupByQuery
                   query.getContext()
               ).withOverriddenContext(
                   ImmutableMap.<String, Object>of(
-                      "finalize", false
+                      "finalize", false,
+                      //setting sort to false avoids unnecessary sorting while merging results. we only need to sort
+                      //in the end when returning results to user.
+                      GroupByQueryHelper.CTX_KEY_SORT_RESULTS, false
                   )
               )
               , context