populate cache without materializing results

server/src/main/java/io/druid/client/CachePopulatingQueryRunner.java

@@ -20,6 +20,8 @@
 package io.druid.client;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.base.Function;
+import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Lists;
 import com.metamx.common.guava.Accumulator;
 import com.metamx.common.guava.Sequence;
@@ -33,6 +35,7 @@ import io.druid.query.QueryRunner;
 import io.druid.query.QueryToolChest;
 import io.druid.query.SegmentDescriptor;
 
+import javax.annotation.Nullable;
 import java.util.ArrayList;
 
 public class CachePopulatingQueryRunner<T> implements QueryRunner<T>
@@ -72,26 +75,33 @@ public class CachePopulatingQueryRunner<T> implements QueryRunner<T>
 
     final boolean populateCache = Boolean.parseBoolean(query.getContextValue(CacheConfig.POPULATE_CACHE, "true"))
                                   && strategy != null
-                                  && cacheConfig.isPopulateCache()
-                                  // historical only populates distributed cache since the cache lookups are done at broker.
-                                  && !(cache instanceof MapCache);
+                                  && cacheConfig.isPopulateCache();
+
+    final Sequence<T> results = base.run(query);
+
     if (populateCache) {
-      Sequence<T> results = base.run(query);
-      Cache.NamedKey key = CacheUtil.computeSegmentCacheKey(
+      final Cache.NamedKey key = CacheUtil.computeSegmentCacheKey(
           segmentIdentifier,
           segmentDescriptor,
           strategy.computeCacheKey(query)
       );
-      ArrayList<T> resultAsList = Sequences.toList(results, new ArrayList<T>());
-      CacheUtil.populate(
-          cache,
-          mapper,
-          key,
-          Lists.transform(resultAsList, strategy.prepareForCache())
+
+      final Function cacheFn = strategy.prepareForCache();
+      return Sequences.map(
+          results,
+          new Function<T, T>()
+          {
+            @Nullable
+            @Override
+            public T apply(@Nullable T input)
+            {
+              CacheUtil.populate(cache, mapper, key, ImmutableList.of(cacheFn.apply(input)));
+              return input;
+            }
+          }
       );
-      return Sequences.simple(resultAsList);
     } else {
-      return base.run(query);
+      return results;
     }
   }