Better groupBy error messages and docs around resource limits. (#4162)

* Better groupBy error messages and docs around resource limits.

* Fix BufferGrouper test from datasketches.

* Further clarify.

docs/content/querying/groupbyquery.md

@@ -141,8 +141,11 @@ on-heap by default, but it can optionally store aggregated values off-heap.
 Query API and results are compatible between the two engines; however, there are some differences from a cluster
 configuration perspective:
 
-- groupBy v1 merges results in heap, whereas groupBy v2 merges results off-heap. As a result, optimal configuration for
-your Druid nodes may involve less heap (-Xmx, -Xms) and more direct memory (-XX:MaxDirectMemorySize).
+- groupBy v1 controls resource usage using a row-based limit (maxResults) whereas groupBy v2 uses bytes-based limits.
+In addition, groupBy v1 merges results on-heap, whereas groupBy v2 merges results off-heap. These factors mean that
+memory tuning and resource limits behave differently between v1 and v2. In particular, due to this, some queries
+that can complete successfully in one engine may exceed resource limits and fail with the other engine. See the
+"Memory tuning and resource limits" section for more details.
 - groupBy v1 imposes no limit on the number of concurrently running queries, whereas groupBy v2 controls memory usage
 by using a finite-sized merge buffer pool. By default, the number of merge buffers is 1/4 the number of processing
 threads. You can adjust this as necessary to balance concurrency and memory usage.
@@ -151,6 +154,37 @@ historical nodes.
 - groupBy v1 supports using [chunkPeriod](query-context.html) to parallelize merging on the broker, whereas groupBy v2
 ignores chunkPeriod.
 
+#### Memory tuning and resource limits
+
+When using groupBy v2, three parameters control resource usage and limits:
+
+- druid.processing.buffer.sizeBytes: size of the off-heap hash table used for aggregation, per query, in bytes. At
+most druid.processing.numMergeBuffers of these will be created at once, which also serves as an upper limit on the
+number of concurrently running groupBy queries.
+- druid.query.groupBy.maxMergingDictionarySize: size of the on-heap dictionary used when grouping on strings, per query,
+in bytes. Note that this is based on a rough estimate of the dictionary size, not the actual size.
+- druid.query.groupBy.maxOnDiskStorage: amount of space on disk used for aggregation, per query, in bytes. By default,
+this is 0, which means aggregation will not use disk.
+
+If maxOnDiskStorage is 0 (the default) then a query that exceeds either the on-heap dictionary limit, or the off-heap
+aggregation table limit, will fail with a "Resource limit exceeded" error describing the limit that was exceeded.
+
+If maxOnDiskStorage is greater than 0, queries that exceed the in-memory limits will start using disk for aggregation.
+In this case, when either the on-heap dictionary or off-heap hash table fills up, partially aggregated records will be
+sorted and flushed to disk. Then, both in-memory structures will be cleared out for further aggregation. Queries that
+then go on to exceed maxOnDiskStorage will fail with a "Resource limit exceeded" error indicating that they ran out of
+disk space.
+
+With groupBy v2, cluster operators should make sure that the off-heap hash tables and on-heap merging dictionaries
+will not exceed available memory for the maximum possible concurrent query load (given by
+druid.processing.numMergeBuffers).
+
+When using groupBy v1, all aggregation is done on-heap, and resource limits are done through the parameter
+druid.query.groupBy.maxResults. This is a cap on the maximum number of results in a result set. Queries that exceed
+this limit will fail with a "Resource limit exceeded" error indicating they exceeded their row limit. Cluster
+operators should make sure that the on-heap aggregations will not exceed available JVM heap space for the expected
+concurrent query load.
+
 #### Alternatives
 
 There are some situations where other query types may be a better choice than groupBy.

extensions-core/datasketches/src/test/java/io/druid/query/aggregation/datasketches/theta/BufferGrouperUsingSketchMergeAggregatorFactoryTest.java

@@ -75,14 +75,14 @@ public class BufferGrouperUsingSketchMergeAggregatorFactoryTest
       columnSelectorFactory.setRow(new MapBasedRow(0, ImmutableMap.<String, Object>of("sketch", sketchHolder)));
 
       for (int i = 0; i < expectedMaxSize; i++) {
-        Assert.assertTrue(String.valueOf(i), grouper.aggregate(i));
+        Assert.assertTrue(String.valueOf(i), grouper.aggregate(i).isOk());
       }
 
       updateSketch.update(3);
       columnSelectorFactory.setRow(new MapBasedRow(0, ImmutableMap.<String, Object>of("sketch", sketchHolder)));
 
       for (int i = 0; i < expectedMaxSize; i++) {
-        Assert.assertTrue(String.valueOf(i), grouper.aggregate(i));
+        Assert.assertTrue(String.valueOf(i), grouper.aggregate(i).isOk());
       }
 
       Object[] holders = Lists.newArrayList(grouper.iterator(true)).get(0).getValues();

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

@@ -256,18 +256,4 @@ public class GroupByQueryHelper
     // Don't include post-aggregators since we don't know what types they are.
     return types.build();
   }
-
-  /**
-   * Throw a {@link ResourceLimitExceededException}. Only used by groupBy v2 when accumulation resources
-   * are exceeded, triggered by false return from {@link io.druid.query.groupby.epinephelinae.Grouper#aggregate(Object)}.
-   *
-   * @return nothing will ever be returned; this return type is for your convenience, similar to
-   * Throwables.propagate in Guava.
-   */
-  public static ResourceLimitExceededException throwAccumulationResourceLimitExceededException()
-  {
-    throw new ResourceLimitExceededException(
-        "Not enough resources to execute this query. Try increasing druid.query.groupBy.maxOnDiskStorage, "
-        + "druid.query.groupBy.maxMergingDictionarySize, or druid.processing.buffer.sizeBytes.");
-  }
 }

processing/src/main/java/io/druid/query/groupby/epinephelinae/AggregateResult.java

@@ -0,0 +1,85 @@
+/*
+ * Licensed to Metamarkets Group Inc. (Metamarkets) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. Metamarkets licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package io.druid.query.groupby.epinephelinae;
+
+import java.util.Objects;
+
+public class AggregateResult
+{
+  private static final AggregateResult OK = new AggregateResult(true, null);
+
+  private final boolean ok;
+  private final String reason;
+
+  public static AggregateResult ok()
+  {
+    return OK;
+  }
+
+  public static AggregateResult failure(final String reason)
+  {
+    return new AggregateResult(false, reason);
+  }
+
+  private AggregateResult(final boolean ok, final String reason)
+  {
+    this.ok = ok;
+    this.reason = reason;
+  }
+
+  public boolean isOk()
+  {
+    return ok;
+  }
+
+  public String getReason()
+  {
+    return reason;
+  }
+
+  @Override
+  public boolean equals(final Object o)
+  {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    final AggregateResult that = (AggregateResult) o;
+    return ok == that.ok &&
+           Objects.equals(reason, that.reason);
+  }
+
+  @Override
+  public int hashCode()
+  {
+    return Objects.hash(ok, reason);
+  }
+
+  @Override
+  public String toString()
+  {
+    return "AggregateResult{" +
+           "ok=" + ok +
+           ", reason='" + reason + '\'' +
+           '}';
+  }
+}

processing/src/main/java/io/druid/query/groupby/epinephelinae/BufferGrouper.java

@@ -59,6 +59,16 @@ import java.util.List;
 public class BufferGrouper<KeyType> implements Grouper<KeyType>
 {
   private static final Logger log = new Logger(BufferGrouper.class);
+  private static final AggregateResult DICTIONARY_FULL = AggregateResult.failure(
+      "Not enough dictionary space to execute this query. Try increasing "
+      + "druid.query.groupBy.maxMergingDictionarySize or enable disk spilling by setting "
+      + "druid.query.groupBy.maxOnDiskStorage to a positive number."
+  );
+  private static final AggregateResult HASHTABLE_FULL = AggregateResult.failure(
+      "Not enough aggregation table space to execute this query. Try increasing "
+      + "druid.processing.buffer.sizeBytes or enable disk spilling by setting "
+      + "druid.query.groupBy.maxOnDiskStorage to a positive number."
+  );
 
   private static final int MIN_INITIAL_BUCKETS = 4;
   private static final int DEFAULT_INITIAL_BUCKETS = 1024;
@@ -146,11 +156,13 @@ public class BufferGrouper<KeyType> implements Grouper<KeyType>
   }
 
   @Override
-  public boolean aggregate(KeyType key, int keyHash)
+  public AggregateResult aggregate(KeyType key, int keyHash)
   {
     final ByteBuffer keyBuffer = keySerde.toByteBuffer(key);
     if (keyBuffer == null) {
-      return false;
+      // This may just trigger a spill and get ignored, which is ok. If it bubbles up to the user, the message will
+      // be correct.
+      return DICTIONARY_FULL;
     }
 
     if (keyBuffer.remaining() != keySize) {
@@ -178,7 +190,9 @@ public class BufferGrouper<KeyType> implements Grouper<KeyType>
       }
 
       if (bucket < 0) {
-        return false;
+        // This may just trigger a spill and get ignored, which is ok. If it bubbles up to the user, the message will
+        // be correct.
+        return HASHTABLE_FULL;
       }
     }
 
@@ -203,11 +217,11 @@ public class BufferGrouper<KeyType> implements Grouper<KeyType>
       aggregators[i].aggregate(tableBuffer, offset + aggregatorOffsets[i]);
     }
 
-    return true;
+    return AggregateResult.ok();
   }
 
   @Override
-  public boolean aggregate(final KeyType key)
+  public AggregateResult aggregate(final KeyType key)
   {
     return aggregate(key, Groupers.hash(key));
   }

processing/src/main/java/io/druid/query/groupby/epinephelinae/ConcurrentGrouper.java

@@ -145,7 +145,7 @@ public class ConcurrentGrouper<KeyType> implements Grouper<KeyType>
   }
 
   @Override
-  public boolean aggregate(KeyType key, int keyHash)
+  public AggregateResult aggregate(KeyType key, int keyHash)
   {
     if (!initialized) {
       throw new ISE("Grouper is not initialized");
@@ -160,8 +160,8 @@ public class ConcurrentGrouper<KeyType> implements Grouper<KeyType>
 
       synchronized (hashBasedGrouper) {
         if (!spilling) {
-          if (hashBasedGrouper.aggregate(key, keyHash)) {
-            return true;
+          if (hashBasedGrouper.aggregate(key, keyHash).isOk()) {
+            return AggregateResult.ok();
           } else {
             spilling = true;
           }
@@ -179,7 +179,7 @@ public class ConcurrentGrouper<KeyType> implements Grouper<KeyType>
   }
 
   @Override
-  public boolean aggregate(KeyType key)
+  public AggregateResult aggregate(KeyType key)
   {
     return aggregate(key, Groupers.hash(key));
   }

processing/src/main/java/io/druid/query/groupby/epinephelinae/GroupByMergingQueryRunnerV2.java

@@ -52,10 +52,10 @@ import io.druid.query.QueryContextKeys;
 import io.druid.query.QueryInterruptedException;
 import io.druid.query.QueryRunner;
 import io.druid.query.QueryWatcher;
+import io.druid.query.ResourceLimitExceededException;
 import io.druid.query.aggregation.AggregatorFactory;
 import io.druid.query.groupby.GroupByQuery;
 import io.druid.query.groupby.GroupByQueryConfig;
-import io.druid.query.groupby.GroupByQueryHelper;
 import io.druid.query.groupby.epinephelinae.RowBasedGrouperHelper.RowBasedKey;
 
 import java.io.Closeable;
@@ -107,7 +107,7 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner<Row>
   }
 
   @Override
-  public Sequence<Row> run(final Query queryParam, final Map responseContext)
+  public Sequence<Row> run(final Query<Row> queryParam, final Map<String, Object> responseContext)
   {
     final GroupByQuery query = (GroupByQuery) queryParam;
     final GroupByQueryConfig querySpecificConfig = config.withOverrides(query);
@@ -180,7 +180,7 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner<Row>
                 throw new QueryInterruptedException(e);
               }
 
-              Pair<Grouper<RowBasedKey>, Accumulator<Grouper<RowBasedKey>, Row>> pair = RowBasedGrouperHelper.createGrouperAccumulatorPair(
+              Pair<Grouper<RowBasedKey>, Accumulator<AggregateResult, Row>> pair = RowBasedGrouperHelper.createGrouperAccumulatorPair(
                   query,
                   false,
                   null,
@@ -192,21 +192,21 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner<Row>
                   combiningAggregatorFactories
               );
               final Grouper<RowBasedKey> grouper = pair.lhs;
-              final Accumulator<Grouper<RowBasedKey>, Row> accumulator = pair.rhs;
+              final Accumulator<AggregateResult, Row> accumulator = pair.rhs;
               grouper.init();
 
               final ReferenceCountingResourceHolder<Grouper<RowBasedKey>> grouperHolder =
                   ReferenceCountingResourceHolder.fromCloseable(grouper);
               resources.add(grouperHolder);
 
-              ListenableFuture<List<Boolean>> futures = Futures.allAsList(
+              ListenableFuture<List<AggregateResult>> futures = Futures.allAsList(
                   Lists.newArrayList(
                       Iterables.transform(
                           queryables,
-                          new Function<QueryRunner<Row>, ListenableFuture<Boolean>>()
+                          new Function<QueryRunner<Row>, ListenableFuture<AggregateResult>>()
                           {
                             @Override
-                            public ListenableFuture<Boolean> apply(final QueryRunner<Row> input)
+                            public ListenableFuture<AggregateResult> apply(final QueryRunner<Row> input)
                             {
                               if (input == null) {
                                 throw new ISE(
@@ -214,21 +214,24 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner<Row>
                                 );
                               }
 
-                              ListenableFuture<Boolean> future = exec.submit(
-                                  new AbstractPrioritizedCallable<Boolean>(priority)
+                              ListenableFuture<AggregateResult> future = exec.submit(
+                                  new AbstractPrioritizedCallable<AggregateResult>(priority)
                                   {
                                     @Override
-                                    public Boolean call() throws Exception
+                                    public AggregateResult call() throws Exception
                                     {
                                       try (
                                           Releaser bufferReleaser = mergeBufferHolder.increment();
                                           Releaser grouperReleaser = grouperHolder.increment()
                                       ) {
-                                        final Object retVal = input.run(queryForRunners, responseContext)
-                                                                   .accumulate(grouper, accumulator);
+                                        final AggregateResult retVal = input.run(queryForRunners, responseContext)
+                                                                            .accumulate(
+                                                                                AggregateResult.ok(),
+                                                                                accumulator
+                                                                            );
 
                                         // Return true if OK, false if resources were exhausted.
-                                        return retVal == grouper;
+                                        return retVal;
                                       }
                                       catch (QueryInterruptedException e) {
                                         throw e;
@@ -295,7 +298,7 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner<Row>
 
   private void waitForFutureCompletion(
       GroupByQuery query,
-      ListenableFuture<List<Boolean>> future,
+      ListenableFuture<List<AggregateResult>> future,
       long timeout
   )
   {
@@ -308,12 +311,12 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner<Row>
         throw new TimeoutException();
       }
 
-      final List<Boolean> results = future.get(timeout, TimeUnit.MILLISECONDS);
+      final List<AggregateResult> results = future.get(timeout, TimeUnit.MILLISECONDS);
 
-      for (Boolean result : results) {
-        if (!result) {
+      for (AggregateResult result : results) {
+        if (!result.isOk()) {
           future.cancel(true);
-          throw GroupByQueryHelper.throwAccumulationResourceLimitExceededException();
+          throw new ResourceLimitExceededException(result.getReason());
         }
       }
     }

processing/src/main/java/io/druid/query/groupby/epinephelinae/GroupByQueryEngineV2.java

@@ -299,7 +299,7 @@ outer:
           // Aggregate additional grouping for this row
           if (doAggregate) {
             keyBuffer.rewind();
-            if (!grouper.aggregate(keyBuffer)) {
+            if (!grouper.aggregate(keyBuffer).isOk()) {
               // Buffer full while aggregating; break out and resume later
               currentRowWasPartiallyAggregated = true;
               break outer;

processing/src/main/java/io/druid/query/groupby/epinephelinae/GroupByRowProcessor.java

@@ -33,12 +33,12 @@ import io.druid.java.util.common.guava.CloseQuietly;
 import io.druid.java.util.common.guava.FilteredSequence;
 import io.druid.java.util.common.guava.Sequence;
 import io.druid.query.Query;
+import io.druid.query.ResourceLimitExceededException;
 import io.druid.query.aggregation.AggregatorFactory;
 import io.druid.query.filter.Filter;
 import io.druid.query.filter.ValueMatcher;
 import io.druid.query.groupby.GroupByQuery;
 import io.druid.query.groupby.GroupByQueryConfig;
-import io.druid.query.groupby.GroupByQueryHelper;
 import io.druid.query.groupby.RowBasedColumnSelectorFactory;
 import io.druid.query.groupby.epinephelinae.RowBasedGrouperHelper.RowBasedKey;
 import io.druid.query.groupby.resource.GroupByQueryResource;
@@ -139,7 +139,7 @@ public class GroupByRowProcessor
 
               closeOnExit.add(temporaryStorage);
 
-              Pair<Grouper<RowBasedKey>, Accumulator<Grouper<RowBasedKey>, Row>> pair = RowBasedGrouperHelper.createGrouperAccumulatorPair(
+              Pair<Grouper<RowBasedKey>, Accumulator<AggregateResult, Row>> pair = RowBasedGrouperHelper.createGrouperAccumulatorPair(
                   query,
                   true,
                   rowSignature,
@@ -160,15 +160,12 @@ public class GroupByRowProcessor
                   aggregatorFactories
               );
               final Grouper<RowBasedKey> grouper = pair.lhs;
-              final Accumulator<Grouper<RowBasedKey>, Row> accumulator = pair.rhs;
+              final Accumulator<AggregateResult, Row> accumulator = pair.rhs;
               closeOnExit.add(grouper);
 
-              final Grouper<RowBasedKey> retVal = filteredSequence.accumulate(
-                  grouper,
-                  accumulator
-              );
-              if (retVal != grouper) {
-                throw GroupByQueryHelper.throwAccumulationResourceLimitExceededException();
+              final AggregateResult retVal = filteredSequence.accumulate(AggregateResult.ok(), accumulator);
+              if (!retVal.isOk()) {
+                throw new ResourceLimitExceededException(retVal.getReason());
               }
 
               return RowBasedGrouperHelper.makeGrouperIterator(

processing/src/main/java/io/druid/query/groupby/epinephelinae/Grouper.java

@@ -60,9 +60,9 @@ public interface Grouper<KeyType> extends Closeable
    * @param key     key object
    * @param keyHash result of {@link Groupers#hash(Object)} on the key
    *
-   * @return true if the row was aggregated, false if not due to hitting resource limits
+   * @return result that is ok if the row was aggregated, not ok if a resource limit was hit
    */
-  boolean aggregate(KeyType key, int keyHash);
+  AggregateResult aggregate(KeyType key, int keyHash);
 
   /**
    * Aggregate the current row with the provided key. Some implementations are thread-safe and
@@ -70,9 +70,9 @@ public interface Grouper<KeyType> extends Closeable
    *
    * @param key key
    *
-   * @return true if the row was aggregated, false if not due to hitting resource limits
+   * @return result that is ok if the row was aggregated, not ok if a resource limit was hit
    */
-  boolean aggregate(KeyType key);
+  AggregateResult aggregate(KeyType key);
 
   /**
    * Reset the grouper to its initial state.

processing/src/main/java/io/druid/query/groupby/epinephelinae/RowBasedGrouperHelper.java

@@ -72,7 +72,7 @@ public class RowBasedGrouperHelper
    * been applied to the input rows yet, for example, in a nested query, if an extraction function is being
    * applied in the outer query to a field of the inner query. This method must apply those transformations.
    */
-  public static Pair<Grouper<RowBasedKey>, Accumulator<Grouper<RowBasedKey>, Row>> createGrouperAccumulatorPair(
+  public static Pair<Grouper<RowBasedKey>, Accumulator<AggregateResult, Row>> createGrouperAccumulatorPair(
       final GroupByQuery query,
       final boolean isInputRaw,
       final Map<String, ValueType> rawInputRowSignature,
@@ -144,23 +144,23 @@ public class RowBasedGrouperHelper
         valueTypes
     );
 
-    final Accumulator<Grouper<RowBasedKey>, Row> accumulator = new Accumulator<Grouper<RowBasedKey>, Row>()
+    final Accumulator<AggregateResult, Row> accumulator = new Accumulator<AggregateResult, Row>()
     {
       @Override
-      public Grouper<RowBasedKey> accumulate(
-          final Grouper<RowBasedKey> theGrouper,
+      public AggregateResult accumulate(
+          final AggregateResult priorResult,
           final Row row
       )
       {
         BaseQuery.checkInterrupted();
 
-        if (theGrouper == null) {
-          // Pass-through null returns without doing more work.
-          return null;
+        if (priorResult != null && !priorResult.isOk()) {
+          // Pass-through error returns without doing more work.
+          return priorResult;
         }
 
-        if (!theGrouper.isInitialized()) {
-          theGrouper.init();
+        if (!grouper.isInitialized()) {
+          grouper.init();
         }
 
         columnSelectorRow.set(row);
@@ -168,14 +168,10 @@ public class RowBasedGrouperHelper
         final Comparable[] key = new Comparable[keySize];
         valueExtractFn.apply(row, key);
 
-        final boolean didAggregate = theGrouper.aggregate(new RowBasedKey(key));
-        if (!didAggregate) {
-          // null return means grouping resources were exhausted.
-          return null;
-        }
+        final AggregateResult aggregateResult = grouper.aggregate(new RowBasedKey(key));
         columnSelectorRow.set(null);
 
-        return theGrouper;
+        return aggregateResult;
       }
     };
 
@@ -346,7 +342,7 @@ public class RowBasedGrouperHelper
               Object dimVal = entry.getKey().getKey()[i];
               theMap.put(
                   query.getDimensions().get(i - dimStart).getOutputName(),
-                  dimVal instanceof String ? Strings.emptyToNull((String)dimVal) : dimVal
+                  dimVal instanceof String ? Strings.emptyToNull((String) dimVal) : dimVal
               );
             }
 
@@ -636,7 +632,7 @@ public class RowBasedGrouperHelper
       }
 
       return 0;
-    };
+    }
   }
 
   private static class RowBasedKeySerde implements Grouper.KeySerde<RowBasedKey>
@@ -935,6 +931,7 @@ public class RowBasedGrouperHelper
        *
        * @param key RowBasedKey containing the grouping key values for a row.
        * @param idx Index of the grouping key column within that this SerdeHelper handles
+       *
        * @return true if the value was added to the key, false otherwise
        */
       boolean putToKeyBuffer(RowBasedKey key, int idx);
@@ -945,11 +942,11 @@ public class RowBasedGrouperHelper
        *
        * The value to be read resides in the buffer at position (`initialOffset` + the SerdeHelper's keyBufferPosition).
        *
-       * @param buffer ByteBuffer containing an array of grouping keys for a row
+       * @param buffer        ByteBuffer containing an array of grouping keys for a row
        * @param initialOffset Offset where non-timestamp grouping key columns start, needed because timestamp is not
        *                      always included in the buffer.
-       * @param dimValIdx Index within dimValues to store the value read from the buffer
-       * @param dimValues Output array containing grouping key values for a row
+       * @param dimValIdx     Index within dimValues to store the value read from the buffer
+       * @param dimValues     Output array containing grouping key values for a row
        */
       void getFromByteBuffer(ByteBuffer buffer, int initialOffset, int dimValIdx, Comparable[] dimValues);
 
@@ -957,10 +954,11 @@ public class RowBasedGrouperHelper
        * Compare the values at lhsBuffer[lhsPosition] and rhsBuffer[rhsPosition] using the natural ordering
        * for this SerdeHelper's value type.
        *
-       * @param lhsBuffer ByteBuffer containing an array of grouping keys for a row
-       * @param rhsBuffer ByteBuffer containing an array of grouping keys for a row
+       * @param lhsBuffer   ByteBuffer containing an array of grouping keys for a row
+       * @param rhsBuffer   ByteBuffer containing an array of grouping keys for a row
        * @param lhsPosition Position of value within lhsBuffer
        * @param rhsPosition Position of value within rhsBuffer
+       *
        * @return Negative number if lhs < rhs, positive if lhs > rhs, 0 if lhs == rhs
        */
       int compare(ByteBuffer lhsBuffer, ByteBuffer rhsBuffer, int lhsPosition, int rhsPosition);

processing/src/main/java/io/druid/query/groupby/epinephelinae/SpillingGrouper.java

@@ -51,6 +51,10 @@ import java.util.List;
  */
 public class SpillingGrouper<KeyType> implements Grouper<KeyType>
 {
+  private static final AggregateResult DISK_FULL = AggregateResult.failure(
+      "Not enough disk space to execute this query. Try raising druid.query.groupBy.maxOnDiskStorage."
+  );
+
   private final BufferGrouper<KeyType> grouper;
   private final KeySerde<KeyType> keySerde;
   private final LimitedTemporaryStorage temporaryStorage;
@@ -106,29 +110,31 @@ public class SpillingGrouper<KeyType> implements Grouper<KeyType>
   }
 
   @Override
-  public boolean aggregate(KeyType key, int keyHash)
+  public AggregateResult aggregate(KeyType key, int keyHash)
   {
-    if (grouper.aggregate(key, keyHash)) {
-      return true;
-    } else if (spillingAllowed) {
+    final AggregateResult result = grouper.aggregate(key, keyHash);
+
+    if (result.isOk() || temporaryStorage.maxSize() <= 0 || !spillingAllowed) {
+      return result;
+    } else {
       // Warning: this can potentially block up a processing thread for a while.
       try {
         spill();
       }
       catch (TemporaryStorageFullException e) {
-        return false;
+        return DISK_FULL;
       }
       catch (IOException e) {
         throw Throwables.propagate(e);
       }
+
+      // Try again.
       return grouper.aggregate(key, keyHash);
-    } else {
-      return false;
     }
   }
 
   @Override
-  public boolean aggregate(KeyType key)
+  public AggregateResult aggregate(KeyType key)
   {
     return aggregate(key, Groupers.hash(key));
   }

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

@@ -1263,7 +1263,112 @@ public class GroupByQueryRunnerTest
     List<Row> expectedResults = null;
     if (config.getDefaultStrategy().equals(GroupByStrategySelector.STRATEGY_V2)) {
       expectedException.expect(ResourceLimitExceededException.class);
-      expectedException.expectMessage("Not enough resources to execute this query");
+      expectedException.expectMessage("Not enough aggregation table space to execute this query");
+    } else {
+      expectedResults = Arrays.asList(
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "automotive", "rows", 1L, "idx", 135L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "business", "rows", 1L, "idx", 118L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "entertainment", "rows", 1L, "idx", 158L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "health", "rows", 1L, "idx", 120L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "mezzanine", "rows", 3L, "idx", 2870L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "news", "rows", 1L, "idx", 121L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "premium", "rows", 3L, "idx", 2900L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "technology", "rows", 1L, "idx", 78L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "travel", "rows", 1L, "idx", 119L),
+
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "automotive", "rows", 1L, "idx", 147L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "business", "rows", 1L, "idx", 112L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "entertainment", "rows", 1L, "idx", 166L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "health", "rows", 1L, "idx", 113L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "mezzanine", "rows", 3L, "idx", 2447L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "news", "rows", 1L, "idx", 114L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "premium", "rows", 3L, "idx", 2505L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "technology", "rows", 1L, "idx", 97L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "travel", "rows", 1L, "idx", 126L)
+      );
+    }
+
+    Iterable<Row> results = GroupByQueryRunnerTestHelper.runQuery(factory, runner, query);
+    TestHelper.assertExpectedObjects(expectedResults, results, "");
+  }
+
+  @Test
+  public void testNotEnoughDictionarySpaceThroughContextOverride()
+  {
+    GroupByQuery query = GroupByQuery
+        .builder()
+        .setDataSource(QueryRunnerTestHelper.dataSource)
+        .setQuerySegmentSpec(QueryRunnerTestHelper.firstToThird)
+        .setDimensions(Lists.<DimensionSpec>newArrayList(new DefaultDimensionSpec("quality", "alias")))
+        .setAggregatorSpecs(
+            Arrays.asList(
+                QueryRunnerTestHelper.rowsCount,
+                new LongSumAggregatorFactory("idx", "index")
+            )
+        )
+        .setGranularity(QueryRunnerTestHelper.dayGran)
+        .setContext(ImmutableMap.<String, Object>of("maxOnDiskStorage", 0, "maxMergingDictionarySize", 1))
+        .build();
+
+    List<Row> expectedResults = null;
+    if (config.getDefaultStrategy().equals(GroupByStrategySelector.STRATEGY_V2)) {
+      expectedException.expect(ResourceLimitExceededException.class);
+      expectedException.expectMessage("Not enough dictionary space to execute this query");
+    } else {
+      expectedResults = Arrays.asList(
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "automotive", "rows", 1L, "idx", 135L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "business", "rows", 1L, "idx", 118L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "entertainment", "rows", 1L, "idx", 158L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "health", "rows", 1L, "idx", 120L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "mezzanine", "rows", 3L, "idx", 2870L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "news", "rows", 1L, "idx", 121L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "premium", "rows", 3L, "idx", 2900L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "technology", "rows", 1L, "idx", 78L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "travel", "rows", 1L, "idx", 119L),
+
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "automotive", "rows", 1L, "idx", 147L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "business", "rows", 1L, "idx", 112L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "entertainment", "rows", 1L, "idx", 166L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "health", "rows", 1L, "idx", 113L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "mezzanine", "rows", 3L, "idx", 2447L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "news", "rows", 1L, "idx", 114L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "premium", "rows", 3L, "idx", 2505L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "technology", "rows", 1L, "idx", 97L),
+          GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-02", "alias", "travel", "rows", 1L, "idx", 126L)
+      );
+    }
+
+    Iterable<Row> results = GroupByQueryRunnerTestHelper.runQuery(factory, runner, query);
+    TestHelper.assertExpectedObjects(expectedResults, results, "");
+  }
+
+  @Test
+  public void testNotEnoughDiskSpaceThroughContextOverride()
+  {
+    GroupByQuery query = GroupByQuery
+        .builder()
+        .setDataSource(QueryRunnerTestHelper.dataSource)
+        .setQuerySegmentSpec(QueryRunnerTestHelper.firstToThird)
+        .setDimensions(Lists.<DimensionSpec>newArrayList(new DefaultDimensionSpec("quality", "alias")))
+        .setAggregatorSpecs(
+            Arrays.asList(
+                QueryRunnerTestHelper.rowsCount,
+                new LongSumAggregatorFactory("idx", "index")
+            )
+        )
+        .setGranularity(QueryRunnerTestHelper.dayGran)
+        .setContext(ImmutableMap.<String, Object>of("maxOnDiskStorage", 1, "maxMergingDictionarySize", 1))
+        .build();
+
+    List<Row> expectedResults = null;
+    if (config.getDefaultStrategy().equals(GroupByStrategySelector.STRATEGY_V2)) {
+      expectedException.expect(ResourceLimitExceededException.class);
+      if (config.getMaxOnDiskStorage() > 0) {
+        // The error message always mentions disk if you have spilling enabled (maxOnDiskStorage > 0)
+        expectedException.expectMessage("Not enough disk space to execute this query");
+      } else {
+        expectedException.expectMessage("Not enough dictionary space to execute this query");
+      }
     } else {
       expectedResults = Arrays.asList(
           GroupByQueryRunnerTestHelper.createExpectedRow("2011-04-01", "alias", "automotive", "rows", 1L, "idx", 135L),
@@ -1336,7 +1441,7 @@ public class GroupByQueryRunnerTest
       GroupByQueryRunnerTestHelper.runQuery(factory, runner, query);
     } else {
       expectedException.expect(ResourceLimitExceededException.class);
-      expectedException.expectMessage("Not enough resources to execute this query");
+      expectedException.expectMessage("Not enough aggregation table space to execute this query");
       GroupByQueryRunnerTestHelper.runQuery(factory, runner, query);
     }
   }

processing/src/test/java/io/druid/query/groupby/epinephelinae/BufferGrouperTest.java

@@ -97,16 +97,16 @@ public class BufferGrouperTest
 
     columnSelectorFactory.setRow(new MapBasedRow(0, ImmutableMap.<String, Object>of("value", 10L)));
     for (int i = 0; i < expectedMaxSize; i++) {
-      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i));
+      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i).isOk());
     }
-    Assert.assertFalse(grouper.aggregate(expectedMaxSize));
+    Assert.assertFalse(grouper.aggregate(expectedMaxSize).isOk());
 
     // Aggregate slightly different row
     columnSelectorFactory.setRow(new MapBasedRow(0, ImmutableMap.<String, Object>of("value", 11L)));
     for (int i = 0; i < expectedMaxSize; i++) {
-      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i));
+      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i).isOk());
     }
-    Assert.assertFalse(grouper.aggregate(expectedMaxSize));
+    Assert.assertFalse(grouper.aggregate(expectedMaxSize).isOk());
 
     final List<Grouper.Entry<Integer>> expected = Lists.newArrayList();
     for (int i = 0; i < expectedMaxSize; i++) {
@@ -125,16 +125,16 @@ public class BufferGrouperTest
 
     columnSelectorFactory.setRow(new MapBasedRow(0, ImmutableMap.<String, Object>of("value", 10L)));
     for (int i = 0; i < expectedMaxSize; i++) {
-      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i));
+      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i).isOk());
     }
-    Assert.assertFalse(grouper.aggregate(expectedMaxSize));
+    Assert.assertFalse(grouper.aggregate(expectedMaxSize).isOk());
 
     // Aggregate slightly different row
     columnSelectorFactory.setRow(new MapBasedRow(0, ImmutableMap.<String, Object>of("value", 11L)));
     for (int i = 0; i < expectedMaxSize; i++) {
-      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i));
+      Assert.assertTrue(String.valueOf(i), grouper.aggregate(i).isOk());
     }
-    Assert.assertFalse(grouper.aggregate(expectedMaxSize));
+    Assert.assertFalse(grouper.aggregate(expectedMaxSize).isOk());
 
     final List<Grouper.Entry<Integer>> expected = Lists.newArrayList();
     for (int i = 0; i < expectedMaxSize; i++) {