Github user zuyu commented on a diff in the pull request:
https://github.com/apache/incubator-quickstep/pull/239#discussion_r112885190
--- Diff: storage/ThreadPrivateCompactKeyHashTable.hpp ---
@@ -0,0 +1,230 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF 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.
+ **/
+
+#ifndef QUICKSTEP_STORAGE_THREAD_PRIVATE_COMPACT_KEY_HASH_TABLE_HPP_
+#define QUICKSTEP_STORAGE_THREAD_PRIVATE_COMPACT_KEY_HASH_TABLE_HPP_
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <unordered_map>
+#include <vector>
+
+#include "catalog/CatalogTypedefs.hpp"
+#include "storage/HashTableBase.hpp"
+#include "storage/StorageBlob.hpp"
+#include "storage/StorageConstants.hpp"
+#include "storage/ValueAccessorMultiplexer.hpp"
+#include "storage/ValueAccessorUtil.hpp"
+#include "types/containers/ColumnVector.hpp"
+#include "utility/Macros.hpp"
+
+namespace quickstep {
+
+class AggregationHandle;
+class StorageManager;
+class Type;
+
+/**
+ * @brief Specialized aggregation hash table that is preferable for
two-phase
+ * aggregation with small-cardinality group-by keys. To use this
hash
+ * table, it also requires that the group-by keys have fixed-length
types
+ * with total byte size no greater than 8 (so that the keys can be
packed
+ * into a 64-bit QWORD).
+ */
+class ThreadPrivateCompactKeyHashTable : public
AggregationStateHashTableBase {
--- End diff --
Could we mark `final` so that all `override` methods would have performance
benefits?
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