Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/6959#discussion_r33517761
--- Diff:
sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/util/UniqueObjectPool.java
---
@@ -0,0 +1,59 @@
+/*
+ * 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.
+ */
+
+package org.apache.spark.sql.catalyst.util;
+
+import java.util.HashMap;
+
+/**
+ * An unique object pool stores a collection of unique objects in it.
+ */
+public class UniqueObjectPool extends ObjectPool {
+
+ /**
+ * A hash map from objects to their indexes in the array.
+ */
+ private HashMap<Object, Integer> objIndex;
--- End diff --
It looks like this is an append-only map where entries are immutable, so we
might be able to use core's `AppendOnlyMap` for this instead. OTOH,
AppendOnlyMap doesn't cache hashCode values and this map does. Probably fine
to leave as is.
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