[GitHub] [calcite] zabetak commented on a change in pull request #2080: [CALCITE-3920] Improve ORDER BY computation in Enumerable convention by exploiting LIMIT

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zabetak commented on a change in pull request #2080:
URL: https://github.com/apache/calcite/pull/2080#discussion_r460301691



##########
File path: linq4j/src/main/java/org/apache/calcite/linq4j/TopNHeap.java
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@@ -0,0 +1,242 @@
+/*
+ * 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.calcite.linq4j;
+
+import org.apache.calcite.linq4j.function.Function1;
+
+import java.util.Comparator;
+
+/**
+ * Implementation of a stable binary heap with a fetch and an offset.
+ * Stable means that if two items are considered equal,
+ * they will appear in the same order as they were offered to the heap.
+ *
+ * @param <TSource> type of the element that will be added to the heap
+ * @param <TKey> type of the key, which the comparator will use for comparisons
+ */
+public class TopNHeap<TSource, TKey> {

Review comment:
       I had the same question with @julianhyde :) Apart from the priority queues I had also in mind a plain `TreeMap`. I did some preliminary benchmarks at some point (WIP in https://github.com/zabetak/jmh-micro-benchmarks/blob/master/core-java/src/main/java/com/github/zabetak/benchmark/PartialSortBenchmark.java) and `TreeMap` was not bad at all. Plus the risk for bugs is much smaller. 




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