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https://issues.apache.org/jira/browse/FLINK-5956?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15895914#comment-15895914
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ASF GitHub Bot commented on FLINK-5956:
---------------------------------------
Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/3470#discussion_r104298708
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/functions/aggfunctions/MaxAggFunctionWithRetract.scala
---
@@ -0,0 +1,205 @@
+/*
+ * 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.flink.table.functions.aggfunctions
+
+import java.math.BigDecimal
+import java.util.{HashMap => JHashMap, List => JList}
+
+import org.apache.flink.api.common.typeinfo.{BasicTypeInfo,
TypeInformation}
+import org.apache.flink.api.java.tuple.{Tuple3 => JTuple3}
+import org.apache.flink.api.java.typeutils.{MapTypeInfo, TupleTypeInfo}
+import org.apache.flink.table.api.TableException
+import org.apache.flink.table.functions.{Accumulator, AggregateFunction}
+
+/** The initial accumulator for Max with retraction aggregate function */
+class MaxWithRetractAccumulator[T] extends JTuple3[T, Long, JHashMap[T,
Long]] with Accumulator
+
+/**
+ * Base class for built-in Max with retraction aggregate function
+ *
+ * @tparam T the type for the aggregation result
+ */
+abstract class MaxWithRetractAggFunction[T](implicit ord: Ordering[T])
extends AggregateFunction[T] {
+
+ override def createAccumulator(): Accumulator = {
+ val acc = new MaxWithRetractAccumulator[T]
+ acc.f0 = getInitValue //max
+ acc.f1 = 0L //total count
+ acc.f2 = new JHashMap[T, Long]() //store the count for each value
--- End diff --
I spent some time thinking whether a `HashMap` is the best data structure
here because searching for the second largest element is quite expensive. But
since many more values should be added O(1) and retracted without changing the
max element (also O(1)), a `HashMap` should be a good choice. I noticed that
the `MapSerializer` of the `MapTypeInfo` serializes and additional `Boolean`
for value null checks. This is unnecessary overhead for us, but should be OK
for now, IMO.
> Add retract method into the aggregateFunction
> ---------------------------------------------
>
> Key: FLINK-5956
> URL: https://issues.apache.org/jira/browse/FLINK-5956
> Project: Flink
> Issue Type: Sub-task
> Components: Table API & SQL
> Reporter: Shaoxuan Wang
> Assignee: Shaoxuan Wang
>
> Retraction method is help for processing updated message. It will also very
> helpful for window Aggregation. This PR will first add retraction methods
> into the aggregateFunctions, such that on-going over window Aggregation can
> get benefit from it.
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