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From fhueske <...@git.apache.org>
Subject [GitHub] flink pull request #4585: [FLINK-7491] [Table API & SQL] add MultiSetTypeInf...
Date Wed, 20 Sep 2017 16:48:22 GMT
Github user fhueske commented on a diff in the pull request:

    https://github.com/apache/flink/pull/4585#discussion_r140009815
  
    --- Diff: flink-libraries/flink-table/src/main/scala/org/apache/flink/table/functions/aggfunctions/CollectAggFunction.scala
---
    @@ -0,0 +1,121 @@
    +/*
    + * 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.lang.{Iterable => JIterable}
    +import java.util
    +import java.util.function.BiFunction
    +
    +import org.apache.flink.api.common.typeinfo.TypeInformation
    +import org.apache.flink.api.java.tuple.{Tuple1 => JTuple1}
    +import org.apache.flink.api.java.typeutils.{GenericTypeInfo, TupleTypeInfo}
    +import org.apache.flink.table.functions.AggregateFunction
    +
    +import scala.collection.JavaConverters._
    +
    +/** The initial accumulator for Collect aggregate function */
    +class CollectAccumulator[E] extends JTuple1[util.Map[E, Integer]]
    --- End diff --
    
    We can use a `MapView` here. This feature was recently added and automatically backs the
Map with a MapState if possible. Otherwise, it uses a Java HashMap (as right now). The benefit
of backing the accumulator by MapState is that only the keys and values that are accessed
need to be deserialized. In contrast, a regular HashMap is completely de/serialized every
time the accumulator is read. Using MapView would require that the accumulator is implemented
as a POJO (instead of a Tuple1). 
    
    Check this class for details [MapView](https://github.com/apache/flink/blob/master/flink-libraries/flink-table/src/main/scala/org/apache/flink/table/api/dataview/MapView.scala)
and let me know if you have questions.


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