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https://issues.apache.org/jira/browse/FLINK-7491?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16181113#comment-16181113
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ASF GitHub Bot commented on FLINK-7491:
---------------------------------------
Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/4585#discussion_r141103652
--- Diff:
flink-core/src/main/java/org/apache/flink/api/java/typeutils/MultisetTypeInfo.java
---
@@ -0,0 +1,133 @@
+/*
+ * 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.api.java.typeutils;
+
+import org.apache.flink.annotation.PublicEvolving;
+import org.apache.flink.api.common.typeinfo.BasicTypeInfo;
+import org.apache.flink.api.common.typeinfo.TypeInformation;
+
+import java.util.Map;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * A {@link TypeInformation} for the Multiset types of the Java API.
+ *
+ * @param <T> The type of the elements in the Multiset.
+ */
+@PublicEvolving
+public final class MultisetTypeInfo<T> extends MapTypeInfo<T, Integer> {
--- End diff --
Does SQL Multiset also support `null` values? If yes, we would need to wrap
the `MapSerializer`.
Otherwise, the problem would be that we would need to rely on the key
serializer to support `null` which many serializers do not. An solution would
be to wrap the `MapSerializer` and additionally serialize the count for `null`
elements.
> Support COLLECT Aggregate function in Flink SQL
> -----------------------------------------------
>
> Key: FLINK-7491
> URL: https://issues.apache.org/jira/browse/FLINK-7491
> Project: Flink
> Issue Type: New Feature
> Components: Table API & SQL
> Reporter: Shuyi Chen
> Assignee: Shuyi Chen
>
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