pnowojski commented on a change in pull request #6787: [FLINK-8577][table] 
Implement proctime DataStream to Table upsert conversion
URL: https://github.com/apache/flink/pull/6787#discussion_r262481756
 
 

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 File path: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/runtime/UpsertToRetractionProcessFunction.scala
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 @@ -0,0 +1,104 @@
+/*
+ * 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.runtime
+
+import org.apache.flink.api.common.state.{ValueState, ValueStateDescriptor}
+import org.apache.flink.api.java.typeutils.RowTypeInfo
+import org.apache.flink.configuration.Configuration
+import org.apache.flink.streaming.api.functions.ProcessFunction
+import org.apache.flink.table.api.StreamQueryConfig
+import org.apache.flink.table.runtime.aggregate.ProcessFunctionWithCleanupState
+import org.apache.flink.table.runtime.types.CRow
+import org.apache.flink.table.util.Logging
+import org.apache.flink.types.Row
+import org.apache.flink.util.Collector
+
+/**
+  * Function used to convert upsert to retractions.
+  *
+  * @param rowTypeInfo the output row type info.
+  * @param queryConfig the configuration for the query.
+  */
+class UpsertToRetractionProcessFunction(
 
 Review comment:
   The only reference in SQL standard that I have found was this `SQL:2016-2: 
ยง10.9, General Rule 12gii, note 510.`, which explicitly states that `array_agg` 
can not ignore nulls. Also I haven't found in the standard  references to 
something stating the opposite, so I would be inclined to re-use aggregation 
node in planning as well if you are fine with that.
   
   Next question would be wether to simply create an aggregation node or to 
extend either `Aggregate` or `FlinkLogicalAggregate` classes (to re-use all of 
their logic), but to keep the distinct name of the node `UpsertToRetraction`?

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