Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/3354#discussion_r102220814
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/functions/AggregateFunction.scala
---
@@ -0,0 +1,73 @@
+/*
+ * 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
+
+/**
+ * Base class for User-Defined Aggregates.
+ *
+ * @tparam T the type of the aggregation result
+ */
+abstract class AggregateFunction[T] extends UserDefinedFunction {
+ /**
+ * Create and init the Accumulator for this [[AggregateFunction]].
+ *
+ * @return the accumulator with the initial value
+ */
+ def createAccumulator(): Accumulator
+
+ /**
+ * Called every time when an aggregation result should be materialized.
+ * The returned value could be either a speculative result (periodically
+ * emitted as data arrive) or the final result of the aggregation (when
+ * the state of the aggregation is completely removed).
--- End diff --
How would the function distinguish these cases?
I think we should not mention that the state is removed because it is not
the responsibility of the method.
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