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https://issues.apache.org/jira/browse/FLINK-6465?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16179244#comment-16179244
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ASF GitHub Bot commented on FLINK-6465:
---------------------------------------
Github user haohui commented on a diff in the pull request:
https://github.com/apache/flink/pull/4556#discussion_r140823719
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/functions/aggfunctions/FirstValueAggFunction.scala
---
@@ -0,0 +1,107 @@
+/*
+ * 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 org.apache.flink.api.common.typeinfo.{BasicTypeInfo,
TypeInformation}
+import org.apache.flink.api.java.tuple.{Tuple1 => JTuple1}
+import org.apache.flink.api.java.typeutils.TupleTypeInfo
+import org.apache.flink.table.functions.AggregateFunction
+
+/**
+ * The initial accumulator for first value aggregate function
+ *
+ * @tparam T the type for the aggregation result
+ */
+class FirstValueAccumulator[T] extends JTuple1[T]
+
+abstract class FirstValueAggFunction[T] extends AggregateFunction[T,
FirstValueAccumulator[T]] {
+
+ override def createAccumulator(): FirstValueAccumulator[T] = {
+ val acc = new FirstValueAccumulator[T]
+ acc.f0 = getInitValue
+ acc
+ }
+
+ def accumulate(acc: FirstValueAccumulator[T], value: Any): Unit = {
+ if (null == acc.f0) {
+ acc.f0 = value.asInstanceOf[T]
+ }
+ }
+
+ override def getValue(acc: FirstValueAccumulator[T]): T = {
+ acc.f0
+ }
+
+ def resetAccumulator(acc: FirstValueAccumulator[T]): Unit = {
+ acc.f0 = getInitValue
+ }
+
+ def getInitValue: T = {
+ null.asInstanceOf[T]
+ }
+
+ def getValueTypeInfo: TypeInformation[T]
--- End diff --
Looks like the parameter can be a generic parameter
> support FIRST_VALUE on Table API & SQL
> --------------------------------------
>
> Key: FLINK-6465
> URL: https://issues.apache.org/jira/browse/FLINK-6465
> Project: Flink
> Issue Type: Sub-task
> Components: Table API & SQL
> Reporter: Hequn Cheng
> Assignee: sunjincheng
>
> {{FIRST_VALUE}} is a OVER WINDOW function. In this JIRA. will add
> {{FIRST_VALUE}} function support on TableAPI & SQL.
> *Syntax:*
> FIRST_VALUE ( [scalar_expression ] )
> OVER ( [ partition_by_clause ] order_by_clause [ rows_range_clause ] )
> [About OVER
> WINDOWS|https://ci.apache.org/projects/flink/flink-docs-release-1.4/dev/table/tableApi.html#over-windows]
> scalar_expression
> Is the value to be returned. scalar_expression can be a column, or other
> arbitrary expression that results in a single value. Other analytic functions
> are not permitted.
> *NOTE:*
> * {{FIRST_VALUE}} if used for OVER WINDOW, e.g.:
> {code}
> SELECT A, B, FIRST_VALUE(C) OVER (ORDER BY E) AS firstValue FROM tab
> {code}
> * OVER WINDOW's retraction is expensive(currently not supported yet), and
> this JIRA. does not implement Retract logic of {{FIRST_VALUE}}.
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