viirya commented on a change in pull request #28087: [SPARK-31319][SQL][DOCS]
Document UDFs/UDAFs in SQL Reference
URL: https://github.com/apache/spark/pull/28087#discussion_r402068616
##########
File path: docs/sql-ref-functions-udf-aggregate.md
##########
@@ -1,22 +1,130 @@
---
layout: global
-title: User defined Aggregate Functions (UDAF)
-displayTitle: User defined Aggregate Functions (UDAF)
+title: User Defined Aggregate Functions (UDAFs)
+displayTitle: User Defined Aggregate Functions (UDAFs)
license: |
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.
---
-**This page is under construction**
+### Description
+
+User-Defined Aggregate Functions (UDAFs) are user-programmable routines that
act on multiple rows at once and return a single aggregated value as a result.
This documentation lists the classes that are required for creating and
registering UDAFs. It also contains examples that demonstrate how to define and
register UDAFs in Scala and invoke them in Spark SQL.
+
+### org.apache.spark.sql.expressions.Aggregator[-IN, BUF, OUT]
+
+A base class for user-defined aggregations, which can be used in Dataset
operations to take all of the elements of a group and reduce them to a single
value.
+- IN The input type for the aggregation.
+
+- BUF The type of the intermediate value of the reduction.
+
+- OUT The type of the final output result.
+
+<dl>
+ <dt><code><em>bufferEncoder: Encoder[BUF]</em></code></dt>
+ <dd>
+ Register a deterministic Java UDF(0-22) instance as user-defined function
(UDF).
+ </dd>
+</dl>
Review comment:
Looks not correct. Isn't `bufferEncoder` the Encoder for the intermediate
value?
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