[GitHub] [spark] AngersZhuuuu commented on a change in pull request #31010: [SPARK-33976][SQL] Spark script TRANSFORM related change doc

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AngersZhuuuu commented on a change in pull request #31010:
URL: https://github.com/apache/spark/pull/31010#discussion_r606266958



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File path: docs/sql-ref-syntax-qry-select-transform.md
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+---
+layout: global
+title: TRANSFORM
+displayTitle: TRANSFORM
+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.
+---
+
+### Description
+
+The `TRANSFORM` clause is used to specifies a hive-style transform (SELECT TRANSFORM/MAP/REDUCE)
+query specification to transform the input by forking and running the specified script. Users can
+plug in their own custom mappers and reducers in the data stream by using features natively supported
+in the Spark/Hive language. e.g. in order to run a custom mapper script - map_script - and a custom
+reducer script - reduce_script - the user can issue the following command which uses the TRANSFORM
+clause to embed the mapper and the reducer scripts.
+
+Currently, Spark's script transform support two mode:
+
+    1. Without Hive: It means we run Spark SQL without hive support, in this mode, we can use default format 
+       by treating data as STRING and use Spark's own SerDe.
+    2. WIth Hive: It means we run Spark SQL with Hive support, in this mode, when we use default format, 
+       it will be treated as Hive default fomat. And we can use Hive supported SerDe to process data.
+
+In both mode with default format, columns will be transformed to STRING and delimited by TAB before feeding
+to the user script, Similarly, all NULL values will be converted to the literal string \N in order to

Review comment:
       > Quote \N - what ASCII value is it, for reference? 0?
   
   Just '\N' a string




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