AngersZhuuuu commented on a change in pull request #31010: URL: https://github.com/apache/spark/pull/31010#discussion_r606318583
########## File path: docs/sql-ref-syntax-qry-select-transform.md ########## @@ -0,0 +1,297 @@ +--- +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 spark run Spark SQL without Hive support, in this mode, spark can use default + `ROW FORMAT DELIMITED` by treating data as STRING. + 2. WIth Hive: It means spark run Spark SQL with Hive support, in this mode, when spark use default format, + it will be treated as Hive default fomat. And spark can use Hive supported SerDe to process data. + +In both modes with default format, columns will be transformed to STRING and delimited by tabs before feeding +to the user script, Similarly, all `NULL` values will be converted to the literal string `\N` in order to +differentiate `NULL` values from empty strings. The standard output of the user script will be treated as +TAB-separated STRING columns, any cell containing only `\N` will be re-interpreted as a `NULL`, and then the +resulting STRING column will be cast to the data type specified in the table declaration in the usual way. +User scripts can output debug information to standard error which will be shown on the task detail page on hadoop. +These defaults can be overridden with `ROW FORMAT DELIMITED`. + +### Syntax + +```sql +rowFormat + : ROW FORMAT SERDE serde_class [ WITH SERDEPROPERTIES serde_props ] + | ROW FORMAT DELIMITED + [ FIELDS TERMINATED BY fields_terminated_char [ ESCAPED BY escapedBy ] ] + [ COLLECTION ITEMS TERMINATED BY collectionItemsTerminatedBy ] + [ MAP KEYS TERMINATED BY keysTerminatedBy ] + [ LINES TERMINATED BY linesSeparatedBy ] + [ NULL DEFINED AS nullDefinedAs ] + +inRowFormat=rowFormat +outRowFormat=rowFormat +namedExpressionSeq = named_expression [ , ... ] + +transformClause: + SELECT [ TRANSFORM ( namedExpressionSeq ) | MAP namedExpressionSeq | REDUCE namedExpressionSeq ] + [ inRowFormat ] + [ RECORDWRITER recordWriter_class ] + USING script + [ AS ( [ col_name [ col_type ]] [ , ... ] ) ] + [ outRowFormat ] + [ RECORDREADER recordReader_class ] + [ WHERE boolean_expression ] + [ GROUP BY expression [ , ... ] ] + [ HAVING boolean_expression ] +``` + +### Parameters + +* **named_expression** + + An expression with an assigned name. In general, it denotes a column expression. + + **Syntax:** `expression [AS] [alias]` + +* **row_format** + + Use the `SERDE` clause to specify a custom SerDe for one table. Otherwise, use the `DELIMITED` clause to use the native SerDe and specify the delimiter, escape character, null character and so on. + +* **SERDE** + + Specifies a custom SerDe for one table. + +* **serde_class** + + Specifies a fully-qualified class name of a custom SerDe. + +* **SERDEPROPERTIES** + + A list of key-value pairs that is used to tag the SerDe definition. + +* **DELIMITED** + + The `DELIMITED` clause can be used to specify the native SerDe and state the delimiter, escape character, null character and so on. + +* **FIELDS TERMINATED BY** + + Used to define a column separator. + +* **COLLECTION ITEMS TERMINATED BY** + + Used to define a collection item separator. + +* **MAP KEYS TERMINATED BY** + + Used to define a map key separator. + +* **LINES TERMINATED BY** + + Used to define a row separator. + +* **NULL DEFINED AS** + + Used to define the specific value for NULL. + +* **ESCAPED BY** + + Used for escape mechanism. + +* **RECORDREADER** + + Specifies a custom RecordReader for one table. + +* **RECORDWRITER** + + Specifies a custom RecordWriter for one table. + +* **recordReader_class** + + Specifies a fully-qualified class name of a custom RecordReader. + Default value is `org.apache.hadoop.hive.ql.exec.TextRecordReader` + +* **recordWriter_class** + + Specifies a fully-qualified class name of a custom RecordWriter. + Default value is `org.apache.hadoop.hive.ql.exec.TextRecordWriter`. + +* **script** + + Specify a command to process data. + +* **boolean_expression** + + Specifies any expression that evaluates to a result type `boolean`. Two or + more expressions may be combined together using the logical + operators ( `AND`, `OR` ). + +* **expression** + + Specifies combination of one or more values, operators and SQL functions that results in a value. + +### Without Hive support Mode + +Spark scripts transform can run without `-Phive` or `SparkSession.builder.enableHiveSupport()`. +In this case, now spark only use script transform with `ROW FORMAT DELIMITED` and treat all value passed +to script as a string. + +### With Hive Support Mode + +When built Spark with `-Phive` and started Spark SQL with `enableHiveSupport()`, spark can use script +transform with Hive SerDe and both `ROW FORMAT DELIMITED`. Review comment: Rmeoved -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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