[SPARK-21485][SQL][DOCS] Spark SQL documentation generation for built-in functions

## What changes were proposed in this pull request?

This generates a documentation for Spark SQL built-in functions.

One drawback is, this requires a proper build to generate built-in function list.
Once it is built, it only takes few seconds by `sql/create-docs.sh`.

Please see https://spark-test.github.io/sparksqldoc/ that I hosted to show the output documentation.

There are few more works to be done in order to make the documentation pretty, for example, separating `Arguments:` and `Examples:` but I guess this should be done within `ExpressionDescription` and `ExpressionInfo` rather than manually parsing it. I will fix these in a follow up.

This requires `pip install mkdocs` to generate HTMLs from markdown files.

## How was this patch tested?

Manually tested:

```
cd docs
jekyll build
```
,

```
cd docs
jekyll serve
```

and

```
cd sql
create-docs.sh
```

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18702 from HyukjinKwon/SPARK-21485.

.gitignore

@@ -47,6 +47,8 @@ dev/pr-deps/
 dist/
 docs/_site
 docs/api
+sql/docs
+sql/site
 lib_managed/
 lint-r-report.log
 log/

docs/README.md

@@ -68,6 +68,6 @@ jekyll plugin to run `build/sbt unidoc` before building the site so if you haven
 may take some time as it generates all of the scaladoc.  The jekyll plugin also generates the
 PySpark docs using [Sphinx](http://sphinx-doc.org/).
 
-NOTE: To skip the step of building and copying over the Scala, Python, R API docs, run `SKIP_API=1
-jekyll`. In addition, `SKIP_SCALADOC=1`, `SKIP_PYTHONDOC=1`, and `SKIP_RDOC=1` can be used to skip a single
-step of the corresponding language.
+NOTE: To skip the step of building and copying over the Scala, Python, R and SQL API docs, run `SKIP_API=1
+jekyll`. In addition, `SKIP_SCALADOC=1`, `SKIP_PYTHONDOC=1`, `SKIP_RDOC=1` and `SKIP_SQLDOC=1` can be used
+to skip a single step of the corresponding language.

docs/_layouts/global.html

@@ -86,6 +86,7 @@
                                 <li><a href="api/java/index.html">Java</a></li>
                                 <li><a href="api/python/index.html">Python</a></li>
                                 <li><a href="api/R/index.html">R</a></li>
+                                <li><a href="api/sql/index.html">SQL, Built-in Functions</a></li>
                             </ul>
                         </li>
 

docs/_plugins/copy_api_dirs.rb

@@ -150,4 +150,31 @@ if not (ENV['SKIP_API'] == '1')
     cp("../R/pkg/DESCRIPTION", "api")
   end
 
+  if not (ENV['SKIP_SQLDOC'] == '1')
+    # Build SQL API docs
+
+    puts "Moving to project root and building API docs."
+    curr_dir = pwd
+    cd("..")
+
+    puts "Running 'build/sbt clean package' from " + pwd + "; this may take a few minutes..."
+    system("build/sbt clean package") || raise("SQL doc generation failed")
+
+    puts "Moving back into docs dir."
+    cd("docs")
+
+    puts "Moving to SQL directory and building docs."
+    cd("../sql")
+    system("./create-docs.sh") || raise("SQL doc generation failed")
+
+    puts "Moving back into docs dir."
+    cd("../docs")
+
+    puts "Making directory api/sql"
+    mkdir_p "api/sql"
+
+    puts "cp -r ../sql/site/. api/sql"
+    cp_r("../sql/site/.", "api/sql")
+  end
+
 end

docs/api.md

@@ -9,3 +9,4 @@ Here you can read API docs for Spark and its submodules.
 - [Spark Java API (Javadoc)](api/java/index.html)
 - [Spark Python API (Sphinx)](api/python/index.html)
 - [Spark R API (Roxygen2)](api/R/index.html)
+- [Spark SQL, Built-in Functions (MkDocs)](api/sql/index.html)

docs/index.md

@@ -100,6 +100,7 @@ options for deployment:
 * [Spark Java API (Javadoc)](api/java/index.html)
 * [Spark Python API (Sphinx)](api/python/index.html)
 * [Spark R API (Roxygen2)](api/R/index.html)
+* [Spark SQL, Built-in Functions (MkDocs)](api/sql/index.html)
 
 **Deployment Guides:**
 

sql/README.md

@@ -8,3 +8,5 @@ Spark SQL is broken up into four subprojects:
  - Execution (sql/core) - A query planner / execution engine for translating Catalyst's logical query plans into Spark RDDs.  This component also includes a new public interface, SQLContext, that allows users to execute SQL or LINQ statements against existing RDDs and Parquet files.
  - Hive Support (sql/hive) - Includes an extension of SQLContext called HiveContext that allows users to write queries using a subset of HiveQL and access data from a Hive Metastore using Hive SerDes.  There are also wrappers that allows users to run queries that include Hive UDFs, UDAFs, and UDTFs.
  - HiveServer and CLI support (sql/hive-thriftserver) - Includes support for the SQL CLI (bin/spark-sql) and a HiveServer2 (for JDBC/ODBC) compatible server.
+
+Running `sql/create-docs.sh` generates SQL documentation for built-in functions under `sql/site`.

sql/core/src/main/scala/org/apache/spark/sql/api/python/PythonSQLUtils.scala

@@ -17,9 +17,16 @@
 
 package org.apache.spark.sql.api.python
 
+import org.apache.spark.sql.catalyst.analysis.FunctionRegistry
+import org.apache.spark.sql.catalyst.expressions.ExpressionInfo
 import org.apache.spark.sql.catalyst.parser.CatalystSqlParser
 import org.apache.spark.sql.types.DataType
 
 private[sql] object PythonSQLUtils {
   def parseDataType(typeText: String): DataType = CatalystSqlParser.parseDataType(typeText)
+
+  // This is needed when generating SQL documentation for built-in functions.
+  def listBuiltinFunctionInfos(): Array[ExpressionInfo] = {
+    FunctionRegistry.functionSet.flatMap(f => FunctionRegistry.builtin.lookupFunction(f)).toArray
+  }
 }

sql/create-docs.sh

+#!/bin/bash
+
+#
+# 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.
+#
+
+# Script to create SQL API docs. This requires `mkdocs` and to build
+# Spark first. After running this script the html docs can be found in
+# $SPARK_HOME/sql/site
+
+set -o pipefail
+set -e
+
+FWDIR="$(cd "`dirname "${BASH_SOURCE[0]}"`"; pwd)"
+SPARK_HOME="$(cd "`dirname "${BASH_SOURCE[0]}"`"/..; pwd)"
+
+if ! hash python 2>/dev/null; then
+  echo "Missing python in your path, skipping SQL documentation generation."
+  exit 0
+fi
+
+if ! hash mkdocs 2>/dev/null; then
+  echo "Missing mkdocs in your path, skipping SQL documentation generation."
+  exit 0
+fi
+
+# Now create the markdown file
+rm -fr docs
+mkdir docs
+echo "Generating markdown files for SQL documentation."
+"$SPARK_HOME/bin/spark-submit" gen-sql-markdown.py
+
+# Now create the HTML files
+echo "Generating HTML files for SQL documentation."
+mkdocs build --clean
+rm -fr docs

sql/gen-sql-markdown.py

+#
+# 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.
+#
+
+import sys
+import os
+from collections import namedtuple
+
+ExpressionInfo = namedtuple("ExpressionInfo", "className usage name extended")
+
+
+def _list_function_infos(jvm):
+    """
+    Returns a list of function information via JVM. Sorts wrapped expression infos by name
+    and returns them.
+    """
+
+    jinfos = jvm.org.apache.spark.sql.api.python.PythonSQLUtils.listBuiltinFunctionInfos()
+    infos = []
+    for jinfo in jinfos:
+        name = jinfo.getName()
+        usage = jinfo.getUsage()
+        usage = usage.replace("_FUNC_", name) if usage is not None else usage
+        extended = jinfo.getExtended()
+        extended = extended.replace("_FUNC_", name) if extended is not None else extended
+        infos.append(ExpressionInfo(
+            className=jinfo.getClassName(),
+            usage=usage,
+            name=name,
+            extended=extended))
+    return sorted(infos, key=lambda i: i.name)
+
+
+def _make_pretty_usage(usage):
+    """
+    Makes the usage description pretty and returns a formatted string.
+    Otherwise, returns None.
+    """
+
+    if usage is not None and usage.strip() != "":
+        usage = "\n".join(map(lambda u: u.strip(), usage.split("\n")))
+        return "%s\n\n" % usage
+
+
+def _make_pretty_extended(extended):
+    """
+    Makes the extended description pretty and returns a formatted string.
+    Otherwise, returns None.
+    """
+
+    if extended is not None and extended.strip() != "":
+        extended = "\n".join(map(lambda u: u.strip(), extended.split("\n")))
+        return "```%s```\n\n" % extended
+
+
+def generate_sql_markdown(jvm, path):
+    """
+    Generates a markdown file after listing the function information. The output file
+    is created in `path`.
+    """
+
+    with open(path, 'w') as mdfile:
+        for info in _list_function_infos(jvm):
+            mdfile.write("### %s\n\n" % info.name)
+            usage = _make_pretty_usage(info.usage)
+            extended = _make_pretty_extended(info.extended)
+            if usage is not None:
+                mdfile.write(usage)
+            if extended is not None:
+                mdfile.write(extended)
+
+
+if __name__ == "__main__":
+    from pyspark.java_gateway import launch_gateway
+
+    jvm = launch_gateway().jvm
+    markdown_file_path = "%s/docs/index.md" % os.path.dirname(sys.argv[0])
+    generate_sql_markdown(jvm, markdown_file_path)

sql/mkdocs.yml

+# 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.
+
+site_name: Spark SQL, Built-in Functions
+theme: readthedocs
+pages:
+  - 'Functions': 'index.md'