[SPARK-28793][DOC][SQL] Document CREATE FUNCTION in SQL Reference

docs/sql-getting-started.md

@@ -346,6 +346,9 @@ For example:
 
 </div>
 
+## Scalar Functions
+(to be filled soon)
+
 ## Aggregations
 
 The [built-in DataFrames functions](api/scala/index.html#org.apache.spark.sql.functions$) provide common

docs/sql-ref-syntax-ddl-create-function.md

@@ -19,4 +19,153 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+The `CREATE FUNCTION` statement is used to create a temporary or permanent function
+in Spark. Temporary functions are scoped at a session level where as permanent
+functions are created in the persistent catalog and are made available to
+all sessions. The resources specified in the `USING` clause are made available
+to all executors when they are executed for the first time. In addition to the
+SQL interface, spark allows users to create custom user defined scalar and
+aggregate functions using Scala, Python and Java APIs. Please refer to 
+[scalar_functions](sql-getting-started.html#scalar-functions) and 
+[aggregate functions](sql-getting-started#aggregations) for more information.
+
+### Syntax
+{% highlight sql %}
+CREATE [ OR REPLACE ] [ TEMPORARY ] FUNCTION [ IF NOT EXISTS ]
+    function_name AS class_name [ resource_locations ]
+{% endhighlight %}
+
+### Parameters
+<dl>
+  <dt><code><em>OR REPLACE</em></code></dt>
+  <dd>
+    If specified, the resources for function are reloaded. This is mainly useful
+    to pick up any changes made to the implementation of the function. This
+    parameter is mutually exclusive to <code>IF NOT EXISTS</code> and can not
+    be specified together.
+  </dd>
+  <dt><code><em>TEMPORARY</em></code></dt>
+  <dd>
+    Indicates the scope of function being created. When TEMPORARY is specified, the
+    created function is valid and visible in the current session. No persistent
+    entry is made in the catalog for these kind of functions.
+  </dd>
+  <dt><code><em>IF NOT EXISTS</em></code></dt>
+  <dd>
+    If specified, creates the function only when it does not exist. The creation
+    of function succeeds (no error is thrown), if the specified function already
+    exists in the system. This parameter is mutually exclusive to <code> OR REPLACE</code> 
+    and can not be specified together.
+  </dd>
+  <dt><code><em>function_name</em></code></dt>
+  <dd>
+    Specifies a name of funnction to be created. The function name may be
+    optionally qualified with a database name. <br><br>
+    <b>Syntax:</b>
+      <code>
+        [database_name.]function_name
+      </code>
+  </dd>
+  <dt><code><em>class_name</em></code></dt>
+  <dd>
+    Specifies the name of the class that provides the implementation for function to be created.
+    The implementing class should extend from one of the base classes as follows:
+    <ul>
+      <li>Should extend <code>UDF</code> or <code>UDAF</code> in <code>org.apache.hadoop.hive.ql.exec</code> package.</li>
+      <li>Should extend <code>AbstractGenericUDAFResolver</code>, <code>GenericUDF</code>, or
+          <code>GenericUDTF</code> in <code>org.apache.hadoop.hive.ql.udf.generic</code> package.</li>
+      <li>Should extend <code>UserDefinedAggregateFunction</code> in <code>org.apache.spark.sql.expressions</code> package.</li>
+    </ul>
+  </dd>
+  <dt><code><em>resource_locations</em></code></dt>
+  <dd>
+    Specifies the list of resources that contain the implementation of the function
+    along with its dependencies. <br><br>
+    <b>Syntax:</b>
+      <code>
+        USING { { (JAR | FILE ) resource_uri} , ...}
+      </code>
+  </dd>
+</dl>
+
+### Examples
+{% highlight sql %}
+-- 1. Create a simple UDF `SimpleUdf` that adds the supplied integral value by 10.
+--    import org.apache.hadoop.hive.ql.exec.UDF;
+--    public class SimpleUdf extends UDF {
+--      public int evaluate(int value) {
+--      return value + 10;
+--      }
+--    }
+-- 2. Compile and place it in a jar file called `SimpleUdf.jar` in /tmp.
+
+-- Create a table called `test` and insert two rows.
+CREATE TABLE test(c1 INT);
+INSERT INTO test VALUES (1), (2);
+
+-- Create a permanent function called `simple_udf`. 
+CREATE FUNCTION simple_udf AS 'SimpleUdf'
+  USING JAR '/tmp/SimpleUdf.jar';
+
+-- Verify that the function is in the registry.
+SHOW USER FUNCTIONS;
+  +------------------+
+  |          function|
+  +------------------+
+  |default.simple_udf|
+  +------------------+
+
+-- Invoke the function. Every selected value should be incremented by 10.
+SELECT simple_udf(c1) AS function_return_value FROM t1;
+  +---------------------+                                                         
+  |function_return_value|
+  +---------------------+
+  |                   11|
+  |                   12|
+  +---------------------+
+
+-- Created a temporary function.
+CREATE TEMPORARY FUNCTION simple_temp_udf AS 'SimpleUdf' 
+  USING JAR '/tmp/SimpleUdf.jar';
+
+-- Verify that the newly created temporary function is in the registry.
+-- Please note that the temporary function does not have a qualified
+-- database associated with it.
+SHOW USER FUNCTIONS;
+  +------------------+
+  |          function|
+  +------------------+
+  |default.simple_udf|
+  |   simple_temp_udf|
+  +------------------+
+
+-- 1. Mofify `SimpleUdf`'s implementation to add supplied integral value by 20.
+--    import org.apache.hadoop.hive.ql.exec.UDF;
+
+--    public class SimpleUdfR extends UDF {
+--      public int evaluate(int value) {
+--      return value + 20;
+--      }
+--    }
+-- 2. Compile and place it in a jar file called `SimpleUdfR.jar` in /tmp.
+
+-- Replace the implementation of `simple_udf`
+CREATE OR REPLACE FUNCTION simple_udf AS 'SimpleUdfR'
+  USING JAR '/tmp/SimpleUdfR.jar';
+
+-- Invoke the function. Every selected value should be incremented by 20.
+SELECT simple_udf(c1) AS function_return_value FROM t1;
++---------------------+                                                         
+|function_return_value|
++---------------------+
+|                   21|
+|                   22|
++---------------------+
+
+{% endhighlight %}
+
+### Related statements
+- [SHOW FUNCTIONS](sql-ref-syntax-aux-show-functions.html)
+- [DESCRIBE FUNCTION](sql-ref-syntax-aux-describe-function.html)
+- [DROP FUNCTION](sql-ref-syntax-ddl-drop-function.html)