[SPARK-16046][DOCS] Aggregations in the Spark SQL programming guide

## What changes were proposed in this pull request?

- A separate subsection for Aggregations under “Getting Started” in the Spark SQL programming guide. It mentions which aggregate functions are predefined and how users can create their own.
- Examples of using the `UserDefinedAggregateFunction` abstract class for untyped aggregations in Java and Scala.
- Examples of using the `Aggregator` abstract class for type-safe aggregations in Java and Scala.
- Python is not covered.
- The PR might not resolve the ticket since I do not know what exactly was planned by the author.

In total, there are four new standalone examples that can be executed via `spark-submit` or `run-example`. The updated Spark SQL programming guide references to these examples and does not contain hard-coded snippets.

## How was this patch tested?

The patch was tested locally by building the docs. The examples were run as well.

![image](https://cloud.githubusercontent.com/assets/6235869/21292915/04d9d084-c515-11e6-811a-999d598dffba.png)

Author: aokolnychyi <okolnychyyanton@gmail.com>

Closes #16329 from aokolnychyi/SPARK-16046.

1 files changed, 46 insertions, 0 deletions

diff --git a/docs/sql-programming-guide.md b/docs/sql-programming-guide.md
index f4c89e58fa..c60088d58c 100644
--- a/docs/sql-programming-guide.md
+++ b/docs/sql-programming-guide.md
@@ -382,6 +382,52 @@ For example:
 
 </div>
 
+## Aggregations
+
+The [built-in DataFrames functions](api/scala/index.html#org.apache.spark.sql.functions$) provide common
+aggregations such as `count()`, `countDistinct()`, `avg()`, `max()`, `min()`, etc.
+While those functions are designed for DataFrames, Spark SQL also has type-safe versions for some of them in 
+[Scala](api/scala/index.html#org.apache.spark.sql.expressions.scalalang.typed$) and 
+[Java](api/java/org/apache/spark/sql/expressions/javalang/typed.html) to work with strongly typed Datasets.
+Moreover, users are not limited to the predefined aggregate functions and can create their own.
+
+### Untyped User-Defined Aggregate Functions
+
+<div class="codetabs">
+
+<div data-lang="scala"  markdown="1">
+
+Users have to extend the [UserDefinedAggregateFunction](api/scala/index.html#org.apache.spark.sql.expressions.UserDefinedAggregateFunction) 
+abstract class to implement a custom untyped aggregate function. For example, a user-defined average
+can look like:
+
+{% include_example untyped_custom_aggregation scala/org/apache/spark/examples/sql/UserDefinedUntypedAggregation.scala%}
+</div>
+
+<div data-lang="java"  markdown="1">
+
+{% include_example untyped_custom_aggregation java/org/apache/spark/examples/sql/JavaUserDefinedUntypedAggregation.java%}
+</div>
+
+</div>
+
+### Type-Safe User-Defined Aggregate Functions
+
+User-defined aggregations for strongly typed Datasets revolve around the [Aggregator](api/scala/index.html#org.apache.spark.sql.expressions.Aggregator) abstract class.
+For example, a type-safe user-defined average can look like:
+<div class="codetabs">
+
+<div data-lang="scala"  markdown="1">
+
+{% include_example typed_custom_aggregation scala/org/apache/spark/examples/sql/UserDefinedTypedAggregation.scala%}
+</div>
+
+<div data-lang="java"  markdown="1">
+
+{% include_example typed_custom_aggregation java/org/apache/spark/examples/sql/JavaUserDefinedTypedAggregation.java%}
+</div>
+
+</div>
 
 # Data Sources