[SPARK-12460][SQL]Add ExpressionDescription to aggregate functions

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Average.scala

@@ -23,6 +23,8 @@ import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.util.TypeUtils
 import org.apache.spark.sql.types._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the average of the values in a group.")
 case class Average(child: Expression) extends DeclarativeAggregate {
 
   override def prettyName: String = "avg"

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Corr.scala

@@ -29,6 +29,8 @@ import org.apache.spark.sql.types._
  * Definition of Pearson correlation can be found at
  * http://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient
  */
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the Pearson Correlation Coefficient for two columns..")
 case class Corr(
     left: Expression,
     right: Expression,

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Count.scala

@@ -21,6 +21,8 @@ import org.apache.spark.sql.catalyst.dsl.expressions._
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.types._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the number of items in a group.")
 case class Count(children: Seq[Expression]) extends DeclarativeAggregate {
 
   override def nullable: Boolean = false

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/First.scala

@@ -28,6 +28,8 @@ import org.apache.spark.sql.types._
  * is used) its result will not be deterministic (unless the input table is sorted and has
  * a single partition, and we use a single reducer to do the aggregation.).
  */
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the first value in a group.")
 case class First(child: Expression, ignoreNullsExpr: Expression) extends DeclarativeAggregate {
 
   def this(child: Expression) = this(child, Literal.create(false, BooleanType))

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/HyperLogLogPlusPlus.scala

@@ -48,6 +48,8 @@ import org.apache.spark.sql.types._
  * @param relativeSD the maximum estimation error allowed.
  */
 // scalastyle:on
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the approximate number of distinct items in a group.")
 case class HyperLogLogPlusPlus(
     child: Expression,
     relativeSD: Double = 0.05,

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Kurtosis.scala

@@ -19,6 +19,8 @@ package org.apache.spark.sql.catalyst.expressions.aggregate
 
 import org.apache.spark.sql.catalyst.expressions._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the kurtosis of the values in a group.")
 case class Kurtosis(child: Expression,
     mutableAggBufferOffset: Int = 0,
     inputAggBufferOffset: Int = 0)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Last.scala

@@ -28,6 +28,8 @@ import org.apache.spark.sql.types._
  * is used) its result will not be deterministic (unless the input table is sorted and has
  * a single partition, and we use a single reducer to do the aggregation.).
  */
+@ExpressionDescription(
+  usage = "_FUNC_(column) -  Aggregate function: returns the last value in a group.")
 case class Last(child: Expression, ignoreNullsExpr: Expression) extends DeclarativeAggregate {
 
   def this(child: Expression) = this(child, Literal.create(false, BooleanType))

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Max.scala

@@ -22,6 +22,8 @@ import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.util.TypeUtils
 import org.apache.spark.sql.types._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) -  Aggregate function: returns the maximum value of the expression in a group.")
 case class Max(child: Expression) extends DeclarativeAggregate {
 
   override def children: Seq[Expression] = child :: Nil

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Min.scala

@@ -22,7 +22,8 @@ import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.util.TypeUtils
 import org.apache.spark.sql.types._
 
-
+@ExpressionDescription(
+  usage = "_FUNC_(column) -  Aggregate function: returns the minimum value of the expression in a group.")
 case class Min(child: Expression) extends DeclarativeAggregate {
 
   override def children: Seq[Expression] = child :: Nil

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Skewness.scala

@@ -19,6 +19,8 @@ package org.apache.spark.sql.catalyst.expressions.aggregate
 
 import org.apache.spark.sql.catalyst.expressions._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the skewness of the values in a group.")
 case class Skewness(child: Expression,
     mutableAggBufferOffset: Int = 0,
     inputAggBufferOffset: Int = 0)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Stddev.scala

@@ -19,6 +19,9 @@ package org.apache.spark.sql.catalyst.expressions.aggregate
 
 import org.apache.spark.sql.catalyst.expressions._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) -  Aggregate function: returns the sample standard deviation of " +
+    "the expression in a group.")
 case class StddevSamp(child: Expression,
     mutableAggBufferOffset: Int = 0,
     inputAggBufferOffset: Int = 0)
@@ -50,6 +53,9 @@ case class StddevSamp(child: Expression,
   }
 }
 
+@ExpressionDescription(
+usage = "_FUNC_(column) -  Aggregate function: returns the population standard deviation of" +
+" the expression in a group.")
 case class StddevPop(
     child: Expression,
     mutableAggBufferOffset: Int = 0,

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Sum.scala

@@ -22,6 +22,8 @@ import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.util.TypeUtils
 import org.apache.spark.sql.types._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the sum of all values in the expression.")
 case class Sum(child: Expression) extends DeclarativeAggregate {
 
   override def children: Seq[Expression] = child :: Nil

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Variance.scala

@@ -19,6 +19,8 @@ package org.apache.spark.sql.catalyst.expressions.aggregate
 
 import org.apache.spark.sql.catalyst.expressions._
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) -Aggregate function: returns the unbiased variance of the values in a group.")
 case class VarianceSamp(child: Expression,
     mutableAggBufferOffset: Int = 0,
     inputAggBufferOffset: Int = 0)
@@ -50,6 +52,8 @@ case class VarianceSamp(child: Expression,
   }
 }
 
+@ExpressionDescription(
+  usage = "_FUNC_(column) - Aggregate function: returns the population variance of the values in a group.")
 case class VariancePop(
     child: Expression,
     mutableAggBufferOffset: Int = 0,