[SPARK-14830][SQL] Add RemoveRepetitionFromGroupExpressions optimizer.

## What changes were proposed in this pull request?

This PR aims to optimize GroupExpressions by removing repeating expressions. `RemoveRepetitionFromGroupExpressions` is added.

**Before**
```scala
scala> sql("select a+1 from values 1,2 T(a) group by a+1, 1+a, A+1, 1+A").explain()
== Physical Plan ==
WholeStageCodegen
:  +- TungstenAggregate(key=[(a#0 + 1)#6,(1 + a#0)#7,(A#0 + 1)#8,(1 + A#0)#9], functions=[], output=[(a + 1)#5])
:     +- INPUT
+- Exchange hashpartitioning((a#0 + 1)#6, (1 + a#0)#7, (A#0 + 1)#8, (1 + A#0)#9, 200), None
   +- WholeStageCodegen
      :  +- TungstenAggregate(key=[(a#0 + 1) AS (a#0 + 1)#6,(1 + a#0) AS (1 + a#0)#7,(A#0 + 1) AS (A#0 + 1)#8,(1 + A#0) AS (1 + A#0)#9], functions=[], output=[(a#0 + 1)#6,(1 + a#0)#7,(A#0 + 1)#8,(1 + A#0)#9])
      :     +- INPUT
      +- LocalTableScan [a#0], [[1],[2]]
```

**After**
```scala
scala> sql("select a+1 from values 1,2 T(a) group by a+1, 1+a, A+1, 1+A").explain()
== Physical Plan ==
WholeStageCodegen
:  +- TungstenAggregate(key=[(a#0 + 1)#6], functions=[], output=[(a + 1)#5])
:     +- INPUT
+- Exchange hashpartitioning((a#0 + 1)#6, 200), None
   +- WholeStageCodegen
      :  +- TungstenAggregate(key=[(a#0 + 1) AS (a#0 + 1)#6], functions=[], output=[(a#0 + 1)#6])
      :     +- INPUT
      +- LocalTableScan [a#0], [[1],[2]]
```

## How was this patch tested?

Pass the Jenkins tests (with a new testcase)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes apache#12590 from dongjoon-hyun/SPARK-14830.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -68,7 +68,8 @@ abstract class Optimizer(sessionCatalog: SessionCatalog, conf: CatalystConf)
       ReplaceExceptWithAntiJoin,
       ReplaceDistinctWithAggregate) ::
     Batch("Aggregate", fixedPoint,
-      RemoveLiteralFromGroupExpressions) ::
+      RemoveLiteralFromGroupExpressions,
+      RemoveRepetitionFromGroupExpressions) ::
     Batch("Operator Optimizations", fixedPoint,
       // Operator push down
       SetOperationPushDown,
@@ -1439,6 +1440,18 @@ object RemoveLiteralFromGroupExpressions extends Rule[LogicalPlan] {
   }
 }
 
+/**
+ * Removes repetition from group expressions in [[Aggregate]], as they have no effect to the result
+ * but only makes the grouping key bigger.
+ */
+object RemoveRepetitionFromGroupExpressions extends Rule[LogicalPlan] {
+  def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+    case a @ Aggregate(grouping, _, _) =>
+      val newGrouping = ExpressionSet(grouping).toSeq
+      a.copy(groupingExpressions = newGrouping)
+  }
+}
+
 /**
  * Computes the current date and time to make sure we return the same result in a single query.
  */

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/AggregateOptimizeSuite.scala

@@ -17,6 +17,9 @@
 
 package org.apache.spark.sql.catalyst.optimizer
 
+import org.apache.spark.sql.catalyst.SimpleCatalystConf
+import org.apache.spark.sql.catalyst.analysis.{Analyzer, EmptyFunctionRegistry}
+import org.apache.spark.sql.catalyst.catalog.{InMemoryCatalog, SessionCatalog}
 import org.apache.spark.sql.catalyst.dsl.expressions._
 import org.apache.spark.sql.catalyst.dsl.plans._
 import org.apache.spark.sql.catalyst.expressions.Literal
@@ -25,10 +28,14 @@ import org.apache.spark.sql.catalyst.plans.logical.{LocalRelation, LogicalPlan}
 import org.apache.spark.sql.catalyst.rules.RuleExecutor
 
 class AggregateOptimizeSuite extends PlanTest {
+  val conf = new SimpleCatalystConf(caseSensitiveAnalysis = false)
+  val catalog = new SessionCatalog(new InMemoryCatalog, EmptyFunctionRegistry, conf)
+  val analyzer = new Analyzer(catalog, conf)
 
   object Optimize extends RuleExecutor[LogicalPlan] {
     val batches = Batch("Aggregate", FixedPoint(100),
-      RemoveLiteralFromGroupExpressions) :: Nil
+      RemoveLiteralFromGroupExpressions,
+      RemoveRepetitionFromGroupExpressions) :: Nil
   }
 
   test("remove literals in grouping expression") {
@@ -42,4 +49,15 @@ class AggregateOptimizeSuite extends PlanTest {
 
     comparePlans(optimized, correctAnswer)
   }
+
+  test("remove repetition in grouping expression") {
+    val input = LocalRelation('a.int, 'b.int, 'c.int)
+
+    val query = input.groupBy('a + 1, 'b + 2, Literal(1) + 'A, Literal(2) + 'B)(sum('c))
+    val optimized = Optimize.execute(analyzer.execute(query))
+
+    val correctAnswer = analyzer.execute(input.groupBy('a + 1, 'b + 2)(sum('c)))
+
+    comparePlans(optimized, correctAnswer)
+  }
 }