[SPARK-7225][SQL] CombineLimits optimizer does not work

SQL
```
select key from (select key from src limit 100) t2 limit 10
```
Optimized Logical Plan before modifying
```
== Optimized Logical Plan ==
Limit 10
Limit 100
Project key#3
MetastoreRelation default, src, None
```
Optimized Logical Plan after modifying
```
== Optimized Logical Plan ==
Limit 10
 Project [key#1]
  MetastoreRelation default, src, None
```

Author: Zhongshuai Pei <[email protected]>
Author: DoingDone9 <[email protected]>

Closes apache#5770 from DoingDone9/limitOptimizer and squashes the following commits:

c68eaa7 [Zhongshuai Pei] Update CombiningLimitsSuite.scala
97e18cf [Zhongshuai Pei] Update Optimizer.scala
19ab875 [Zhongshuai Pei] Update CombiningLimitsSuite.scala
7db4566 [Zhongshuai Pei] Update CombiningLimitsSuite.scala
e2a491d [Zhongshuai Pei] Update Optimizer.scala
f03fe7f [Zhongshuai Pei] Merge pull request #12 from apache/master
f12fa50 [Zhongshuai Pei] Merge pull request #10 from apache/master
f61210c [Zhongshuai Pei] Merge pull request #9 from apache/master
34b1a9a [Zhongshuai Pei] Merge pull request #8 from apache/master
802261c [DoingDone9] Merge pull request #7 from apache/master
d00303b [DoingDone9] Merge pull request #6 from apache/master
98b134f [DoingDone9] Merge pull request #5 from apache/master
161cae3 [DoingDone9] Merge pull request #4 from apache/master
c87e8b6 [DoingDone9] Merge pull request #3 from apache/master
cb1852d [DoingDone9] Merge pull request #2 from apache/master
c3f046f [DoingDone9] Merge pull request #1 from apache/master

Author: pzzs

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

@@ -36,7 +36,13 @@ object DefaultOptimizer extends Optimizer {
     // SubQueries are only needed for analysis and can be removed before execution.
     Batch("Remove SubQueries", FixedPoint(100),
       EliminateSubQueries) ::
-    Batch("Combine Limits", FixedPoint(100),
+    Batch("Operator Reordering", FixedPoint(100),
+      UnionPushdown,
+      CombineFilters,
+      PushPredicateThroughProject,
+      PushPredicateThroughJoin,
+      PushPredicateThroughGenerate,
+      ColumnPruning,
       CombineLimits) ::
     Batch("ConstantFolding", FixedPoint(100),
       NullPropagation,
@@ -49,13 +55,6 @@ object DefaultOptimizer extends Optimizer {
       OptimizeIn) ::
     Batch("Decimal Optimizations", FixedPoint(100),
       DecimalAggregates) ::
-    Batch("Filter Pushdown", FixedPoint(100),
-      UnionPushdown,
-      CombineFilters,
-      PushPredicateThroughProject,
-      PushPredicateThroughJoin,
-      PushPredicateThroughGenerate,
-      ColumnPruning) ::
     Batch("LocalRelation", FixedPoint(100),
       ConvertToLocalRelation) :: Nil
 }

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

@@ -27,6 +27,8 @@ class CombiningLimitsSuite extends PlanTest {
 
   object Optimize extends RuleExecutor[LogicalPlan] {
     val batches =
+      Batch("Filter Pushdown", FixedPoint(100),
+        ColumnPruning) ::
       Batch("Combine Limit", FixedPoint(10),
         CombineLimits) ::
       Batch("Constant Folding", FixedPoint(10),
@@ -69,4 +71,21 @@ class CombiningLimitsSuite extends PlanTest {
 
     comparePlans(optimized, correctAnswer)
   }
+  
+  test("limits: combines two limits after ColumnPruning") {
+    val originalQuery =
+      testRelation
+        .select('a)
+        .limit(2)
+        .select('a)
+        .limit(5)
+        
+    val optimized = Optimize.execute(originalQuery.analyze)
+    val correctAnswer =
+      testRelation
+        .select('a)
+        .limit(2).analyze
+
+    comparePlans(optimized, correctAnswer)
+  }
 }