[SPARK-17480][SQL] Improve performance by removing or caching List.length which is O(n)

## What changes were proposed in this pull request?
Scala's List.length method is O(N) and it makes the gatherCompressibilityStats function O(N^2). Eliminate the List.length calls by writing it in Scala way.

https://github.com/scala/scala/blob/2.10.x/src/library/scala/collection/LinearSeqOptimized.scala#L36

As suggested. Extended the fix to HiveInspectors and AggregationIterator classes as well.

## How was this patch tested?
Profiled a Spark job and found that CompressibleColumnBuilder is using 39% of the CPU. Out of this 39% CompressibleColumnBuilder->gatherCompressibilityStats is using 23% of it. 6.24% of the CPU is spend on List.length which is called inside gatherCompressibilityStats.

After this change we started to save 6.24% of the CPU.

Author: Ergin Seyfe <eseyfe@fb.com>

Closes #15032 from seyfe/gatherCompressibilityStats.

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveInspectors.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveInspectors.scala
index bf5cc17a68..4e74452f6c 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveInspectors.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveInspectors.scala
@@ -756,7 +756,8 @@ private[hive] trait HiveInspectors {
       cache: Array[AnyRef],
       dataTypes: Array[DataType]): Array[AnyRef] = {
     var i = 0
-    while (i < inspectors.length) {
+    val length = inspectors.length
+    while (i < length) {
       cache(i) = wrap(row.get(i, dataTypes(i)), inspectors(i), dataTypes(i))
       i += 1
     }
@@ -769,7 +770,8 @@ private[hive] trait HiveInspectors {
       cache: Array[AnyRef],
       dataTypes: Array[DataType]): Array[AnyRef] = {
     var i = 0
-    while (i < inspectors.length) {
+    val length = inspectors.length
+    while (i < length) {
       cache(i) = wrap(row(i), inspectors(i), dataTypes(i))
       i += 1
     }