[SPARK-13185][SQL] Reuse Calendar object in DateTimeUtils.StringToDate method...

[SPARK-13185][SQL] Reuse Calendar object in DateTimeUtils.StringToDate method to improve performance

The java `Calendar` object is expensive to create. I have a sub query like this `SELECT a, b, c FROM table UV WHERE (datediff(UV.visitDate, '1997-01-01')>=0 AND datediff(UV.visitDate, '2015-01-01')<=0))`

The table stores `visitDate` as String type and has 3 billion records. A `Calendar` object is created every time `DateTimeUtils.stringToDate` is called. By reusing the `Calendar` object, I saw about 20 seconds performance improvement for this stage.

Author: Carson Wang <carson.wang@intel.com>

Closes #11090 from carsonwang/SPARK-13185.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala

@@ -59,6 +59,13 @@ object DateTimeUtils {
 
   @transient lazy val defaultTimeZone = TimeZone.getDefault
 
+  // Reuse the Calendar object in each thread as it is expensive to create in each method call.
+  private val threadLocalGmtCalendar = new ThreadLocal[Calendar] {
+    override protected def initialValue: Calendar = {
+      Calendar.getInstance(TimeZoneGMT)
+    }
+  }
+
   // Java TimeZone has no mention of thread safety. Use thread local instance to be safe.
   private val threadLocalLocalTimeZone = new ThreadLocal[TimeZone] {
     override protected def initialValue: TimeZone = {
@@ -408,7 +415,8 @@ object DateTimeUtils {
         segments(2) < 1 || segments(2) > 31) {
       return None
     }
-    val c = Calendar.getInstance(TimeZoneGMT)
+    val c = threadLocalGmtCalendar.get()
+    c.clear()
     c.set(segments(0), segments(1) - 1, segments(2), 0, 0, 0)
     c.set(Calendar.MILLISECOND, 0)
     Some((c.getTimeInMillis / MILLIS_PER_DAY).toInt)