Github user ssonker commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21505#discussion_r194305723
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala
 ---
    @@ -111,6 +113,23 @@ object DateTimeUtils {
         computedTimeZones.computeIfAbsent(timeZoneId, computeTimeZone)
       }
     
    +  private val threadLocalComputedCalendarsMap =
    +    new ThreadLocal[mutable.Map[TimeZone, Calendar]] {
    --- End diff --
    
    @kiszk @viirya I've tried running benchmarks for with/without 
```mutable.Map``` implementation. Looks like setting timezone in a calendar 
instance is a costly operation and it drags the performance down. As the number 
of timezones cannot be large, maintaining a map will not be a huge memory 
overhead. So, I suggest going with the ```mutable.Map``` approach. Comments?


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