Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16308#discussion_r95344871
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/datetimeExpressions.scala
 ---
    @@ -30,21 +30,48 @@ import org.apache.spark.sql.types._
     import org.apache.spark.unsafe.types.{CalendarInterval, UTF8String}
     
     /**
    + * Common base class for time zone aware expressions.
    + */
    +trait TimeZoneAwareExpression extends Expression {
    +
    +  /** the timezone ID. */
    +  def timeZoneId: String
    +
    +  /**
    +   * Returns true if the timezone ID is resolved.
    +   * If not resolved, the analyzer [[ResolveTimeZone]] will resolve with 
session local timezone.
    +   */
    +  def timeZoneResolved: Boolean = timeZoneId != null
    --- End diff --
    
    Does it worth to add such a concept? What if we just set session local 
timezone for every `TimeZoneAwareExpression`? It seems to me that it's ok to 
call `withTimeZone` blindly, as the `ResolveTimeZone` rule will only run once.


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