[MINOR][DOCS] Document when current_date and current_timestamp are evaluated

[MaxGekk]

What changes were proposed in this pull request?

Explicitly document that current_date and current_timestamp are executed at the start of query evaluation. And all calls of current_date/current_timestamp within the same query return the same value

Why are the changes needed?

Users could expect that current_date and current_timestamp return the current date/timestamp at the moment of query execution but in fact the functions are folded by the optimizer at the start of query evaluation:

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

Lines 71 to 91 in 0df8dd6

	/**
	* Computes the current date and time to make sure we return the same result in a single query.
	*/
	object ComputeCurrentTime extends Rule[LogicalPlan] {
	def apply(plan: LogicalPlan): LogicalPlan = {
	val currentDates = mutable.Map.empty[String, Literal]
	val timeExpr = CurrentTimestamp()
	val timestamp = timeExpr.eval(EmptyRow).asInstanceOf[Long]
	val currentTime = Literal.create(timestamp, timeExpr.dataType)
	plan transformAllExpressions {
	case CurrentDate(Some(timeZoneId)) =>
	currentDates.getOrElseUpdate(timeZoneId, {
	Literal.create(
	LocalDate.now(DateTimeUtils.getZoneId(timeZoneId)),
	DateType)
	})
	case CurrentTimestamp() | Now() => currentTime
	}
	}
	}

Does this PR introduce any user-facing change?

No

How was this patch tested?

by running ./dev/scalastyle.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/33797/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/33797/

[srowen]

Looks good though do you want to merge this into the PR that fixes this behavior?

[SparkQA]

Test build #129181 has finished for PR 29892 at commit df64fae.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

thanks, merging to master/3.0!