Major Description
According to the doc for current_date(), it must compute the current date at the start of query evaluation: http://spark.apache.org/docs/latest/api/sql/#current_date but it can compute it multiple times: https://github.com/apache/spark/blob/0df8dd60733066076967f0525210bbdb5e12415a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/finishAnalysis.scala#L85
