Minor Description
Currently if use UDTF like `explode`, `json_tuple` in `DataFrame.selectExpr`, it will be parsed into `UnresolvedFunction` first, and then alias it with `expr.prettyString`. However, UDTF may need MultiAlias so we will get error if we run:
val df = Seq((Map("1" -> 1), 1)).toDF("a", "b") df.selectExpr("explode(a)").show()
[info] org.apache.spark.sql.AnalysisException: Expect multiple names given for org.apache.spark.sql.catalyst.expressions.Explode,
[info] but only single name ''explode(a)' specified;
