[FLINK-9742] [table] Add helper method to access private Expression.r…

…esultType.

This closes apache#6252.

flink-libraries/flink-table/src/main/scala/org/apache/flink/table/expressions/ExpressionUtils.scala

@@ -22,13 +22,22 @@ import java.lang.{Boolean => JBoolean, Byte => JByte, Double => JDouble, Float =
 import java.math.{BigDecimal => JBigDecimal}
 import java.sql.{Date, Time, Timestamp}
 
-import org.apache.flink.api.common.typeinfo.BasicTypeInfo
+import org.apache.flink.api.common.typeinfo.{BasicTypeInfo, TypeInformation}
 import org.apache.flink.streaming.api.windowing.time.{Time => FlinkTime}
 import org.apache.flink.table.api.ValidationException
 import org.apache.flink.table.calcite.FlinkTypeFactory
 import org.apache.flink.table.typeutils.{RowIntervalTypeInfo, TimeIntervalTypeInfo}
 
 object ExpressionUtils {
+  /**
+    * Retrieve result type of given Expression.
+    *
+    * @param expr The expression which caller is interested about result type
+    * @return     The result type of Expression
+    */
+  def getResultType(expr: Expression): TypeInformation[_] = {
+    expr.resultType
+  }
 
   private[flink] def isTimeIntervalLiteral(expr: Expression): Boolean = expr match {
     case Literal(_, TimeIntervalTypeInfo.INTERVAL_MILLIS) => true