Promote to string type.

Author: rxin

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/HiveTypeCoercion.scala

@@ -58,6 +58,15 @@ object HiveTypeCoercion {
     case _ => None
   }
 
+  /** Similar to [[findTightestCommonType]], but can promote all the way to StringType. */
+  private def findTightestCommonTypeToString(left: DataType, right: DataType): Option[DataType] = {
+    findTightestCommonTypeOfTwo(left, right).orElse((left, right) match {
+      case (StringType, t2: AtomicType) if t2 != BinaryType && t2 != BooleanType => Some(StringType)
+      case (t1: AtomicType, StringType) if t1 != BinaryType && t1 != BooleanType => Some(StringType)
+      case _ => None
+    })
+  }
+
   /**
    * Find the tightest common type of a set of types by continuously applying
    * `findTightestCommonTypeOfTwo` on these types.
@@ -660,7 +669,7 @@ trait HiveTypeCoercion {
     def apply(plan: LogicalPlan): LogicalPlan = plan transformAllExpressions {
       // Find tightest common type for If, if the true value and false value have different types.
       case i @ If(pred, left, right) if left.dataType != right.dataType =>
-        findTightestCommonTypeOfTwo(left.dataType, right.dataType).map { widestType =>
+        findTightestCommonTypeToString(left.dataType, right.dataType).map { widestType =>
           val newLeft = if (left.dataType == widestType) left else Cast(left, widestType)
           val newRight = if (right.dataType == widestType) right else Cast(right, widestType)
           i.makeCopy(Array(pred, newLeft, newRight))