use DebugString for unreferenceable alias child

Author: angelamayxie

sql/expression/alias.go

@@ -137,7 +137,7 @@ func (e *Alias) String() string {
 
 func (e *Alias) DebugString() string {
 	if e.unreferencable {
-		return fmt.Sprintf("%s->%s", e.Child, e.name)
+		return fmt.Sprintf("%s->%s", sql.DebugString(e.Child), e.name)
 	} else {
 		return fmt.Sprintf("%s->%s:%d", sql.DebugString(e.Child), e.name, e.id)
 	}