create.test, create.result, sql_select.cc, item.cc:

  After merge fix for bug #12537

mysql-test/r/create.result

@@ -597,11 +597,10 @@ create table t1(t1.name int);
 create table t2(test.t2.name int);
 drop table t1,t2;
 CREATE TABLE t1 (f1 VARCHAR(255) CHARACTER SET utf8);
-CREATE TABLE t2 AS SELECT LEFT(f1,86) AS f2 FROM t1 UNION SELECT LEFT(f1,86)
-AS f2 FROM t1;
+CREATE TABLE t2 AS SELECT LEFT(f1,171) AS f2 FROM t1 UNION SELECT LEFT(f1,171) AS f2 FROM t1;
 DESC t2;
 Field	Type	Null	Key	Default	Extra
-f2	varchar(86)	YES		NULL	
+f2	varchar(171)	YES		NULL	
 DROP TABLE t1,t2;
 create database mysqltest;
 use mysqltest;

mysql-test/t/create.test

@@ -507,8 +507,7 @@ drop table t1,t2;
 # Bug #12537: UNION produces longtext instead of varchar
 #
 CREATE TABLE t1 (f1 VARCHAR(255) CHARACTER SET utf8);
-CREATE TABLE t2 AS SELECT LEFT(f1,86) AS f2 FROM t1 UNION SELECT LEFT(f1,86)
-AS f2 FROM t1;
+CREATE TABLE t2 AS SELECT LEFT(f1,171) AS f2 FROM t1 UNION SELECT LEFT(f1,171) AS f2 FROM t1;
 DESC t2;
 DROP TABLE t1,t2;
 

sql/item.cc

@@ -3648,7 +3648,8 @@ enum_field_types Item::field_type() const
 
 Field *Item::make_string_field(TABLE *table)
 {
-  if (max_length > CONVERT_IF_BIGGER_TO_BLOB)
+  DBUG_ASSERT(collation.collation);
+  if (max_length/collation.collation->mbmaxlen > CONVERT_IF_BIGGER_TO_BLOB)
     return new Field_blob(max_length, maybe_null, name, table,
                           collation.collation);
   if (max_length > 0)

sql/sql_select.cc

@@ -7963,7 +7963,8 @@ static Field *create_tmp_field_from_item(THD *thd, Item *item, TABLE *table,
     if ((type= item->field_type()) == MYSQL_TYPE_DATETIME ||
         type == MYSQL_TYPE_TIME || type == MYSQL_TYPE_DATE)
       new_field= item->tmp_table_field_from_field_type(table);
-    else if (item->max_length > 255 && convert_blob_length)
+    else if (item->max_length/item->collation.collation->mbmaxlen > 255 &&
+             convert_blob_length)
       new_field= new Field_varstring(convert_blob_length, maybe_null,
                                      item->name, table,
                                      item->collation.collation);