BUG#823930: Wrong result with semijoin materialization and blob fields

- Make subquery_types_allow_materialization() detect a case where 
  create_tmp_table() would create a blob column which would make it 
  impossible to use materialization
  Non-semi-join materialization worked because it detected that this case
  and felt back to use IN->EXISTS. Semi-join Materialization cannot easily
  fallback, so we have to detect this case early.

mysql-test/r/subselect_sj_mat.result

@@ -856,15 +856,16 @@ explain extended select left(a1,7), left(a2,7)
 from t1_1024
 where a1 in (select substring(b1,1,1024) from t2_1024 where b1 > '0');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	PRIMARY	t1_1024	ALL	NULL	NULL	NULL	NULL	3	100.00	
-1	PRIMARY	<subquery2>	eq_ref	NULL	distinct_key	15	func,func	1	100.00	
-2	SUBQUERY	t2_1024	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
+1	PRIMARY	t1_1024	ALL	NULL	NULL	NULL	NULL	3	100.00	Start temporary
+1	PRIMARY	t2_1024	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; End temporary; Using join buffer (flat, BNL join)
 Warnings:
 Note	1003	select left(`test`.`t1_1024`.`a1`,7) AS `left(a1,7)`,left(`test`.`t1_1024`.`a2`,7) AS `left(a2,7)` from `test`.`t1_1024` semi join (`test`.`t2_1024`) where ((`test`.`t2_1024`.`b1` > '0') and (`test`.`t1_1024`.`a1` = substr(`test`.`t2_1024`.`b1`,1,1024)))
 select left(a1,7), left(a2,7)
 from t1_1024
 where a1 in (select substring(b1,1,1024) from t2_1024 where b1 > '0');
 left(a1,7)	left(a2,7)
+1 - 01x	2 - 01x
+1 - 02x	2 - 02x
 explain extended select left(a1,7), left(a2,7)
 from t1_1024
 where a1 in (select group_concat(b1) from t2_1024 group by b2);
@@ -950,15 +951,16 @@ explain extended select left(a1,7), left(a2,7)
 from t1_1025
 where a1 in (select substring(b1,1,1025) from t2_1025 where b1 > '0');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	PRIMARY	t1_1025	ALL	NULL	NULL	NULL	NULL	3	100.00	
-1	PRIMARY	<subquery2>	eq_ref	NULL	distinct_key	15	func,func	1	100.00	
-2	SUBQUERY	t2_1025	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
+1	PRIMARY	t1_1025	ALL	NULL	NULL	NULL	NULL	3	100.00	Start temporary
+1	PRIMARY	t2_1025	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; End temporary; Using join buffer (flat, BNL join)
 Warnings:
 Note	1003	select left(`test`.`t1_1025`.`a1`,7) AS `left(a1,7)`,left(`test`.`t1_1025`.`a2`,7) AS `left(a2,7)` from `test`.`t1_1025` semi join (`test`.`t2_1025`) where ((`test`.`t2_1025`.`b1` > '0') and (`test`.`t1_1025`.`a1` = substr(`test`.`t2_1025`.`b1`,1,1025)))
 select left(a1,7), left(a2,7)
 from t1_1025
 where a1 in (select substring(b1,1,1025) from t2_1025 where b1 > '0');
 left(a1,7)	left(a2,7)
+1 - 01x	2 - 01x
+1 - 02x	2 - 02x
 explain extended select left(a1,7), left(a2,7)
 from t1_1025
 where a1 in (select group_concat(b1) from t2_1025 group by b2);

sql/item.cc

@@ -5191,6 +5191,10 @@ Field *Item::make_string_field(TABLE *table)
 {
   Field *field;
   DBUG_ASSERT(collation.collation);
+  /* 
+    Note: the following check is repeated in 
+    subquery_types_allow_materialization():
+  */
   if (max_length/collation.collation->mbmaxlen > CONVERT_IF_BIGGER_TO_BLOB)
     field= new Field_blob(max_length, maybe_null, name,
                           collation.collation);

sql/opt_subselect.cc

@@ -559,6 +559,16 @@ bool subquery_types_allow_materialization(Item_in_subselect *in_subs)
       if (inner->field_type() == MYSQL_TYPE_BLOB || 
           inner->field_type() == MYSQL_TYPE_GEOMETRY)
         DBUG_RETURN(FALSE);
+      /* 
+        Materialization also is unable to work when create_tmp_table() will
+        create a blob column because item->max_length is too big.
+        The following check is copied from Item::make_string_field():
+      */ 
+      if (inner->max_length / inner->collation.collation->mbmaxlen > 
+          CONVERT_IF_BIGGER_TO_BLOB)
+      {
+        DBUG_RETURN(FALSE);
+      }
       break;
     case TIME_RESULT:
       if (mysql_type_to_time_type(outer->field_type()) !=