Description:
A simple query on an indexed column encoded with the utf8mb4 character set with an "order by" clause, such as the query below, unexpectedly results in a filesort:
mysql> explain select * from test force index (B) where B = 'foo' order by B asc;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
| 1 | SIMPLE | test | NULL | ref | B | B | 82 | const | 1 | 100.00 | Using where; Using index; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
1 row in set, 1 warning (0.00 sec)
This is only observed when the connection character set is also utf8mb4. If the connection character set is set to utf8 then a sort isn't performed. Forcing the index doesn't change the plan.
I would expect that a filesort would not be required when the character sets of the index and connection are the same.
Query optimizer trace showing index_provides_order is false is attached.
How to repeat:
Step 1: Create the table
CREATE TABLE `test` (
`A` varchar(10) COLLATE utf8mb4_bin NOT NULL,
`B` varchar(20) COLLATE utf8mb4_bin NOT NULL,
PRIMARY KEY (`A`),
KEY `B` (`B`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
Step 2: Query with character_set_connection set to 'utf8mb4' and see filesort being used;
mysql> show variables like 'character\_set\_%';
+--------------------------+---------+
| Variable_name | Value |
+--------------------------+---------+
| character_set_client | utf8mb4 |
| character_set_connection | utf8mb4 |
| character_set_database | utf8mb4 |
| character_set_filesystem | binary |
| character_set_results | utf8mb4 |
| character_set_server | utf8mb4 |
| character_set_system | utf8 |
+--------------------------+---------+
7 rows in set (0.00 sec)
mysql> explain select * from test where B = 'foo' order by B asc;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
| 1 | SIMPLE | test | NULL | ref | B | B | 82 | const | 1 | 100.00 | Using where; Using index; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
1 row in set, 1 warning (0.00 sec)
Forcing the index doesn't help either:
mysql> explain select * from test force index (B) where B = 'foo' order by B asc;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
| 1 | SIMPLE | test | NULL | ref | B | B | 82 | const | 1 | 100.00 | Using where; Using index; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+------------------------------------------+
1 row in set, 1 warning (0.00 sec)
Step 3: Query with character_set_connection set to 'utf8' and see that filesort isn't used:
mysql> set character_set_connection='utf8';
Query OK, 0 rows affected (0.00 sec)
mysql> explain select * from test where (B = 'foo') order by B asc;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | test | NULL | ref | B | B | 82 | const | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
