MySQL 5.7のEXPLAINでようやく変なUsing whereが消えた

や、EXPLAINの説明用の資料作ってて気付いたんですが、こんなサンプルテーブルに対して

mysql56> show create table cards\G
*************************** 1. row ***************************
       Table: cards
Create Table: CREATE TABLE `cards` (
  `seq` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `suite` varchar(6) NOT NULL,
  `number` tinyint(3) unsigned NOT NULL,
  UNIQUE KEY `seq` (`seq`),
  KEY `suite` (`suite`)
) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql56> SELECT * FROM cards ORDER BY _rowid LIMIT 3;
+-----+--------------+--------+
| seq | suite        | number |
+-----+--------------+--------+
|   1 | ダイヤ       |      2 |
|   2 | クラブ       |      9 |
|   3 | スペード     |      3 |
+-----+--------------+--------+
3 rows in set (0.01 sec)

WHERE句だけをインデックスで解決するクエリーをEXPLAIN取ると

mysql56> explain extended SELECT * FROM cards WHERE suite= 'ハート' ORDER BY number;
+----+-------------+-------+------+---------------+-------+---------+-------+------+----------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra                                              |
+----+-------------+-------+------+---------------+-------+---------+-------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | cards | ref  | suite         | suite | 20      | const |   31 |   100.00 | Using index condition; Using where; Using filesort |
+----+-------------+-------+------+---------------+-------+---------+-------+------+----------+----------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql56> SELECT * FROM cards WHERE suite= 'ハート' ORDER BY number;
..
31 rows in set (0.00 sec)

mysql56> SHOW SESSION STATUS LIKE 'handler%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Handler_commit             | 1     |
| Handler_delete             | 0     |
| Handler_discover           | 0     |
| Handler_external_lock      | 2     |
| Handler_mrr_init           | 0     |
| Handler_prepare            | 0     |
| Handler_read_first         | 0     |
| Handler_read_key           | 1     |
| Handler_read_last          | 0     |
| Handler_read_next          | 31    |
| Handler_read_prev          | 0     |
| Handler_read_rnd           | 0     |
| Handler_read_rnd_next      | 0     |
| Handler_rollback           | 0     |
| Handler_savepoint          | 0     |
| Handler_savepoint_rollback | 0     |
| Handler_update             | 0     |
| Handler_write              | 0     |
+----------------------------+-------+
18 rows in set (0.00 sec)

こんな風になるじゃないですか。31行をインデックスからフェッチしてUsing filesort。

それはいいんですが、インデックスだけでWHERE句を解決しているにも関わらずUsing whereが出るし、filteredは100.00以外の値を見た記憶がない。これ5.5からずっとそうだったし、バグなのか仕様なのかよくわからない。

これが5.7では、

mysql57> explain extended SELECT * FROM cards WHERE suite= 'ハート' ORDER BY number;
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+---------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra                                 |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+---------------------------------------+
|  1 | SIMPLE      | cards | NULL       | ref  | idx_suite     | idx_suite | 17      | const |  100 |    31.00 | Using index condition; Using filesort |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+---------------------------------------+
1 row in set, 2 warnings (0.00 sec)

mysql57> SELECT * FROM cards WHERE suite= 'ハート' ORDER BY number;
..
31 rows in set (0.00 sec)

mysql57> SHOW SESSION STATUS LIKE 'handler%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Handler_commit             | 1     |
| Handler_delete             | 0     |
| Handler_discover           | 0     |
| Handler_external_lock      | 2     |
| Handler_mrr_init           | 0     |
| Handler_prepare            | 0     |
| Handler_read_first         | 0     |
| Handler_read_key           | 1     |
| Handler_read_last          | 0     |
| Handler_read_next          | 31    |
| Handler_read_prev          | 0     |
| Handler_read_rnd           | 0     |
| Handler_read_rnd_next      | 0     |
| Handler_rollback           | 0     |
| Handler_savepoint          | 0     |
| Handler_savepoint_rollback | 0     |
| Handler_update             | 0     |
| Handler_write              | 0     |
+----------------------------+-------+
18 rows in set (0.00 sec)

こうなってました。
テーブル全体がrowsの100件で、31.00%がフィルターされてってことなんでしょうかね？ 変なUsing whereは姿を見せなくなっています。


とはいえなんかこのfilteredカラム、ちょっと謎で、インデックスだけで解決できないクエリーにすると

mysql57> explain extended SELECT * FROM cards WHERE suite = 'ハート' AND number = 1 ORDER BY number;
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | cards | NULL       | ref  | idx_suite     | idx_suite | 17      | const |   31 |    10.00 | Using where |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)

mysql57> SELECT * FROM cards WHERE suite = 'ハート' AND number = 1 ORDER BY number;
+-----+-----------+--------+
| seq | suite     | number |
+-----+-----------+--------+
|  41 | ハート    |      1 |
|  47 | ハート    |      1 |
|  50 | ハート    |      1 |
|  59 | ハート    |      1 |
|  96 | ハート    |      1 |
+-----+-----------+--------+
5 rows in set (0.00 sec)

rows= インデックスで解決できた31件、filterd= 10.00%は31 * 10.00% < 5とちょっと誤差が大きすぎる気がします。どうやって計算してるんだろ。