有人可以向我解释为什么我会看到以下行为：
mysql> show index from history_historyentry;
+----------------------+------------+------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------------------+------------+------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| history_historyentry | 0 | PRIMARY | 1 | id | A | 48609 | NULL | NULL | | BTREE | |
+----------------------+------------+------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
1 row in set (0.00 sec)
mysql> explain SELECT COUNT(*) FROM `history_historyentry` WHERE `history_historyentry`.`is_deleted` = False;
+----+-------------+----------------------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | history_historyentry | ALL | NULL | NULL | NULL | NULL | 48612 | Using where |
+----+-------------+----------------------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)
mysql> explain SELECT COUNT(*) FROM `history_historyentry` WHERE `history_historyentry`.`is_deleted` = True;
+----+-------------+----------------------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | history_historyentry | ALL | NULL | NULL | NULL | NULL | 48613 | Using where |
+----+-------------+----------------------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)
mysql> create index deleted on history_historyentry (is_deleted) ;
Query OK, 48627 rows affected (0.38 sec)
Records: 48627 Duplicates: 0 Warnings: 0
mysql> explain SELECT COUNT(*) FROM `history_historyentry` WHERE `history_historyentry`.`is_deleted` = False;
+----+-------------+----------------------+-------+---------------+---------+---------+------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------------+-------+---------------+---------+---------+------+-------+--------------------------+
| 1 | SIMPLE | history_historyentry | index | deleted | deleted | 1 | NULL | 36471 | Using where; Using index |
+----+-------------+----------------------+-------+---------------+---------+---------+------+-------+--------------------------+
1 row in set (0.00 sec)
mysql> explain SELECT COUNT(*) FROM `history_historyentry` WHERE `history_historyentry`.`is_deleted` = True;
+----+-------------+----------------------+------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------------+------+---------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | history_historyentry | ref | deleted | deleted | 1 | const | 166 | Using index |
+----+-------------+----------------------+------+---------------+---------+---------+-------+------+-------------+
1 row in set (0.00 sec)
为什么 True 与 False 的索引使用率存在差异？具体来说，在false的情况下，ref列是NULL，额外的列是Using where；使用索引。但在真实情况下，ref 列是 const，而额外的列是 Using index。