MySQL的真VS假優化

Question

爲什麼我看到下面的行爲可以有人向我解釋：

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) 

爲什麼在使用索引的descrepancy爲真VS假？具體來說，在假的情況下，ref列是NULL，額外的列是使用where;使用索引。但在真實情況下，ref列是const，額外的列是使用索引。

Answer 1

大概是因爲一個提供了很好的選擇性，另一個沒有提供，即只有一小部分的行被刪除。

基於成本的優化器只有在提供了良好的選擇性（通常爲10％）或者可能是覆蓋索引（無需進一步表或書籤查詢）的情況下才會使用索引。