MySQL是自动会选择它认为好的执行划，但是MySQL毕竟是程序，还没有达到像人类思考这么智能，还是通过一些按部就班的算法实现最优执行计划(基于cost)的选择。下面就是一个真实的案例，带你来看看MySQL也有失误的时候，这种情况不在少数。
注意：一下分析是在MySQl5.6.16版本下，其它版本未验证。
表结构：
CREATE TABLE `test_tab` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `token` varchar(100) NOT NULL, `user_id` int(10) NOT NULL DEFAULT '0', `a` int(10) NOT NULL DEFAULT '0', `b` int(10) NOT NULL, `t_logo` varchar(255) NOT NULL DEFAULT '', `t_name` varchar(50) NOT NULL, `t_contact_name` varchar(20) NOT NULL, `z` varchar(20) NOT NULL, `c` varchar(30) DEFAULT NULL, `d` varchar(100) NOT NULL DEFAULT '0', `e` varchar(100) NOT NULL DEFAULT '0', `t_province` varchar(50) DEFAULT NULL, `f` varchar(50) DEFAULT NULL, `t_district` varchar(50) DEFAULT NULL, `g` varchar(100) NOT NULL, `t_info` text NOT NULL, `h` char(1) NOT NULL DEFAULT '0', `i` tinyint(3) DEFAULT '0', `j` decimal(10,2) NOT NULL DEFAULT '0.00' , `t_add_time` int(10) NOT NULL DEFAULT '0' , `t_update_time` int(10) NOT NULL DEFAULT '0' ', `t_begin_time` int(10) NOT NULL DEFAULT '0' , `t_end_time` int(10) NOT NULL DEFAULT '0' , `k` char(1) NOT NULL DEFAULT '1' , `t_is_check` char(1) NOT NULL DEFAULT '0' , `l` int(10) NOT NULL DEFAULT '0', `u` int(10) NOT NULL DEFAULT '0', `is_delete` tinyint(1) unsigned NOT NULL DEFAULT '0' , `p` varchar(50) DEFAULT NULL, `sort` int(11) NOT NULL DEFAULT '999', PRIMARY KEY (`id`), KEY `z` (`z`), KEY `t_name` (`t_name`), KEY `token` (`token`,`sort`), KEY `idx_0` (`token`,`user_id`,`is_delete`), KEY `idx_doc_time` (`t_add_time`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8
表记录数：
select count(*) from user.`test_tab`;
+----------+
| count(*) |
+----------+
| 2865011 |
+----------+
执行sql： SELECT `id`,`t_name` FROM user.`test_tab` WHERE `token` = 'xx_8cilc73a22hm' AND `t_is_check` = 1 AND `is_delete` = 0 ORDER BY id desc LIMIT 5; +---------+--------------------------------+ | id | t_name | +---------+--------------------------------+ | 2983295 | 《管理之道》 | | 2983289 | 《气功是怎么炼成丹》 | | 2925294 | 小推车配送 | | 2925292 | 特色小拉面 | | 1101709 | 惠世本源 | +---------+--------------------------------+ 5 rows in set (1.25 sec) --可以看到这个sql花了1.25秒查询出来了结果， 这是不可接受的速度 我们来看看执行计划： explain SELECT `id`,`t_name` FROM user.`test_tab` WHERE `token` = 'xx_8cilc73a22hm' AND `t_is_check` = 1 AND `is_delete` = 0 ORDER BY id desc LIMIT 5 +----+-------------+--------------------+-------+-----------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------------------+-------+-----------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | test_tab | index | token,idx_0 | PRIMARY | 4 | NULL | 1185 | Using where | +----+-------------+--------------------+-------+-----------------+---------+---------+------+------+-------------+
说明：执行计划看着很正常，使用了主键索引，没啥大问题，为什么速度这么慢呢？因为主键字段没有范围条件限制，要把所有数据读出来，所以慢了！！！
我们换个索引看看执行效率： SELECT `id`,`t_name` FROM user.`test_tab` force index(token) WHERE `token` = 'xx_8cilc73a22hm' AND `t_is_check` = 1 AND `is_delete` = 0 ORDER BY id desc LIMIT 5; +---------+--------------------------------+ | id | t_name | +---------+--------------------------------+ | 2983295 | 《管理之道》 | | 2983289 | 《气功是怎么炼成丹》 | | 2925294 | 小推车配送 | | 2925292 | 特色小拉面 | | 1101709 | 惠世本源 | +---------+--------------------------------+ 5 rows in set (0.03 sec) --速度这么快，比走主键索引快多了
看下执行计划： explain SELECT `id`,`t_name` FROM user.`test_tab` force index(token) WHERE `token` = 'xx_8cilc73a22hm' AND `t_is_check` = 1 AND `is_delete` = 0 ORDER BY id desc LIMIT 5 +----+-------------+--------------------+------+---------------+-------+---------+-------+-------+----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------------------+------+---------------+-------+---------+-------+-------+----------------------------------------------------+ | 1 | SIMPLE | test_tab | ref | token | token | 302 | const | 11506 | Using index condition; Using where; Using filesort | +----+-------------+--------------------+------+---------------+-------+---------+-------+-------+----------------------------------------------------+
说明：看着执行计划有Using filesort为什么速度会快呢？看看Using index condition这个原来使用了索引下推，也就是说从从存储引擎获得的数据只有5行，然后再进行排序，所以速度很快，哈哈！！这也就是为什么快的原因了。 那为什么MySQL没有选择这个执行token索引呢？
下面我们看看具体原因，如果要看详细执行计划，我们之前是不是说过要看什么呢？optimizer_trace!!回答正确。
set optimizer_trace=1;
sql语句
set
optimizer_trace=0;
select * from information_schema.optimizer_trace;
下面就是我们要看的走主键索引慢的详细的执行计划：
| SELECT `id`,`t_name` FROM user.`test_tab` WHERE `token` = 'xx_8cilc73a22hm' AND `t_is_check` = 1 AND `is_delete` = 0 ORDER BY id desc LIMIT 5 | { "steps": [ { "join_preparation": { "select#": 1, "steps": [ { "expanded_query": "/* select#1 */ select `user`.`test_tab`.`id` AS `id`,`user`.`test_tab`.`t_name` AS `t_name` from `user`.`test_tab` where ((`user`.`test_tab`.`token` = 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1) and (`user`.`test_tab`.`is_delete` = 0)) order by `user`.`test_tab`.`id` desc limit 5" } ] } }, { "join_optimization": { "select#": 1, "steps": [ { "condition_processing": { "condition": "WHERE", "original_condition": "((`user`.`test_tab`.`token` = 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1) and (`user`.`test_tab`.`is_delete` = 0))", "steps": [ { "transformation": "equality_propagation", "resulting_condition": "((`user`.`test_tab`.`token` = 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1) and multiple equal(0, `user`.`test_tab`.`is_delete`))" }, { "transformation": "constant_propagation", "resulting_condition": "((`user`.`test_tab`.`token` = 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1) and multiple equal(0, `user`.`test_tab`.`is_delete`))" }, { "transformation": "trivial_condition_removal", "resulting_condition": "((`user`.`test_tab`.`token` = 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1) and multiple equal(0, `user`.`test_tab`.`is_delete`))" } ] } }, { "table_dependencies": [ { "table": "`user`.`test_tab`", "row_may_be_null": false, "map_bit": 0, "depends_on_map_bits": [ ] } ] }, { "ref_optimizer_key_uses": [ { "table": "`user`.`test_tab`", "field": "token", "equals": "'xx_8cilc73a22hm'", "null_rejecting": false }, { "table": "`user`.`test_tab`", "field": "token", "equals": "'xx_8cilc73a22hm'", "null_rejecting": false } ] }, { "rows_estimation": [ { "table": "`user`.`test_tab`", "range_analysis": { "table_scan": { "rows": 2597778, "cost": 572230 }, "potential_range_indices": [
{ "index": "PRIMARY", "usable": false, --可以看到根据范围条件选择，主键索引是false的，也就是范围条件选择主键是不好，那为啥实际为啥还使用主键呢？往下看。。。
"cause": "not_applicable" }, { "index": "z", "usable": false, "cause": "not_applicable" }, { "index": "t_name", "usable": false, "cause": "not_applicable" }, { "index": "token", "usable": true, --范围查询意向选择token索引。这没有问题啊！！！再往下看
"key_parts": [ "token", "sort", "id" ] }, { "index": "idx_0", "usable": true, "key_parts": [ "token", "user_id", "is_delete", "id" ] }, { "index": "idx_doc_time", "usable": false, "cause": "not_applicable" } ], "setup_range_conditions": [ ], "group_index_range": { "chosen": false, "cause": "not_group_by_or_distinct" }, "analyzing_range_alternatives": { "range_scan_alternatives": [ { "index": "token", "ranges": [ "xx_8cilc73a22hm <= token <= xx_8cilc73a22hm" ], "index_dives_for_eq_ranges": true, "rowid_ordered": false, "using_mrr": false, "index_only": false, "rows": 11506, "cost": 13808, "chosen": true }, { "index": "idx_0", "ranges": [ "xx_8cilc73a22hm <= token <= xx_8cilc73a22hm" ], "index_dives_for_eq_ranges": true, "rowid_ordered": false, "using_mrr": false, "index_only": false, "rows": 10960, "cost": 13153, "chosen": true } ], "analyzing_roworder_intersect": { "usable": false, "cause": "too_few_roworder_scans" } }, "chosen_range_access_summary": { "range_access_plan": { "type": "range_scan", "index": "idx_0", "rows": 10960, "ranges": [ "xx_8cilc73a22hm <= token <= xx_8cilc73a22hm" ] }, "rows_for_plan": 10960, "cost_for_plan": 13153, "chosen": true } } } ] }, { "considered_execution_plans": [ { "plan_prefix": [ ], "table": "`user`.`test_tab`", "best_access_path": { "considered_access_paths": [ { "access_type": "ref", "index": "token", "rows": 11506, "cost": 13807, "chosen": true }, { "access_type": "ref", "index": "idx_0", "rows": 10960, "cost": 13152, "chosen": true }, { "access_type": "range", "cause": "heuristic_index_cheaper", "chosen": false } ] }, "cost_for_plan": 13152, "rows_for_plan": 10960, "chosen": true } ] }, { "attaching_conditions_to_tables": { "original_condition": "((`user`.`test_tab`.`is_delete` = 0) and (`user`.`test_tab`.`token` = 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1))", "attached_conditions_computation": [ ], "attached_conditions_summary": [ { "table": "`user`.`test_tab`", "attached": "((`user`.`test_tab`.`is_delete` = 0) and (`user`.`test_tab`.`token` = 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1))" } ] } }, { "clause_processing": { "clause": "ORDER BY", "original_clause": "`user`.`test_tab`.`id` desc", "items": [ { "item": "`user`.`test_tab`.`id`" } ], "resulting_clause_is_simple": true, "resulting_clause": "`user`.`test_tab`.`id` desc" } }, { "refine_plan": [ { "table": "`user`.`test_tab`", "pushed_index_condition": "((`user`.`test_tab`.`is_delete` = 0) and (`user`.`test_tab`.`token` = 'xx_8cilc73a22hm'))", "table_condition_attached": "(`user`.`test_tab`.`t_is_check` = 1)" } ] }, { "added_back_ref_condition": "((`user`.`test_tab`.`token` <=> 'xx_8cilc73a22hm') and (`user`.`test_tab`.`t_is_check` = 1))" }, { "reconsidering_access_paths_for_index_ordering": { "clause": "ORDER BY", "index_order_summary": { "table": "`user`.`test_tab`", "index_provides_order": true, --关键来了，在判断排序的时候发现主键字段id能够排序，所以MySQL认为排序是花费很大的操作，使用这个主键字段是有序的，不用排序了，就使用它吧！！！！！我靠这不就是错了吗。虽然排序消耗时间，但是你为啥不判断下排序结果集大小啊，扫描所有数据（2865011）和排序5行（这里是排序1000多行还是5行不确定，就当mysql最优5行吧）数据哪个消耗更低?
"order_direction": "desc", "disabled_pushed_condition_on_old_index": true, "index": "PRIMARY", "plan_changed": true, "access_type": "index_scan" } } } ] } }, { "join_execution": { "select#": 1, "steps": [ ] } } ] }
总结下：
1.MySQL在进行分析的时候会把等值、范围的执行消耗都分析出来。
2.在最后选择上选择了不用排序的主键索引。
为什么会选择错误？我推测原因：可能下索引条件下推还没有完善到执行计划中，没有判断索引下推的情况。
给我们的启发：不要完全相信MySQL，需要自己去验证，发现问题，MySQL不是十全十美的，还需要有很多改善！！！！！！