转自：
https://www.cnblogs.com/katec/p/9276746.html
专注MySQL、Python。
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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 |
+---------+--------------------------------+
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 |
+---------+--------------------------------+
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！！！！！！
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