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MySQL选择的执行计划性能底下原因分析--实战案例分析

时间:2018-07-07 12:29:36      阅读:169      评论:0      收藏:0      [点我收藏+]

标签:col   美的   思考   原因分析   tran   art   setup   original   字段   

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!!!!!!

 


MySQL选择的执行计划性能底下原因分析--实战案例分析

标签:col   美的   思考   原因分析   tran   art   setup   original   字段   

原文地址:https://www.cnblogs.com/katec/p/9276746.html

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