经验分享 – 【MySQL高级】优化SQL步骤

show status like 'Com_______';

复制

show status like 'Innodb_rows_%';

复制

1） id列，用户登录mysql时，系统分配的"connection_id"，可以使用函数connection_id()查看
​
2） user列，显示当前用户。如果不是root，这个命令就只显示用户权限范围的sql语句
​
3） host列，显示这个语句是从哪个ip的哪个端口上发的，可以用来跟踪出现问题语句的用户
​
4） db列，显示这个进程目前连接的是哪个数据库
​
5） command列，显示当前连接的执行的命令，一般取值为休眠（sleep），查询（query），连接（connect）等
​
6） time列，显示这个状态持续的时间，单位是秒
​
7） state列，显示使用当前连接的sql语句的状态，很重要的列。state描述的是语句执行中的某一个状态。一个sql语句，以查询为例，可能需要经过copying to tmp table、sorting result、sending data等状态才可以完成
​
8） info列，显示这个sql语句，是判断问题语句的一个重要依据

复制

explain  select * from tb_item where id = 1;

复制

explain  select * from tb_item where title = '阿尔卡特 (OT-979) 冰川白 联通3G手机3';

复制

CREATE TABLE `t_role` (
  `id` varchar(32) NOT NULL,
  `role_name` varchar(255) DEFAULT NULL,
  `role_code` varchar(255) DEFAULT NULL,
  `description` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique_role_name` (`role_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
​
​
CREATE TABLE `t_user` (
  `id` varchar(32) NOT NULL,
  `username` varchar(45) NOT NULL,
  `password` varchar(96) NOT NULL,
  `name` varchar(45) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique_user_username` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
​
​
CREATE TABLE `user_role` (
  `id` int(11) NOT NULL auto_increment ,
  `user_id` varchar(32) DEFAULT NULL,
  `role_id` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_ur_user_id` (`user_id`),
  KEY `fk_ur_role_id` (`role_id`),
  CONSTRAINT `fk_ur_role_id` FOREIGN KEY (`role_id`) REFERENCES `t_role` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `fk_ur_user_id` FOREIGN KEY (`user_id`) REFERENCES `t_user` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
​
​
​
​
insert into `t_user` (`id`, `username`, `password`, `name`) values('1','super','$2a$10$TJ4TmCdK.X4wv/tCqHW14.w70U3CC33CeVncD3SLmyMXMknstqKRe','超级管理员');
insert into `t_user` (`id`, `username`, `password`, `name`) values('2','admin','$2a$10$TJ4TmCdK.X4wv/tCqHW14.w70U3CC33CeVncD3SLmyMXMknstqKRe','系统管理员');
insert into `t_user` (`id`, `username`, `password`, `name`) values('3','itcast','$2a$10$8qmaHgUFUAmPR5pOuWhYWOr291WJYjHelUlYn07k5ELF8ZCrW0Cui','test02');
insert into `t_user` (`id`, `username`, `password`, `name`) values('4','stu1','$2a$10$pLtt2KDAFpwTWLjNsmTEi.oU1yOZyIn9XkziK/y/spH5rftCpUMZa','学生1');
insert into `t_user` (`id`, `username`, `password`, `name`) values('5','stu2','$2a$10$nxPKkYSez7uz2YQYUnwhR.z57km3yqKn3Hr/p1FR6ZKgc18u.Tvqm','学生2');
insert into `t_user` (`id`, `username`, `password`, `name`) values('6','t1','$2a$10$TJ4TmCdK.X4wv/tCqHW14.w70U3CC33CeVncD3SLmyMXMknstqKRe','老师1');
​
​
​
INSERT INTO `t_role` (`id`, `role_name`, `role_code`, `description`) VALUES('5','学生','student','学生');
INSERT INTO `t_role` (`id`, `role_name`, `role_code`, `description`) VALUES('7','老师','teacher','老师');
INSERT INTO `t_role` (`id`, `role_name`, `role_code`, `description`) VALUES('8','教学管理员','teachmanager','教学管理员');
INSERT INTO `t_role` (`id`, `role_name`, `role_code`, `description`) VALUES('9','管理员','admin','管理员');
INSERT INTO `t_role` (`id`, `role_name`, `role_code`, `description`) VALUES('10','超级管理员','super','超级管理员');
​
​
INSERT INTO user_role(id,user_id,role_id) VALUES(NULL, '1', '5'),(NULL, '1', '7'),(NULL, '2', '8'),(NULL, '3', '9'),(NULL, '4', '8'),(NULL, '5', '10') ;
​
​

复制

explain select * from t_role r, t_user u, user_role ur where r.id = ur.role_id and u.id = ur.user_id ;

复制

EXPLAIN SELECT * FROM t_role WHERE id = (SELECT role_id FROM user_role WHERE user_id = (SELECT id FROM t_user WHERE username = 'stu1'))

复制

EXPLAIN SELECT * FROM t_role r , (SELECT * FROM user_role ur WHERE ur.`user_id` = '2') a WHERE r.id = a.role_id ; 

复制

NULL > system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
​
​
system > const > eq_ref > ref > range > index > ALL

复制

possible_keys : 显示可能应用在这张表的索引， 一个或多个。 
​
key ： 实际使用的索引， 如果为NULL， 则没有使用索引。
​
key_len : 表示索引中使用的字节数， 该值为索引字段最大可能长度，并非实际使用长度，在不损失精确性的前提下， 长度越短越好 。

复制

set profiling=1; //开启profiling 开关；

复制

show databases;
​
use db01;
​
show tables;
​
select * from tb_item where id < 5;
​
select count(*) from tb_item;

复制

TIP ：
    Sending data 状态表示MySQL线程开始访问数据行并把结果返回给客户端，而不仅仅是返回个客户端。由于在Sending data状态下，MySQL线程往往需要做大量的磁盘读取操作，所以经常是整各查询中耗时最长的状态。

复制

SET optimizer_trace="enabled=on",end_markers_in_json=on;
set optimizer_trace_max_mem_size=1000000;

复制

select * from tb_item where id < 4;

复制

select * from information_schema.optimizer_trace;

复制

*************************** 1. row ***************************
QUERY: select * from tb_item where id < 4
TRACE: {
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `tb_item`.`id` AS `id`,`tb_item`.`title` AS `title`,`tb_item`.`price` AS `price`,`tb_item`.`num` AS `num`,`tb_item`.`categoryid` AS `categoryid`,`tb_item`.`status` AS `status`,`tb_item`.`sellerid` AS `sellerid`,`tb_item`.`createtime` AS `createtime`,`tb_item`.`updatetime` AS `updatetime` from `tb_item` where (`tb_item`.`id` < 4)"
          }
        ] /* steps */
      } /* join_preparation */
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "(`tb_item`.`id` < 4)",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "(`tb_item`.`id` < 4)"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(`tb_item`.`id` < 4)"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "(`tb_item`.`id` < 4)"
                }
              ] /* steps */
            } /* condition_processing */
          },
          {
            "table_dependencies": [
              {
                "table": "`tb_item`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ] /* depends_on_map_bits */
              }
            ] /* table_dependencies */
          },
          {
            "ref_optimizer_key_uses": [
            ] /* ref_optimizer_key_uses */
          },
          {
            "rows_estimation": [
              {
                "table": "`tb_item`",
                "range_analysis": {
                  "table_scan": {
                    "rows": 9816098,
                    "cost": 2.04e6
                  } /* table_scan */,
                  "potential_range_indices": [
                    {
                      "index": "PRIMARY",
                      "usable": true,
                      "key_parts": [
                        "id"
                      ] /* key_parts */
                    }
                  ] /* potential_range_indices */,
                  "setup_range_conditions": [
                  ] /* setup_range_conditions */,
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  } /* group_index_range */,
                  "analyzing_range_alternatives": {
                    "range_scan_alternatives": [
                      {
                        "index": "PRIMARY",
                        "ranges": [
                          "id < 4"
                        ] /* ranges */,
                        "index_dives_for_eq_ranges": true,
                        "rowid_ordered": true,
                        "using_mrr": false,
                        "index_only": false,
                        "rows": 3,
                        "cost": 1.6154,
                        "chosen": true
                      }
                    ] /* range_scan_alternatives */,
                    "analyzing_roworder_intersect": {
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    } /* analyzing_roworder_intersect */
                  } /* analyzing_range_alternatives */,
                  "chosen_range_access_summary": {
                    "range_access_plan": {
                      "type": "range_scan",
                      "index": "PRIMARY",
                      "rows": 3,
                      "ranges": [
                        "id < 4"
                      ] /* ranges */
                    } /* range_access_plan */,
                    "rows_for_plan": 3,
                    "cost_for_plan": 1.6154,
                    "chosen": true
                  } /* chosen_range_access_summary */
                } /* range_analysis */
              }
            ] /* rows_estimation */
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ] /* plan_prefix */,
                "table": "`tb_item`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "range",
                      "rows": 3,
                      "cost": 2.2154,
                      "chosen": true
                    }
                  ] /* considered_access_paths */
                } /* best_access_path */,
                "cost_for_plan": 2.2154,
                "rows_for_plan": 3,
                "chosen": true
              }
            ] /* considered_execution_plans */
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "(`tb_item`.`id` < 4)",
              "attached_conditions_computation": [
              ] /* attached_conditions_computation */,
              "attached_conditions_summary": [
                {
                  "table": "`tb_item`",
                  "attached": "(`tb_item`.`id` < 4)"
                }
              ] /* attached_conditions_summary */
            } /* attaching_conditions_to_tables */
          },
          {
            "refine_plan": [
              {
                "table": "`tb_item`",
                "access_type": "range"
              }
            ] /* refine_plan */
          }
        ] /* steps */
      } /* join_optimization */
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ] /* steps */
      } /* join_execution */
    }
  ] /* steps */
}

复制