CREATE TABLE `employees` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名',
 `age` int(11) NOT NULL DEFAULT '0' COMMENT '年齡',
 `position` varchar(20) NOT NULL DEFAULT '' COMMENT '職位',
 `hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入職時間',
 PRIMARY KEY (`id`),
 KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE
 ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT='員工記錄表';

 INSERT INTO employees(name,age,position,hire_time) VALUES('LiLei',22,'manager',NOW());
 INSERT INTO employees(name,age,position,hire_time) VALUES('HanMeimei', 23,'dev',NOW());
 INSERT INTO employees(name,age,position,hire_time) VALUES('Lucy',23,'dev',NOW());

 ‐‐ 插入一些示例數(shù)據(jù)
 drop procedure if exists insert_emp;
 delimiter ;;
 create procedure insert_emp()
 begin
 declare i int;
 set i=1;
 while(i<=100000)do
 insert into employees(name,age,position) values(CONCAT('zhuge',i),i,'dev');
 set i=i+1;
 end while;
 end;;
 delimiter ;
 call insert_emp();

EXPLAIN SELECT * FROM employees WHERE name > 'LiLei' AND age = 22 AND position ='manager';

EXPLAIN SELECT * FROM employees force index(idx_name_age_position) WHERE name > 'LiLei' AND age = 22 AND position ='manager';

EXPLAIN SELECT name,age,position FROM employees WHERE name > 'LiLei' AND age = 22 AND position ='manager';

EXPLAIN SELECT * FROM employees WHERE name in ('LiLei','HanMeimei','Lucy') AND age = 22 AND position ='manager';

EXPLAIN SELECT * FROM employees WHERE (name = 'LiLei' or name = 'HanMeimei') AND age = 22 AND position ='manager';

EXPLAIN SELECT * FROM employees_copy WHERE name in ('LiLei','HanMeimei','Lucy') AND age = 22 AND position ='manager';

EXPLAIN SELECT * FROM employees_copy WHERE (name = 'LiLei' or name = 'HanMeimei') AND age = 22 AND position ='manager';

EXPLAIN SELECT * FROM employees WHERE name like 'LiLei%' AND age = 22 AND position ='manager';

EXPLAIN SELECT * FROM employees_copy WHERE name like 'LiLei%' AND age = 22 AND position ='manager';

mysql> set session optimizer_trace="enabled=on",end_markers_in_json=on;  --開啟trace

mysql> select * from employees where name > 'a' order by position;

mysql> SELECT * FROM information_schema.OPTIMIZER_TRACE;

{
  "steps": [
    {
      "join_preparation": {    //第一階段：SQL準備階段，格式化sql
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `employees`.`id` AS `id`,`employees`.`name` AS `name`,`employees`.`age` AS `age`,`employees`.`position` AS `position`,`employees`.`hire_time` AS `hire_time` from `employees` where (`employees`.`name` > 'a') order by `employees`.`position`"
          }
        ] /* steps */
      } /* join_preparation */
    },
    {
      "join_optimization": {    //第二階段：SQL優(yōu)化階段
        "select#": 1,
        "steps": [
          {
            "condition_processing": {    //條件處理
              "condition": "WHERE",
              "original_condition": "(`employees`.`name` > 'a')",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "(`employees`.`name` > 'a')"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(`employees`.`name` > 'a')"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "(`employees`.`name` > 'a')"
                }
              ] /* steps */
            } /* condition_processing */
          },
          {
            "substitute_generated_columns": {
            } /* substitute_generated_columns */
          },
          {
            "table_dependencies": [    //表依賴詳情
              {
                "table": "`employees`",
                "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": [    //預(yù)估表的訪問成本
              {
                "table": "`employees`",
                "range_analysis": {
                  "table_scan": {     //全表掃描情況
                    "rows": 10123,    //掃描行數(shù)
                    "cost": 2054.7    //查詢成本
                  } /* table_scan */,
                  "potential_range_indexes": [    //查詢可能使用的索引
                    {
                      "index": "PRIMARY",    //主鍵索引
                      "usable": false,
                      "cause": "not_applicable"
                    },
                    {
                      "index": "idx_name_age_position",    //輔助索引
                      "usable": true,
                      "key_parts": [
                        "name",
                        "age",
                        "position",
                        "id"
                      ] /* key_parts */
                    }
                  ] /* potential_range_indexes */,
                  "setup_range_conditions": [
                  ] /* setup_range_conditions */,
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  } /* group_index_range */,
                  "analyzing_range_alternatives": {    //分析各個索引使用成本
                    "range_scan_alternatives": [
                      {
                        "index": "idx_name_age_position",
                        "ranges": [
                          "a < name"      //索引使用范圍
                        ] /* ranges */,
                        "index_dives_for_eq_ranges": true,
                        "rowid_ordered": false,    //使用該索引獲取的記錄是否按照主鍵排序
                        "using_mrr": false,
                        "index_only": false,       //是否使用覆蓋索引
                        "rows": 5061,              //索引掃描行數(shù)
                        "cost": 6074.2,            //索引使用成本
                        "chosen": false,           //是否選擇該索引
                        "cause": "cost"
                      }
                    ] /* range_scan_alternatives */,
                    "analyzing_roworder_intersect": {
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    } /* analyzing_roworder_intersect */
                  } /* analyzing_range_alternatives */
                } /* range_analysis */
              }
            ] /* rows_estimation */
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ] /* plan_prefix */,
                "table": "`employees`",
                "best_access_path": {    //最優(yōu)訪問路徑
                  "considered_access_paths": [   //最終選擇的訪問路徑
                    {
                      "rows_to_scan": 10123,
                      "access_type": "scan",     //訪問類型：為scan，全表掃描
                      "resulting_rows": 10123,
                      "cost": 2052.6,
                      "chosen": true,            //確定選擇
                      "use_tmp_table": true
                    }
                  ] /* considered_access_paths */
                } /* best_access_path */,
                "condition_filtering_pct": 100,
                "rows_for_plan": 10123,
                "cost_for_plan": 2052.6,
                "sort_cost": 10123,
                "new_cost_for_plan": 12176,
                "chosen": true
              }
            ] /* considered_execution_plans */
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "(`employees`.`name` > 'a')",
              "attached_conditions_computation": [
              ] /* attached_conditions_computation */,
              "attached_conditions_summary": [
                {
                  "table": "`employees`",
                  "attached": "(`employees`.`name` > 'a')"
                }
              ] /* attached_conditions_summary */
            } /* attaching_conditions_to_tables */
          },
          {
            "clause_processing": {
              "clause": "ORDER BY",
              "original_clause": "`employees`.`position`",
              "items": [
                {
                  "item": "`employees`.`position`"
                }
              ] /* items */,
              "resulting_clause_is_simple": true,
              "resulting_clause": "`employees`.`position`"
            } /* clause_processing */
          },
          {
            "reconsidering_access_paths_for_index_ordering": {
              "clause": "ORDER BY",
              "steps": [
              ] /* steps */,
              "index_order_summary": {
                "table": "`employees`",
                "index_provides_order": false,
                "order_direction": "undefined",
                "index": "unknown",
                "plan_changed": false
              } /* index_order_summary */
            } /* reconsidering_access_paths_for_index_ordering */
          },
          {
            "refine_plan": [
              {
                "table": "`employees`"
              }
            ] /* refine_plan */
          }
        ] /* steps */
      } /* join_optimization */
    },
    {
      "join_execution": {    //第三階段：SQL執(zhí)行階段
        "select#": 1,
        "steps": [
        ] /* steps */
      } /* join_execution */
    }
  ] /* steps */
}
// 結(jié)論：全表掃描的成本低于索引掃描，所以mysql最終選擇全表掃描

mysql> select * from employees where name > 'zzz' order by position;
mysql> SELECT * FROM information_schema.OPTIMIZER_TRACE;

# 查看trace字段可知索引掃描的成本低于全表掃描，所以mysql最終選擇索引掃描

mysql> set session optimizer_trace="enabled=off";    //關(guān)閉trace

select * from employees limit 10000,10;

select * from employees where id > 90000 limit 5;

EXPLAIN select * from employees ORDER BY name limit 90000,5;

select * from employees e inner join (select id from employees order by name limit 90000,5) ed on e.id = ed.id;

CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_a` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

create table t2 like t1;

-- 插入一些示例數(shù)據(jù)
-- 往t1表插入1萬行記錄
drop?procedure?if?exists?insert_t1;?
delimiter?;;
create?procedure?insert_t1()
begin
??declare?i?int;
??set?i=1;
??while(i<=10000)do
????insert?into?t1(a,b)?values(i,i);??
????set?i=i+1;
??end?while;
end;;
delimiter?;
call?insert_t1();

-- 往t2表插入100行記錄
drop?procedure?if?exists?insert_t2;?
delimiter?;;
create?procedure?insert_t2()
begin
??declare?i?int;
??set?i=1;
??while(i<=100)do
????insert?into?t2(a,b)?values(i,i);??
????set?i=i+1;
??end?while;
end;;
delimiter?;
call?insert_t2();

EXPLAIN select * from t1 inner join t2 on t1.a= t2.a;

EXPLAIN select * from t1 inner join t2 on t1.b= t2.b;

select * from A where id in (select id from B)
#等價于： 　　
for(select id from B){
    select * from A where A.id = B.id
}

select * from A where exists (select 1 from B where B.id = A.id)
#等價于: 
for(select * from A){
  select * from B where B.id = A.id 
} 
#A表與B表的ID字段應(yīng)建立索引

EXPLAIN select count(1) from employees;
EXPLAIN select count(id) from employees;
EXPLAIN select count(name) from employees;
EXPLAIN select count(*) from employees;