示例表
表结构
DROP TABLE IF EXISTS employees;
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 '年龄',
`position` varchar(20) NOT NULL DEFAULT '' COMMENT '职位',
`hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间',
PRIMARY KEY (`id`),
KEY `idx_name_age_position` (`name`, `age`, `position`) USING BTREE
) ENGINE = InnoDB
AUTO_INCREMENT = 1
DEFAULT CHARSET = utf8 COMMENT ='员工记录表';测试数据
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());
# 插入一些示例数据
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();不同的索引选择
我们用一个查询语句来做示例，同样的查询条件，仅仅是查询参数的不同。
全表扫描示例
EXPLAIN
SELECT *
FROM employees
WHERE name > 'a';[
{
"id": 1,
"select_type": "SIMPLE",
"table": "employees",
"partitions": null,
"type": "ALL",
"possible_keys": "idx_name_age_position",
"key": null,
"key_len": null,
"ref": null,
"rows": 100269,
"filtered": 50,
"Extra": "Using where"
}
]覆盖索引示例
EXPLAIN
SELECT name, age, position
FROM employees
WHERE name > 'a';[
{
"id": 1,
"select_type": "SIMPLE",
"table": "employees",
"partitions": null,
"type": "range",
"possible_keys": "idx_name_age_position",
"key": "idx_name_age_position",
"key_len": "74",
"ref": null,
"rows": 50134,
"filtered": 100,
"Extra": "Using where; Using index"
}
]走索引示例
EXPLAIN
SELECT *
FROM employees
WHERE name > 'zzz';[
{
"id": 1,
"select_type": "SIMPLE",
"table": "employees",
"partitions": null,
"type": "range",
"possible_keys": "idx_name_age_position",
"key": "idx_name_age_position",
"key_len": "74",
"ref": null,
"rows": 1,
"filtered": 100,
"Extra": "Using index condition"
}
]可以看到，上述3种情况，MySQL 选择了不同的查询方案。 mysql最终选择是否走索引或者一张表涉及多个索引最终选择哪个索引，我们可以用 trace 工具来一查究竟。
trace介绍
用法
开启 trace 工具会影响 MySQL 性能，所以只能临时分析 sql 使用，使用后应该立即关闭。
# 开启trace
SET SESSION optimizer_trace = "enabled=on",end_markers_in_json = ON;
# 关闭 trace
SET SESSION optimizer_trace = "enabled=off";使用trace工具查看语句：
SELECT *
FROM employees
WHERE name > 'a'
ORDER BY position;
SELECT *
FROM information_schema.optimizer_trace;结果分析
{
"steps": [
{
/* 第一阶段，sql准备阶段，格式化sql */
"join_preparation": {
"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 */
},
{
/* 第二阶段，sql优化阶段 */
"join_optimization": {
"select#": 1,
"steps": [
{
/* 条件处理 */
"condition_processing": {
"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 */
},
{
/* 表依赖详情 */
"table_dependencies": [
{
"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 */
},
{
/* 预估表的访问成本 */
"rows_estimation": [
{
"table": "`employees`",
"range_analysis": {
/* 全表扫描情况 */
"table_scan": {
/* 扫描行数 */
"rows": 100269,
/* 扫描成本 */
"cost": 20345
} /* table_scan */,
/* 查询可能使用的索引 */
"potential_range_indexes": [
{
/* 主键索引 */
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
/* 辅助索引 */
"index": "idx_name_age_position",
"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 */,
/* 分析各个索引使用成本 */
"analyzing_range_alternatives": {
"range_scan_alternatives": [
{
"index": "idx_name_age_position",
/* 索引使用范围 */
"ranges": [
"a < name"
] /* ranges */,
"index_dives_for_eq_ranges": true,
/* 使用该索引获取的记录是否按照主键排序 */
"rowid_ordered": false,
"using_mrr": false,
/* 是否使用覆盖索引 */
"index_only": false,
/* 索引扫描行数 */
"rows": 50134,
/* 索引扫描成本 */
"cost": 60162,
/* 是否选择该索引 */
"chosen": false,
"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`",
/* 最优访问路径 */
"best_access_path": {
/* 最终选择的访问路径 */
"considered_access_paths": [
{
"rows_to_scan": 100269,
/* 访问类型，scan为全表扫描 */
"access_type": "scan",
"resulting_rows": 100269,
"cost": 20343,
/* 是否选择：是 */
"chosen": true,
"use_tmp_table": true
}
] /* considered_access_paths */
} /* best_access_path */,
"condition_filtering_pct": 100,
"rows_for_plan": 100269,
"cost_for_plan": 20343,
"sort_cost": 100269,
"new_cost_for_plan": 120612,
"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 */
},
{
/* 第三阶段，sql执行阶段 */
"join_execution": {
"select#": 1,
"steps": [
{
"filesort_information": [
{
"direction": "asc",
"table": "`employees`",
"field": "position"
}
] /* filesort_information */,
"filesort_priority_queue_optimization": {
"usable": false,
"cause": "not applicable (no LIMIT)"
} /* filesort_priority_queue_optimization */,
"filesort_execution": [
] /* filesort_execution */,
"filesort_summary": {
"rows": 100003,
"examined_rows": 100003,
"number_of_tmp_files": 30,
"sort_buffer_size": 262056,
"sort_mode": "<sort_key, packed_additional_fields>"
} /* filesort_summary */
}
] /* steps */
} /* join_execution */
}
] /* steps */
}上述 json 结果中给出大部分关键字段的注释，可以看到当前执行语句，MySQL 分析全表扫描比走索引查询的成本 cost 更低，所以选择了全表扫描。