mysql - 第4课 - trace工具用法详解

注：当前测试mysql版本：mysql5.7，编码utf8mb4

trace是MySQL5.6版本后提供的SQL跟踪工具，通过使用trace可以让我们明白optimizer（优化器）如何选择执行计划。

注意：开启trace工具会影响mysql性能，所以只适合临时分析sql使用，用完之后请立即关闭。

测试数据脚本：

DROP TABLE IF EXISTS `t_student`;
CREATE TABLE `t_student` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `std_name` varchar(30) NOT NULL,
  `age` tinyint(3) unsigned NOT NULL,
  `class_id` int(11) unsigned NOT NULL,
  `gmt_create` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `idx_std_age` (`age`),
  KEY `idx_std_name_age_class` (`std_name`,`age`,`class_id`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=100766 DEFAULT CHARSET=utf8mb4;

--添加测试数据的存储过程
drop procedure if exists proc_insert_student; 
create procedure proc_insert_student()        
begin
  declare i int;                    
  set i=1;                          
  while(i<=100000)do                 
    insert into t_student(std_name,age,class_id) values(CONCAT('Li Lei',i), (i mod 120)+1 ,(i mod 3)+1);   
    set i=i+1;                       
  end while;
end;

-- 执行存储过程
call proc_insert_student();

trace工具用法

一.查看trace开关状态，默认关闭的

show variables like 'optimizer_trace';

二.开启face

1.会话级别临时开启，只在当前会话生效。

set session optimizer_trace="enabled=on",end_markers_in_json=on;

2.永久开启（重启失效）

注意用完关闭

set optimizer_trace="enabled=on";

三.用法

在查询sql后加上固定sql，例：

set session optimizer_trace="enabled=on",end_markers_in_json=on;  
select * from t_student where std_name > 'a' order by age;
SELECT * FROM information_schema.OPTIMIZER_TRACE;

执行结果：

把TRACE列的json数据拷贝出来查看。

{
  "steps": [
    {
      "join_preparation": {  -- 第一阶段：SQL准备阶段，格式化sql
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `t_student`.`id` AS `id`,`t_student`.`std_name` AS `std_name`,`t_student`.`age` AS `age`,`t_student`.`class_id` AS `class_id`,`t_student`.`gmt_create` AS `gmt_create` from `t_student` where (`t_student`.`std_name` > 'a') order by `t_student`.`age`"
          }
        ] /* steps */
      } /* join_preparation */
    },
    {
      "join_optimization": {  -- 第二阶段：SQL优化阶段
        "select#": 1,
        "steps": [
          {
            "condition_processing": {  -- 条件处理
              "condition": "WHERE",
              "original_condition": "(`t_student`.`std_name` > 'a')",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "(`t_student`.`std_name` > 'a')"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(`t_student`.`std_name` > 'a')"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "(`t_student`.`std_name` > 'a')"
                }
              ] /* steps */
            } /* condition_processing */
          },
          {
            "substitute_generated_columns": {
            } /* substitute_generated_columns */
          },
          {
            "table_dependencies": [  -- 表依赖详情
              {
                "table": "`t_student`",
                "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": [  -- 预估表的访问成本
              {
                "table": "`t_student`",
                "range_analysis": {
                  "table_scan": {   -- 全表扫描
                    "rows": 100300,  -- 行数
                    "cost": 20351  -- 查询消耗
                  } /* table_scan */,
                  "potential_range_indexes": [  -- 查询可能使用的索引
                    {
                      "index": "PRIMARY",  -- 主键索引
                      "usable": false,  -- 未使用
                      "cause": "not_applicable"  -- 原因：不适合
                    },
                    {
                      "index": "idx_std_age",  -- age索引
                      "usable": false,  -- 未使用
                      "cause": "not_applicable"  -- 原因：不适合
                    },
                    {
                      "index": "idx_std_name_age_class",  -- stdname,age,class的组合索引
                      "usable": true,  -- 使用
                      "key_parts": [
                        "std_name",
                        "age",
                        "class_id",
                        "id"
                      ] /* key_parts */
                    }
                  ] /* potential_range_indexes */,
                  "setup_range_conditions": [
                  ] /* setup_range_conditions */,
                  "group_index_range": {  -- group 用到的索引
                    "chosen": false,  -- 未使用
                    "cause": "not_group_by_or_distinct"  -- 原因：未使用group by 或者 distinct
                  } /* group_index_range */,
                  "analyzing_range_alternatives": {   -- 分析各个索引使用成本
                    "range_scan_alternatives": [
                      {
                        "index": "idx_std_name_age_class",
                        "ranges": [
                          "a < std_name"  -- 索引使用范围
                        ] /* ranges */,
                        "index_dives_for_eq_ranges": true,
                        "rowid_ordered": false,  -- 使用该索引获取的记录是否按照主键排序
                        "using_mrr": false,
                        "index_only": false,  -- 是否使用覆盖索引
                        "rows": 50150,  -- 索引扫描行数
                        "cost": 60181,   -- 索引使用成本
                        "chosen": false,  -- 是否选择该索引：否
                        "cause": "cost"  -- 原因：消耗
                      }
                    ] /* range_scan_alternatives */,
                    "analyzing_roworder_intersect": {  -- 分析使用索引合并的成本
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    } /* analyzing_roworder_intersect */
                  } /* analyzing_range_alternatives */
                } /* range_analysis */
              }
            ] /* rows_estimation */
          },
          {
            "considered_execution_plans": [  -- 分析出的执行计划
              {
                "plan_prefix": [
                ] /* plan_prefix */,
                "table": "`t_student`",
                "best_access_path": {  -- 最优访问路径
                  "considered_access_paths": [  --分析出的最终访问路径
                    {
                      "rows_to_scan": 100300,
                      "access_type": "scan",  -- 访问类型：为scan，全表扫描
                      "resulting_rows": 100300,
                      "cost": 20349,
                      "chosen": true,  -- 确定选择
                      "use_tmp_table": true
                    }
                  ] /* considered_access_paths */
                } /* best_access_path */,
                "condition_filtering_pct": 100,
                "rows_for_plan": 100300,
                "cost_for_plan": 20349,
                "sort_cost": 100300,
                "new_cost_for_plan": 120649,
                "chosen": true
              }
            ] /* considered_execution_plans */
          },
          {
            "attaching_conditions_to_tables": {   -- 为查询的表添加条件
              "original_condition": "(`t_student`.`std_name` > 'a')",
              "attached_conditions_computation": [
              ] /* attached_conditions_computation */,
              "attached_conditions_summary": [    -- 添加条件结果
                {
                  "table": "`t_student`",
                  "attached": "(`t_student`.`std_name` > 'a')"
                }
              ] /* attached_conditions_summary */
            } /* attaching_conditions_to_tables */
          },
          {
            "clause_processing": {   -- order by 处理
              "clause": "ORDER BY",
              "original_clause": "`t_student`.`age`",
              "items": [
                {
                  "item": "`t_student`.`age`"
                }
              ] /* items */,
              "resulting_clause_is_simple": true,
              "resulting_clause": "`t_student`.`age`"
            } /* clause_processing */
          },
          {
            "reconsidering_access_paths_for_index_ordering": {    -- 重构索引处理顺序
              "clause": "ORDER BY",
              "steps": [
              ] /* steps */,
              "index_order_summary": {
                "table": "`t_student`",
                "index_provides_order": false,
                "order_direction": "undefined",
                "index": "unknown",
                "plan_changed": false
              } /* index_order_summary */
            } /* reconsidering_access_paths_for_index_ordering */
          },
          {
            "refine_plan": [
              {
                "table": "`t_student`"
              }
            ] /* refine_plan */
          }
        ] /* steps */
      } /* join_optimization */
    },
    {
      "join_execution": {    -- 第三阶段：SQL执行阶段
        "select#": 1,
        "steps": [
          {
            "filesort_information": [
              {
                "direction": "asc",
                "table": "`t_student`",
                "field": "age"
              }
            ] /* filesort_information */,
            "filesort_priority_queue_optimization": {
              "usable": false,
              "cause": "not applicable (no LIMIT)"
            } /* filesort_priority_queue_optimization */,
            "filesort_execution": [
            ] /* filesort_execution */,
            "filesort_summary": {
              "rows": 100000,
              "examined_rows": 100000,
              "number_of_tmp_files": 14,
              "sort_buffer_size": 262016,
              "sort_mode": "<sort_key, packed_additional_fields>"
            } /* filesort_summary */
          }
        ] /* steps */
      } /* join_execution */
    }
  ] /* steps */
}