Mysql trace命令介绍

trace作用：

• 对SQL的跟踪，可以知道SQL是如何执行的，比EXPLAIN还要强大。

注意点：

• 开启trace工具会影响mysql性能，所以只能临时分析sql使用，用 完之后立即关闭。

开启trace：

set session optimizer_trace="enabled=on",end_markers_in_json=on; #开启
set session optimizer_trace="enabled=off"; #关闭

如果使用索引查询数据，但最终还是走了全表扫描，可能是全表扫描的成本低于索引扫描，所以mysql最终选择全表扫描。
具体根据trace命令就可以根据sql查询成本对比出来。

查看根据记录：

# 这里是需要执行的sql
SELECT * FROM information_schema.OPTIMIZER_TRACE;

出现的结果是json数据：

{
  "steps": [
    {
      "join_preparation": { -- 第一阶段：SQL准备阶段
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `cr_shop_tables`.`id` AS `id`,`cr_shop_tables`.`title` AS `title`,`cr_shop_tables`.`num` AS `num`,`cr_shop_tables`.`qrcode_pic` AS `qrcode_pic`,`cr_shop_tables`.`sort` AS `sort`,`cr_shop_tables`.`create_time` AS `create_time`,`cr_shop_tables`.`modify_time` AS `modify_time`,`cr_shop_tables`.`shop_id` AS `shop_id`,`cr_shop_tables`.`status` AS `status` from `cr_shop_tables` where (`cr_shop_tables`.`title` > 'a') order by `cr_shop_tables`.`num`"
          }
        ] /* 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": "(`cr_shop_tables`.`title` > 'a')",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "(`cr_shop_tables`.`title` > 'a')"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(`cr_shop_tables`.`title` > 'a')"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "(`cr_shop_tables`.`title` > 'a')"
                }
              ] /* steps */
            } /* condition_processing */
          },
          {
            "substitute_generated_columns": {
            } /* substitute_generated_columns */
          },
          {
            "table_dependencies": [ ‐‐表依赖详情
              {
                "table": "`cr_shop_tables`",
                "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": "`cr_shop_tables`",
                "table_scan": {   ‐‐全表扫描情况
                  "rows": 207,  ‐‐扫描行数
                  "cost": 4  ‐‐查询成本
                } /* table_scan */
              }
            ] /* rows_estimation */
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ] /* plan_prefix */,
                "table": "`cr_shop_tables`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "rows_to_scan": 207,
                      "access_type": "scan",
                      "resulting_rows": 207,
                      "cost": 45.4,
                      "chosen": true,
                      "use_tmp_table": true
                    }
                  ] /* considered_access_paths */
                } /* best_access_path */,
                "condition_filtering_pct": 100,
                "rows_for_plan": 207,
                "cost_for_plan": 45.4,
                "sort_cost": 207,
                "new_cost_for_plan": 252.4,
                "chosen": true
              }
            ] /* considered_execution_plans */
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "(`cr_shop_tables`.`title` > 'a')",
              "attached_conditions_computation": [
              ] /* attached_conditions_computation */,
              "attached_conditions_summary": [
                {
                  "table": "`cr_shop_tables`",
                  "attached": "(`cr_shop_tables`.`title` > 'a')"
                }
              ] /* attached_conditions_summary */
            } /* attaching_conditions_to_tables */
          },
          {
            "clause_processing": {
              "clause": "ORDER BY",
              "original_clause": "`cr_shop_tables`.`num`",
              "items": [
                {
                  "item": "`cr_shop_tables`.`num`"
                }
              ] /* items */,
              "resulting_clause_is_simple": true,
              "resulting_clause": "`cr_shop_tables`.`num`"
            } /* clause_processing */
          },
          {
            "refine_plan": [
              {
                "table": "`cr_shop_tables`"
              }
            ] /* refine_plan */
          }
        ] /* steps */
      } /* join_optimization */
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
          {
            "filesort_information": [
              {
                "direction": "asc",
                "table": "`cr_shop_tables`",
                "field": "num"
              }
            ] /* filesort_information */,
            "filesort_priority_queue_optimization": {
              "usable": false,
              "cause": "not applicable (no LIMIT)"
            } /* filesort_priority_queue_optimization */,
            "filesort_execution": [
            ] /* filesort_execution */,
            "filesort_summary": {
              "rows": 133,
              "examined_rows": 207,
              "number_of_tmp_files": 0,
              "sort_buffer_size": 32136,
              "sort_mode": "<sort_key, rowid>"
            } /* filesort_summary */
          }
        ] /* steps */
      } /* join_execution */
    }
  ] /* steps */
}

本文地址：https://blog.csdn.net/xianyun1992/article/details/107650596