MySQL5.6提供了对SQL的跟踪trace, 通过trace文件能够进一步了解为什么优化器选择A计划, 而不是选择B计划。

1.打开trace

打开trace ， 设置格式为 JSON，并设置trace最大能够使用的内存大小，避免解析过程中因为默认内存过小而不能够完整展示。

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

示例1：

mysql> select * from city_innodb;
+---------+-----------+------------+
| city_id | city_name | country_id |
+---------+-----------+------------+
|       1 | Xian      |         10 |
|       2 | NewYork   |          2 |
|       3 | BeiJing   |         10 |
+---------+-----------+------------+
3 rows in set (0.00 sec)

mysql> select * from information_schema.optimizer_trace\G;
*************************** 1. row ***************************
                            QUERY: select * from city_innodb
                            TRACE: {
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `city_innodb`.`city_id` AS `city_id`,`city_innodb`.`city_name` AS `city_name`,`city_innodb`.`country_id` AS `country_id` from `city_innodb`"
          }
        ] /* steps */
      } /* join_preparation */
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "table_dependencies": [
              {
                "table": "`city_innodb`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ] /* depends_on_map_bits */
              }
            ] /* table_dependencies */
          },
          {
            "rows_estimation": [
              {
                "table": "`city_innodb`",
                "table_scan": {
                  "rows": 3,
                  "cost": 0.25
                } /* table_scan */
              }
            ] /* rows_estimation */
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ] /* plan_prefix */,
                "table": "`city_innodb`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "rows_to_scan": 3,
                      "access_type": "scan",
                      "resulting_rows": 3,
                      "cost": 0.55,
                      "chosen": true
                    }
                  ] /* considered_access_paths */
                } /* best_access_path */,
                "condition_filtering_pct": 100,
                "rows_for_plan": 3,
                "cost_for_plan": 0.55,
                "chosen": true
              }
            ] /* considered_execution_plans */
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": null,
              "attached_conditions_computation": [
              ] /* attached_conditions_computation */,
              "attached_conditions_summary": [
                {
                  "table": "`city_innodb`",
                  "attached": null
                }
              ] /* attached_conditions_summary */
            } /* attaching_conditions_to_tables */
          },
          {
            "finalizing_table_conditions": [
            ] /* finalizing_table_conditions */
          },
          {
            "refine_plan": [
              {
                "table": "`city_innodb`"
              }
            ] /* refine_plan */
          }
        ] /* steps */
      } /* join_optimization */
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ] /* steps */
      } /* join_execution */
    }
  ] /* steps */
}
MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0
          INSUFFICIENT_PRIVILEGES: 0
1 row in set (0.00 sec)

ERROR: 
No query specified