13. OPTIMIZER_TRACE

13. OPTIMIZER_TRACE

OPTIMIZER_TRACE表提供由跟踪语句的优化程序跟踪功能生成的信息。 要启用跟踪，请使用optimizer_trace系统变量。 有关详细信息，请参阅MySQL内部：跟踪优化程序。

OPTIMIZER_TRACE表有以下列：

• QUERY ：跟踪的文本
• TRACE ：以JSON格式跟踪
• MISSING_BYTES_BEYOND_MAX_MEM_SIZE：跟踪的每个文件是一个字符串，随着优化的进行而扩展，并将数据附加到该字符串。 optimizer_trace_max_mem_size变量设置所有当前记住的跟踪使用的内存总量的限制。 如果达到此限制，则不会扩展当前跟踪（因此不完整），并且MISSING_BYTES_BEYOND_MAX_MEM_SIZE列显示跟踪中缺少的字节数。
• INSUFFICIENT_PRIVILEGES ：如果跟踪查询使用具有值为DEFINER的SQL SECURITY的视图或存储例程，则可能会拒绝定义者以外的用户查看查询的跟踪。 在这种情况下，跟踪显示为空，INSUFFICIENT_PRIVILEGES的值为1.否则，该值为0。

示例

mysql> SET optimizer_trace="enabled=on";
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM test.t1;
Empty set (0.00 sec)

mysql> SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE\G
*************************** 1. row ***************************
                            QUERY: SELECT * FROM test.t1
                            TRACE: {
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `t1`.`s1` AS `s1`,`t1`.`s2` AS `s2`,`t1`.`s3` AS `s3` from `t1`"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "table_dependencies": [
              {
                "table": "`t1`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`t1`",
                "table_scan": {
                  "rows": 1,
                  "cost": 1
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`t1`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "rows_to_scan": 1,
                      "access_type": "scan",
                      "resulting_rows": 1,
                      "cost": 1.2,
                      "chosen": true
                    }
                  ]
                },
                "condition_filtering_pct": 100,
                "rows_for_plan": 1,
                "cost_for_plan": 1.2,
                "chosen": true
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": null,
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`t1`",
                  "attached": null
                }
              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": "`t1`"
              }
            ]
          }
        ]
      }
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}
MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0
          INSUFFICIENT_PRIVILEGES: 0
1 row in set (0.00 sec)

mysql> SET optimizer_trace="enabled=off";
Query OK, 0 rows affected (0.00 sec)