MySQL实战：为什么选择这个执行计划？（optimizer trace）

CREATE TABLE `demo_table` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `val` int(11) NOT NULL,
  `name` varchar(20) NOT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  KEY `idx_val` (`val`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into demo_table (val, name) values 
(1, 'a'), (1, 'b'), (1, 'c'), (1, 'd'), (1, 'e'), (6, 'f'), (7, 'g'), (8, 'h'), (9, 'i'), (10, 'j');

explain SELECT name from demo_table where val = 6;

# 1. 打开optimizer trace功能 (默认情况下它是关闭的):
SET optimizer_trace="enabled=on";

# 2. 这里输入你自己的查询语句
SELECT name from demo_table where val = 6;

# 3. 从OPTIMIZER_TRACE表中查看上一个查询的优化过程
SELECT * FROM information_schema.OPTIMIZER_TRACE;

# 4. 可能你还要观察其他语句执行的优化过程，重复上边的第2、3步
...

# 5. 当你停止查看语句的优化过程时，把optimizer trace功能关闭
SET optimizer_trace="enabled=off";

# 预估单表不同访问方法的成本
"rows_estimation": [
  {
    "table": "`demo_table`",
    "range_analysis": {
      # 全表扫描
      "table_scan": {
        "rows": 10,
        "cost": 5.1
      },
      # 可能使用到的索引
      "potential_range_indexes": [
        {
          "index": "PRIMARY",
          "usable": false,
          "cause": "not_applicable"
        },
        {
          "index": "idx_val",
          "usable": true,
          "key_parts": [
            "val",
            "id"
          ]
        }
      ],
    # 可能使用到的索引的成本
      "analyzing_range_alternatives": {
        "range_scan_alternatives": [
          {
            "index": "idx_val",
            "ranges": [
              "6 <= val <= 6"
            ],
            "index_dives_for_eq_ranges": true,
            "rowid_ordered": true,
            "using_mrr": false,
            "index_only": false,
            "rows": 1,
            "cost": 2.21,
            "chosen": true
          }
        ]
      },
    # 单表查询的最优方法
      "chosen_range_access_summary": {
        "range_access_plan": {
          "type": "range_scan",
          "index": "idx_val",
          "rows": 1,
          "ranges": [
            "6 <= val <= 6"
          ]
        },
        "rows_for_plan": 1,
        "cost_for_plan": 2.21,
        "chosen": true
      }
    }
  }
]

参考博客

[1]https://zhuanlan.zhihu.com/p/166426713