表数据量影响MySQL索引选择

现象

新建了一张员工表，插入了少量数据，索引中所有的字段均在where条件出现时，正确走到了idx_nap索引，但是where出现部分自左开始的索引时，却进行全表扫描，与MySQL官方所说的最左匹配原则“相悖”。

数据背景

CREATE TABLE `staffs` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名',
  `age` int(11) NOT NULL DEFAULT '0' COMMENT '年龄',
  `pos` varchar(20) NOT NULL DEFAULT '' COMMENT '职位',
  `add_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间',
  PRIMARY KEY (`id`),
  KEY `idx_nap` (`name`,`age`,`pos`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8 COMMENT='员工记录表';

表中数据如下：
id	name	age	pos	    add_time
1	July	23	dev	    2018-06-04 16:02:02
2	Clive	22	dev	    2018-06-04 16:02:32
3	Cleva	24	test	2018-06-04 16:02:38
4	July	23	test	2018-06-04 16:12:22
5	July	23	pre	    2018-06-04 16:12:37
6	Clive	22	pre	    2018-06-04 16:12:48
7	July	25	dev	    2018-06-04 16:30:17

Explain语句看下执行计划

-- 全匹配走了索引
explain select * from staffs where name = 'July' and age = 23 and pos = 'dev';
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	staffs	NULL	ref	idx_nap	idx_nap	140	const,const,const	1	100.00	NULL

开启优化器跟踪优化过程

-- 左侧部分匹配却没有走索引，全表扫描
explain select * from staffs where name = 'July' and age = 23;
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	staffs2	NULL	ALL	idx_nap	NULL	NULL	NULL	6	50.00	Using where

-- 开启优化器跟踪
set session optimizer_trace='enabled=on';
-- 在执行完查询语句后，在执行以下的select语句可以查看具体的优化器执行过程
select * from information_schema.optimizer_trace;

Trace部分的内容

{
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `staffs`.`id` AS `id`,`staffs`.`name` AS `name`,`staffs`.`age` AS `age`,`staffs`.`pos` AS `pos`,`staffs`.`add_time` AS `add_time` from `staffs` where ((`staffs`.`name` = 'July') and (`staffs`.`age` = 23))"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "((`staffs`.`name` = 'July') and (`staffs`.`age` = 23))",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "((`staffs`.`name` = 'July') and multiple equal(23, `staffs`.`age`))"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "((`staffs`.`name` = 'July') and multiple equal(23, `staffs`.`age`))"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "((`staffs`.`name` = 'July') and multiple equal(23, `staffs`.`age`))"
                }
              ]
            }
          },
          {
            "substitute_generated_columns": {
            }
          },
          {
            "table_dependencies": [
              {
                "table": "`staffs`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "`staffs`",
                "field": "name",
                "equals": "'July'",
                "null_rejecting": false
              },
              {
                "table": "`staffs`",
                "field": "age",
                "equals": "23",
                "null_rejecting": false
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`staffs`",
                "range_analysis": {
                  "table_scan": {
                    "rows": 6,
                    "cost": 4.3
                  },
                  "potential_range_indexes": [
                    {
                      "index": "PRIMARY",
                      "usable": false,
                      "cause": "not_applicable"
                    },
                    {
                      "index": "idx_nap",
                      "usable": true,
                      "key_parts": [
                        "name",
                        "age",
                        "pos",
                        "id"
                      ]
                    }
                  ],
                  "setup_range_conditions": [
                  ],
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  },
                  "analyzing_range_alternatives": {
                    "range_scan_alternatives": [
                      {
                        "index": "idx_nap",
                        "ranges": [
                          "July <= name <= July AND 23 <= age <= 23"
                        ],
                        "index_dives_for_eq_ranges": true,
                        "rowid_ordered": false,
                        "using_mrr": false,
                        "index_only": false,
                        "rows": 3,
                        "cost": 4.61,
                        "chosen": false,
                        "cause": "cost"
                      }
                    ],
                    "analyzing_roworder_intersect": {
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    }
                  }
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`staffs`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                    //可以看到这边MySQL计算得到使用索引的成本为2.6
                      "access_type": "ref",
                      "index": "idx_nap",
                      "rows": 3,
                      "cost": 2.6,
                      "chosen": true
                    },
                    {
                    //而全表扫描计算所得的成本为2.2
                      "rows_to_scan": 6,
                      "access_type": "scan",
                      "resulting_rows": 6,
                      "cost": 2.2,
                      "chosen": true
                    }
                  ]
                },
                //因此选择了成本更低的scan
                "condition_filtering_pct": 100,
                "rows_for_plan": 6,
                "cost_for_plan": 2.2,
                "chosen": true
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "((`staffs`.`age` = 23) and (`staffs`.`name` = 'July'))",
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`staffs`",
                  "attached": "((`staffs`.`age` = 23) and (`staffs`.`name` = 'July'))"
                }
              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": "`staffs`"
              }
            ]
          }
        ]
      }
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}

增加表数据量

-- 接下来增大表的数据量
INSERT INTO `staffs` (`name`, `age`, `pos`, `add_time`)
VALUES
	('July', 25, 'dev', '2018-06-04 16:30:17'),
	('July', 23, 'dev1', '2018-06-04 16:02:02'),
	('July', 23, 'dev2', '2018-06-04 16:02:02'),
	('July', 23, 'dev3', '2018-06-04 16:02:02'),
	('July', 23, 'dev4', '2018-06-04 16:02:02'),
	('July', 23, 'dev6', '2018-06-04 16:02:02'),
	('July', 23, 'dev5', '2018-06-04 16:02:02'),
	('July', 23, 'dev7', '2018-06-04 16:02:02'),
	('July', 23, 'dev8', '2018-06-04 16:02:02'),
	('July', 23, 'dev9', '2018-06-04 16:02:02'),
	('July', 23, 'dev10', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev1', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev2', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev3', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev4', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev6', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev5', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev7', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev8', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev9', '2018-06-04 16:02:02'),
	('Clive', 23, 'dev10', '2018-06-04 16:02:02');

执行Explain

-- 再次执行同样的查询语句，会发现走到索引上了
explain select * from staffs where name = 'July' and age = 23;
id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	staffs	NULL	ref	idx_nap	idx_nap	78	const,const	13	100.00	NULL

查看新的Trace内容

-- 再看下优化器执行过程
{
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `staffs`.`id` AS `id`,`staffs`.`name` AS `name`,`staffs`.`age` AS `age`,`staffs`.`pos` AS `pos`,`staffs`.`add_time` AS `add_time` from `staffs` where ((`staffs`.`name` = 'July') and (`staffs`.`age` = 23))"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "((`staffs`.`name` = 'July') and (`staffs`.`age` = 23))",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "((`staffs`.`name` = 'July') and multiple equal(23, `staffs`.`age`))"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "((`staffs`.`name` = 'July') and multiple equal(23, `staffs`.`age`))"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "((`staffs`.`name` = 'July') and multiple equal(23, `staffs`.`age`))"
                }
              ]
            }
          },
          {
            "substitute_generated_columns": {
            }
          },
          {
            "table_dependencies": [
              {
                "table": "`staffs`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "`staffs`",
                "field": "name",
                "equals": "'July'",
                "null_rejecting": false
              },
              {
                "table": "`staffs`",
                "field": "age",
                "equals": "23",
                "null_rejecting": false
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`staffs`",
                "range_analysis": {
                  "table_scan": {
                    "rows": 27,
                    "cost": 8.5
                  },
                  "potential_range_indexes": [
                    {
                      "index": "PRIMARY",
                      "usable": false,
                      "cause": "not_applicable"
                    },
                    {
                      "index": "idx_nap",
                      "usable": true,
                      "key_parts": [
                        "name",
                        "age",
                        "pos",
                        "id"
                      ]
                    }
                  ],
                  "setup_range_conditions": [
                  ],
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  },
                  "analyzing_range_alternatives": {
                    "range_scan_alternatives": [
                      {
                        "index": "idx_nap",
                        "ranges": [
                          "July <= name <= July AND 23 <= age <= 23"
                        ],
                        "index_dives_for_eq_ranges": true,
                        "rowid_ordered": false,
                        "using_mrr": false,
                        "index_only": false,
                        "rows": 13,
                        "cost": 16.61,
                        "chosen": false,
                        "cause": "cost"
                      }
                    ],
                    "analyzing_roworder_intersect": {
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    }
                  }
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`staffs`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                    //使用索引的成本变为了5.3
                      "access_type": "ref",
                      "index": "idx_nap",
                      "rows": 13,
                      "cost": 5.3,
                      "chosen": true
                    },
                    {
                    //scan的成本变为了6.4
                      "rows_to_scan": 27,
                      "access_type": "scan",
                      "resulting_rows": 27,
                      "cost": 6.4,
                      "chosen": false
                    }
                  ]
                },
                //使用索引查询的成本更低，因此选择了走索引
                "condition_filtering_pct": 100,
                "rows_for_plan": 13,
                "cost_for_plan": 5.3,
                "chosen": true
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "((`staffs`.`age` = 23) and (`staffs`.`name` = 'July'))",
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`staffs`",
                  "attached": null
                }
              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": "`staffs`"
              }
            ]
          }
        ]
      }
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}

结论

MySQL表数据量的大小，会影响索引的选择，具体的情况还是通过Explain和Optimizer Trace来查看与分析。