MySQL如何选择合适的索引

先来看一个栗子

EXPLAIN select * from employees where name > 'a';

如果用name索引查找数据需要遍历name字段联合索引树，然后根据遍历出来的主键值去主键索引树里再去查出最终数据，成本比全表扫描还高。

可以用覆盖索引优化，这样只需要遍历name字段的联合索引树就可以拿到所有的结果。

EXPLAIN select name,age,position from employees where name > 'a';

可以看到通过select出的字段是覆盖索引，MySQL底层使用了索引优化。在看另一个case：

EXPLAIN select * from employees where name > 'zzz';

对于上面的这两种 name>'a' 和 name>'zzz'的执行结果， mysql最终是否选择走索引或者一张表涉及多个索引， mysql最终如何选择索引，可以通过trace工具来一查究竟，开启trace工具会影响mysql性能，所以只能临时分析sql使用，用完之后需要立即关闭。

SET SESSION optimizer_trace="enabled=on",end_markers_in_json=on; --开启trace
SELECT * FROM employees WHERE name > 'a' ORDER BY position;
SELECT * FROM information_schema.OPTIMIZER_TRACE;

看trace字段：

{
"steps": [
{
 "join_preparation": { --第一阶段：SQl准备阶段
 "select#": 1,
 "steps": [
  {
  "expanded_query": "/* select#1 */ select `employees`.`id` AS `id`,`employees`.`name` AS `name`,`employees`.`age` AS `age`,`employees`.`position` AS `position`,`employees`.`hire_time` AS `hire_time` from `employees` where (`employees`.`name` > 'a') order by `employees`.`position`"
  }
 ] /* 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": "(`employees`.`name` > 'a')",
   "steps": [
   {
    "transformation": "equality_propagation",
    "resulting_condition": "(`employees`.`name` > 'a')"
   },
   {
    "transformation": "constant_propagation",
    "resulting_condition": "(`employees`.`name` > 'a')"
   },
   {
    "transformation": "trivial_condition_removal",
    "resulting_condition": "(`employees`.`name` > 'a')"
   }
   ] /* steps */
  } /* condition_processing */
  },
  {
  "table_dependencies": [ --表依赖详情
   {
   "table": "`employees`",
   "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": [ --预估标的访问成本
   {
   "table": "`employees`",
   "range_analysis": {
    "table_scan": { --全表扫描情况
    "rows": 3, --扫描行数
    "cost": 3.7 --查询成本
    } /* table_scan */,
    "potential_range_indices": [ --查询可能使用的索引
    {
     "index": "PRIMARY", --主键索引
     "usable": false,
     "cause": "not_applicable"
    },
    {
     "index": "idx_name_age_position", --辅助索引
     "usable": true,
     "key_parts": [
     "name",
     "age",
     "position",
     "id"
     ] /* key_parts */
    },
    {
     "index": "idx_age",
     "usable": false,
     "cause": "not_applicable"
    }
    ] /* potential_range_indices */,
    "setup_range_conditions": [
    ] /* setup_range_conditions */,
    "group_index_range": {
    "chosen": false,
    "cause": "not_group_by_or_distinct"
    } /* group_index_range */,
    "analyzing_range_alternatives": { ‐‐分析各个索引使用成本
    "range_scan_alternatives": [
     {
     "index": "idx_name_age_position",
     "ranges": [
      "a < name"
     ] /* ranges */,
     "index_dives_for_eq_ranges": true,
     "rowid_ordered": false,
     "using_mrr": false,
     "index_only": false, ‐‐是否使用覆盖索引
     "rows": 3, --‐‐索引扫描行数
     "cost": 4.61, --索引使用成本
     "chosen": false, ‐‐是否选择该索引
     "cause": "cost"
     }
    ] /* range_scan_alternatives */,
    "analyzing_roworder_intersect": {
     "usable": false,
     "cause": "too_few_roworder_scans"
    } /* analyzing_roworder_intersect */
    } /* analyzing_range_alternatives */
   } /* range_analysis */
   }
  ] /* rows_estimation */
  },
  {
  "considered_execution_plans": [
   {
   "plan_prefix": [
   ] /* plan_prefix */,
   "table": "`employees`",
   "best_access_path": {
    "considered_access_paths": [
    {
     "access_type": "scan",
     "rows": 3,
     "cost": 1.6,
     "chosen": true,
     "use_tmp_table": true
    }
    ] /* considered_access_paths */
   } /* best_access_path */,
   "cost_for_plan": 1.6,
   "rows_for_plan": 3,
   "sort_cost": 3,
   "new_cost_for_plan": 4.6,
   "chosen": true
   }
  ] /* considered_execution_plans */
  },
  {
  "attaching_conditions_to_tables": {
   "original_condition": "(`employees`.`name` > 'a')",
   "attached_conditions_computation": [
   ] /* attached_conditions_computation */,
   "attached_conditions_summary": [
   {
    "table": "`employees`",
    "attached": "(`employees`.`name` > 'a')"
   }
   ] /* attached_conditions_summary */
  } /* attaching_conditions_to_tables */
  },
  {
  "clause_processing": {
   "clause": "ORDER BY",
   "original_clause": "`employees`.`position`",
   "items": [
   {
    "item": "`employees`.`position`"
   }
   ] /* items */,
   "resulting_clause_is_simple": true,
   "resulting_clause": "`employees`.`position`"
  } /* clause_processing */
  },
  {
  "refine_plan": [
   {
   "table": "`employees`",
   "access_type": "table_scan"
   }
  ] /* refine_plan */
  },
  {
  "reconsidering_access_paths_for_index_ordering": {
   "clause": "ORDER BY",
   "index_order_summary": {
   "table": "`employees`",
   "index_provides_order": false,
   "order_direction": "undefined",
   "index": "unknown",
   "plan_changed": false
   } /* index_order_summary */
  } /* reconsidering_access_paths_for_index_ordering */
  }
 ] /* steps */
 } /* join_optimization */
},
{
 "join_execution": { --第三阶段：SQL执行阶段
 "select#": 1,
 "steps": [
  {
  "filesort_information": [
   {
   "direction": "asc",
   "table": "`employees`",
   "field": "position"
   }
  ] /* filesort_information */,
  "filesort_priority_queue_optimization": {
   "usable": false,
   "cause": "not applicable (no LIMIT)"
  } /* filesort_priority_queue_optimization */,
  "filesort_execution": [
  ] /* filesort_execution */,
  "filesort_summary": {
   "rows": 3,
   "examined_rows": 3,
   "number_of_tmp_files": 0,
   "sort_buffer_size": 200704,
   "sort_mode": "<sort_key, additional_fields>"
  } /* filesort_summary */
  }
 ] /* steps */
 } /* join_execution */
}
] /* steps */
}

全表扫描的成本低于索引扫描， 索引MySQL最终会选择全表扫描。

SELECT * FROM employees WHERE name > 'zzz' ORDER BY position;
SELECT * FROM information_schema.OPTIMIZER_TRACE;

{
"steps": [
{
 "join_preparation": {
 "select#": 1,
 "steps": [
  {
  "expanded_query": "/* select#1 */ select `employees`.`id` AS `id`,`employees`.`name` AS `name`,`employees`.`age` AS `age`,`employees`.`position` AS `position`,`employees`.`hire_time` AS `hire_time` from `employees` where (`employees`.`name` > 'zzz') order by `employees`.`position`"
  }
 ] /* steps */
 } /* join_preparation */
},
{
 "join_optimization": {
 "select#": 1,
 "steps": [
  {
  "condition_processing": {
   "condition": "WHERE",
   "original_condition": "(`employees`.`name` > 'zzz')",
   "steps": [
   {
    "transformation": "equality_propagation",
    "resulting_condition": "(`employees`.`name` > 'zzz')"
   },
   {
    "transformation": "constant_propagation",
    "resulting_condition": "(`employees`.`name` > 'zzz')"
   },
   {
    "transformation": "trivial_condition_removal",
    "resulting_condition": "(`employees`.`name` > 'zzz')"
   }
   ] /* steps */
  } /* condition_processing */
  },
  {
  "table_dependencies": [
   {
   "table": "`employees`",
   "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": [
   {
   "table": "`employees`",
   "range_analysis": {
    "table_scan": {
    "rows": 3,
    "cost": 3.7
    } /* table_scan */,
    "potential_range_indices": [
    {
     "index": "PRIMARY",
     "usable": false,
     "cause": "not_applicable"
    },
    {
     "index": "idx_name_age_position",
     "usable": true,
     "key_parts": [
     "name",
     "age",
     "position",
     "id"
     ] /* key_parts */
    },
    {
     "index": "idx_age",
     "usable": false,
     "cause": "not_applicable"
    }
    ] /* potential_range_indices */,
    "setup_range_conditions": [
    ] /* setup_range_conditions */,
    "group_index_range": {
    "chosen": false,
    "cause": "not_group_by_or_distinct"
    } /* group_index_range */,
    "analyzing_range_alternatives": {
    "range_scan_alternatives": [
     {
     "index": "idx_name_age_position",
     "ranges": [
      "zzz < name"
     ] /* ranges */,
     "index_dives_for_eq_ranges": true,
     "rowid_ordered": false,
     "using_mrr": false,
     "index_only": false,
     "rows": 1,
     "cost": 2.21,
     "chosen": true
     }
    ] /* range_scan_alternatives */,
    "analyzing_roworder_intersect": {
     "usable": false,
     "cause": "too_few_roworder_scans"
    } /* analyzing_roworder_intersect */
    } /* analyzing_range_alternatives */,
    "chosen_range_access_summary": {
    "range_access_plan": {
     "type": "range_scan",
     "index": "idx_name_age_position",
     "rows": 1,
     "ranges": [
     "zzz < name"
     ] /* ranges */
    } /* range_access_plan */,
    "rows_for_plan": 1,
    "cost_for_plan": 2.21,
    "chosen": true
    } /* chosen_range_access_summary */
   } /* range_analysis */
   }
  ] /* rows_estimation */
  },
  {
  "considered_execution_plans": [
   {
   "plan_prefix": [
   ] /* plan_prefix */,
   "table": "`employees`",
   "best_access_path": {
    "considered_access_paths": [
    {
     "access_type": "range",
     "rows": 1,
     "cost": 2.41,
     "chosen": true,
     "use_tmp_table": true
    }
    ] /* considered_access_paths */
   } /* best_access_path */,
   "cost_for_plan": 2.41,
   "rows_for_plan": 1,
   "sort_cost": 1,
   "new_cost_for_plan": 3.41,
   "chosen": true
   }
  ] /* considered_execution_plans */
  },
  {
  "attaching_conditions_to_tables": {
   "original_condition": "(`employees`.`name` > 'zzz')",
   "attached_conditions_computation": [
   ] /* attached_conditions_computation */,
   "attached_conditions_summary": [
   {
    "table": "`employees`",
    "attached": "(`employees`.`name` > 'zzz')"
   }
   ] /* attached_conditions_summary */
  } /* attaching_conditions_to_tables */
  },
  {
  "clause_processing": {
   "clause": "ORDER BY",
   "original_clause": "`employees`.`position`",
   "items": [
   {
    "item": "`employees`.`position`"
   }
   ] /* items */,
   "resulting_clause_is_simple": true,
   "resulting_clause": "`employees`.`position`"
  } /* clause_processing */
  },
  {
  "refine_plan": [
   {
   "table": "`employees`",
   "pushed_index_condition": "(`employees`.`name` > 'zzz')",
   "table_condition_attached": null,
   "access_type": "range"
   }
  ] /* refine_plan */
  },
  {
  "reconsidering_access_paths_for_index_ordering": {
   "clause": "ORDER BY",
   "index_order_summary": {
   "table": "`employees`",
   "index_provides_order": false,
   "order_direction": "undefined",
   "index": "idx_name_age_position",
   "plan_changed": false
   } /* index_order_summary */
  } /* reconsidering_access_paths_for_index_ordering */
  }
 ] /* steps */
 } /* join_optimization */
},
{
 "join_execution": {
 "select#": 1,
 "steps": [
  {
  "filesort_information": [
   {
   "direction": "asc",
   "table": "`employees`",
   "field": "position"
   }
  ] /* filesort_information */,
  "filesort_priority_queue_optimization": {
   "usable": false,
   "cause": "not applicable (no LIMIT)"
  } /* filesort_priority_queue_optimization */,
  "filesort_execution": [
  ] /* filesort_execution */,
  "filesort_summary": {
   "rows": 0,
   "examined_rows": 0,
   "number_of_tmp_files": 0,
   "sort_buffer_size": 200704,
   "sort_mode": "<sort_key, additional_fields>"
  } /* filesort_summary */
  }
 ] /* steps */
 } /* join_execution */
}
] /* steps */
}

查看trace字段可知索引扫描的成本低于全表扫描的成本，所以MySQL最终选择索引扫描。

SET SESSION optimizer_trace="enabled=off"; -- 关闭trace

总结

以上所述是小编给大家介绍的MySQL如何选择合适的索引网站的支持！如果你觉得本文对你有帮助，欢迎转载，烦请注明出处，谢谢！

来源：https://www.cnblogs.com/xiaoqiang-code/archive/2019/09/06/11474036.html