富贵资源网

MySQL如何选择合适的索引
站长资源 2024/11/14 佚名
52 1538
富贵资源网 Design By www.hznty.com

先来看一个栗子

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

MySQL如何选择合适的索引

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';

MySQL如何选择合适的索引

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';

MySQL如何选择合适的索引

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，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对网站的支持！如果你觉得本文对你有帮助，欢迎转载，烦请注明出处，谢谢！

富贵资源网 Design By www.hznty.com
广告合作：本站广告合作请联系QQ：858582 申请时备注：广告合作（否则不回）
免责声明：本站资源来自互联网收集,仅供用于学习和交流,请遵循相关法律法规,本站一切资源不代表本站立场,如有侵权、后门、不妥请联系本站删除！
富贵资源网 Design By www.hznty.com
www.hznty.com 富贵资源网
20,135影音资源
44,641技术资源
2,817软件资源
435,032站长资源

P70系列延期，华为新旗舰将在下月发布

3月20日消息，近期博主@数码闲聊站 透露，原定三月份发布的华为新旗舰P70系列延期发布，预计4月份上市。

而博主@定焦数码 爆料，华为的P70系列在定位上已经超过了Mate60，成为了重要的旗舰系列之一。它肩负着重返影像领域顶尖的使命。那么这次P70会带来哪些令人惊艳的创新呢？

根据目前爆料的消息来看，华为P70系列将推出三个版本，其中P70和P70 Pro采用了三角形的摄像头模组设计，而P70 Art则采用了与上一代P60 Art相似的不规则形状设计。这样的外观是否好看见仁见智，但辨识度绝对拉满。

友情链接

杰晶网络 DDR爱好者之家 桃源资源网 杰网资源 富贵资源网 南强小屋 铁雪资源网 幽灵资源网 万梅资源网 狼山资源网 白云岛资源网 昆仑资源网 相思资源网 明霞山资源网 内蒙古资源网 黑松山资源网 茶园资源网 饿虎岗资源网 大旗谷资源网 常春岛资源网 岱庙资源网 兴国资源网 快活林资源网 蝙蝠岛资源网 帝王谷资源网 白云城资源网 伏龙阁资源网 清风细雨楼 天枫庄资源网 圆月山庄资源网 无争山庄资源网 神水资源网 移花宫资源网 神剑山庄资源网 无为清净楼资源网 金钱帮资源网 丐帮资源网 华山资源网 极乐门资源网 小李飞刀资源网 凤求凰客栈 风云阁资源网 金狮镖局 鸳鸯亭资源网 千金楼资源网 更多链接
富贵资源网 Design By www.hznty.com