先來看一個栗子

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

如果用name索引查詢資料需要遍歷name欄位聯合索引樹，然後根據遍歷出來的主鍵值去主鍵索引樹裡再去查出最終資料，成本比全表掃描還高。

可以用覆蓋索引優化，這樣只需要遍歷name欄位的聯合索引樹就可以拿到所有的結果。

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

可以看到通過select出的欄位是覆蓋索引，MySQL底層使用了索引優化。在看另一個case：

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

對於上面的這兩種 name>'a' 和 name>'zzz'的執行結果， mysql最終是否選擇走索引或者一張表涉及多個索引， mysql最終如何選擇索引，可以通過trace工具來一查究竟，開啟trace工具會影響mysql效能，所以只能臨時分析sql使用，用完之後需要立即關閉。

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

看trace欄位：

{
 "steps": [
 {
  "join_preparation": { --第一階段：SQl準備階段
  "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": { --第二階段：SQL優化階段
  "select#": 1,"steps": [
   {
   "condition_processing": { --條件處理
    "condition": "WHERE","original_condition": "(`employees`.`name` > 'a')","steps": [
    {
     "transformation": "equality_propagation","resulting_condition": "(`employees`.`name` > 'a')"
    },{
     "transformation": "constant_propagation",{
     "transformation": "trivial_condition_removal","resulting_condition": "(`employees`.`name` > 'a')"
    }
    ] /* steps */
   } /* condition_processing */
   },{
   "table_dependencies": [ --表依賴詳情
    {
    "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": [ --預估標的訪問成本
    {
    "table": "`employees`","range_analysis": {
     "table_scan": { --全表掃描情況
     "rows": 3,--掃描行數
     "cost": 3.7 --查詢成本
     } /* table_scan */,"potential_range_indices": [ --查詢可能使用的索引
     {
      "index": "PRIMARY",--主鍵索引
      "usable": false,"cause": "not_applicable"
     },{
      "index": "idx_name_age_position",--輔助索引
      "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": { ‐‐分析各個索引使用成本
     "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,‐‐是否使用覆蓋索引
      "rows": 3,--‐‐索引掃描行數
      "cost": 4.61,--索引使用成本
      "chosen": false,‐‐是否選擇該索引
      "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": { --第三階段：SQL執行階段
  "select#": 1,"steps": [
   {
   "filesort_information": [
    {
    "direction": "asc","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 */
}

全表掃描的成本低於索引掃描， 索引MySQL最終會選擇全表掃描。

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

{
 "steps": [
 {
  "join_preparation": {
  "select#": 1,`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')","resulting_condition": "(`employees`.`name` > 'zzz')"
    },"resulting_condition": "(`employees`.`name` > 'zzz')"
    }
    ] /* steps */
   } /* condition_processing */
   },{
   "table_dependencies": [
    {
    "table": "`employees`",{
   "rows_estimation": [
    {
    "table": "`employees`","range_analysis": {
     "table_scan": {
     "rows": 3,"cost": 3.7
     } /* table_scan */,"potential_range_indices": [
     {
      "index": "PRIMARY","usable": true,"analyzing_range_alternatives": {
     "range_scan_alternatives": [
      {
      "index": "idx_name_age_position","ranges": [
       "zzz < name"
      ] /* ranges */,"rows": 1,"cost": 2.21,"chosen": true
      }
     ] /* range_scan_alternatives */,"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","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 */
   },"best_access_path": {
     "considered_access_paths": [
     {
      "access_type": "range","cost": 2.41,"cost_for_plan": 2.41,"sort_cost": 1,"new_cost_for_plan": 3.41,{
   "attaching_conditions_to_tables": {
    "original_condition": "(`employees`.`name` > 'zzz')","attached": "(`employees`.`name` > 'zzz')"
    }
    ] /* attached_conditions_summary */
   } /* attaching_conditions_to_tables */
   },"pushed_index_condition": "(`employees`.`name` > 'zzz')","table_condition_attached": null,"access_type": "range"
    }
   ] /* refine_plan */
   },{
  "join_execution": {
  "select#": 1,"filesort_summary": {
    "rows": 0,"examined_rows": 0,additional_fields>"
   } /* filesort_summary */
   }
  ] /* steps */
  } /* join_execution */
 }
 ] /* steps */
}

檢視trace欄位可知索引掃描的成本低於全表掃描的成本，所以MySQL最終選擇索引掃描。

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

總結

以上所述是小編給大家介紹的MySQL如何選擇合適的索引，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回覆大家的。在此也非常感謝大家對我們網站的支援！如果你覺得本文對你有幫助，歡迎轉載，煩請註明出處，謝謝！