MySQL如何选择合适的索引

先来看一个栗子

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1	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。

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1	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：

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1	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。

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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;   查看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。

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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。

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1	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服务器之家网站的支持！如果你觉得本文对你有帮助，欢迎转载，烦请注明出处，谢谢！

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