Mysql如何选择合适的索引

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原创最新推荐文章于 2026-06-03 12:44:12 发布 1.5k 阅读 AI 写同款· GEO 优化 ›

【MYSQL】高性能的索引策略 - 选择合适的索引列顺序阅读详情

对于我们之前创建的表：employees

假设我们执行如下SQL：
EXPLAIN select * from employees where name > 'a';

可见上方并未走索引，而当我们执行下方SQL的时候，他走了索引：

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

那么Mysql到底是如何选择索引的呢？我们可以选择trace工具来查看Mysql选择索引的过程。

注意：trace工具开启会影响Mysql性能，使用完毕后需要立即关闭。

trace工具用法：

首先开启trace工具：

set session optimizer_trace="enabled=on",end_markers_in_json=on;

其次执行查询SQL，下方两条需一起执行：

select * from employees where name > 'a' order by position;
SELECT * FROM information_schema.OPTIMIZER_TRACE;

将trace列结果拷贝出来，放到文本工具中查看如下：

查看trace字段：
{
   "steps": [{
           "join_preparation": {‐‐第一阶段： SQL准备阶段，格式化sql
           "select#": 1,
               "steps": [{
                       "expanded_query": "/* select#1 */ select `employees`.`id` AS `id`,`employees`.`name` AS `name`,`empl oyees`.`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 */

                   },
                   {
                       "substitute_generated_columns": {

                       } /* substitute_generated_columns */

                   },
                   {
                       "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": [‐‐预估表的访问成本
                       {
                           "table": "`employees`",
                           "range_analysis": {
                               "table_scan": {‐‐全表扫描情况
                                   "rows": 10123,‐‐扫描行数
                                   "cost": 2054.7‐‐ 查询成本
                               } /* table_scan */ ,
                               "potential_range_indexes": [‐‐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 */
                                   }

                               ] /* potential_range_indexes */ ,
                               "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"‐‐索引使用范围
                                       ] /* ranges */ ,
                                       "index_dives_for_eq_ranges": true,
                                       "rowid_ordered": false,‐‐使用该索引获取的记录是否按照主键排序
                                       "using_mrr": false,
                                       "index_only": false, ‐‐是否使用覆盖索引
                                       "rows": 5061, ‐‐索引扫描行数
                                       "cost": 6074.2, ‐‐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": {‐‐最优访问路径
                           "considered_access_paths": [‐‐最终选择的访问路径
                           {
                                   "rows_to_scan": 10123,
                                   "access_type": "scan",‐‐访问类型：为scan，全表扫描
                                   "resulting_rows": 10123,
                                   "cost": 2052.6,
                                   "chosen": true,‐‐确定选择
                                   "use_tmp_table": true
                               }] /* considered_access_paths */
                           } /* best_access_path */ ,
                           "condition_filtering_pct": 100,
                           "rows_for_plan": 10123,
                           "cost_for_plan": 2052.6,
                           "sort_cost": 10123,
                           "new_cost_for_plan": 12176,
                           "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 */

                   },
                   {
                       "reconsidering_access_paths_for_index_ordering": {
                           "clause": "ORDER BY",
                           "steps": [] /* steps */ ,
                           "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 */

                   },
                   {
                       "refine_plan": [{
                               "table": "`employees`"

                           }

                       ] /* refine_plan */
                   }

               ] /* steps */
           } /* join_optimization */

       },
       {
           "join_execution": {‐‐第三阶段： SQL执行阶段
           "select#": 1,
               "steps": [] /* steps */
           } /* join_execution */

       }

   ] /* steps */
}

结论：全表扫描的成本低于索引扫描，所以mysql最终选择全表扫描

而针对第二条SQL：

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 */
},
{
"substitute_generated_columns": {
} /* substitute_generated_columns */
},
{
"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": 100117,
"cost": 20378
} /* table_scan */,
"potential_range_indexes": [
{
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
"index": "idx_name_age_position",
"usable": true,
"key_parts": [
"name",
"age",
"position",
"id"
] /* key_parts */
}
] /* potential_range_indexes */,
"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": [
{
"rows_to_scan": 1,
"access_type": "range",
"range_details": {
"used_index": "idx_name_age_position"
} /* range_details */,
"resulting_rows": 1,
"cost": 2.41,
"chosen": true,
"use_tmp_table": true
}
] /* considered_access_paths */
} /* best_access_path */,
"condition_filtering_pct": 100,
"rows_for_plan": 1,
"cost_for_plan": 2.41,
"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 */
},
{
"reconsidering_access_paths_for_index_ordering": {
"clause": "ORDER BY",
"steps": [
] /* steps */,
"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 */
},
{
"refine_plan": [
{
"table": "`employees`",
"pushed_index_condition": "(`employees`.`name` > 'zzz')",
"table_condition_attached": null
}
] /* refine_plan */
}
] /* 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": 262056,
"sort_mode": "<sort_key, packed_additional_fields>"
} /* filesort_summary */
}
] /* steps */
} /* join_execution */
}
] /* steps */
}