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;

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。

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

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