MySQL如何选择合适的索引

接下来以一个栗子开始今天的内容:

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