先来看一个栗子
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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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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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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traceselect * 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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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
