MySql性能调优03-[SQL优化]

SQL优化

MySQL优化
- SQL优化-不要写select *
- SQL优化-小表驱动大表，而不是大表驱动小表
- SQL优化-连接查询代替子查询
- SQL优化-提升group by的效率
- SQL优化-使用limit
- SQL优化-union all代替union
- SQL优化-join的表不宜过多

MySQL优化

trace工具

set session optimizer_trace='enabled=on',end_markers_in_json =on; -- 开启trace
select * From employees where  name ='ddd';
select * from information_schema.OPTIMIZER_TRACE;

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trace

{"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`,`employees`.`age` AS `age`,`employees`.`position` AS `position`,`employees`.`hire_time` AS `hire_time`,`employees`.`remark` AS `remark`,`employees`.`salary` AS `salary` from `employees` where (`employees`.`name` = 'ddd')"}] /* 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` = 'ddd')","steps": [{"transformation": "equality_propagation","resulting_condition": "multiple equal('ddd', `employees`.`name`)"},{"transformation": "constant_propagation","resulting_condition": "multiple equal('ddd', `employees`.`name`)"},{"transformation": "trivial_condition_removal","resulting_condition": "multiple equal('ddd', `employees`.`name`)"}] /* 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": [{"table": "`employees`","field": "name","equals": "'ddd'","null_rejecting": true}] /* 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": 10,         -- 扫描行数,"cost": 3.35        -- 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 */,"skip_scan_range": {"potential_skip_scan_indexes": [{"index": "idx_name_age_position","usable": false,"cause": "query_references_nonkey_column"}] /* potential_skip_scan_indexes */} /* skip_scan_range */,"analyzing_range_alternatives": {"range_scan_alternatives": [{"index": "idx_name_age_position","ranges": ["name = 'ddd'"      -- 索引使用范围] /* ranges */,"index_dives_for_eq_ranges": true,"rowid_ordered": false,"using_mrr": false,"index_only": false,"in_memory": 1,"rows": 1,          -- 索引扫描行数"cost": 0.61,       -- 索引使用成本"chosen": true      -- 是否使用该索引}] /* 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": ["name = 'ddd'"] /* ranges */} /* range_access_plan */,"rows_for_plan": 1,"cost_for_plan": 0.61,"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": "ref",   -- 访问类型：ref，使用索引，如果是scan，则为全表扫描"index": "idx_name_age_position","rows": 1,"cost": 0.35,"chosen": true             --确定选择},{"access_type": "range","range_details": {"used_index": "idx_name_age_position"} /* range_details */,"chosen": false,"cause": "heuristic_index_cheaper"}] /* considered_access_paths */} /* best_access_path */,"condition_filtering_pct": 100,"rows_for_plan": 1,"cost_for_plan": 0.35,"chosen": true}] /* considered_execution_plans */},{"attaching_conditions_to_tables": {"original_condition": "(`employees`.`name` = 'ddd')","attached_conditions_computation": [] /* attached_conditions_computation */,"attached_conditions_summary": [{"table": "`employees`","attached": "(`employees`.`name` = 'ddd')"}] /* attached_conditions_summary */} /* attaching_conditions_to_tables */},{"finalizing_table_conditions": [{"table": "`employees`","original_table_condition": "(`employees`.`name` = 'ddd')","final_table_condition   ": null}] /* finalizing_table_conditions */},{"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 */
}