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
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;
- 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 */
}
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
给group by后面的字段添加索引
