trace工具

MySQL5.6版本后提供了对SQL的跟踪工具trace，通过使用trace可以让我们明白optimizer如何选择执行计划的。

⚠️开启trace工具会影响mysql性能，所以只能临时分析sql使用，用 完之后立即关闭

使用方式

开启 trace
set session optimizer_trace=”enabled=on”,end_markers_in_json=on;

使用

select * from ${table};
SELECT * FROM information_schema.OPTIMIZER_TRACE;

案例分析

示例表参见:索引分析实操: 索引分析实操

我们先来看下面两条语句的结果。
EXPLAIN select * from employees where name > ‘a’;

EXPLAIN select * from employees where name > ‘zzz’;

这两条语句除了参数一模一样 但是第一个走了全表扫描而第二个走的索引搜索，这是为什么呢,接下来让我们使用 trace工具 一探究竟。

1. 开启trace

   set session optimizer_trace=”enabled=on”,end_markers_in_json=on;

2. 执行语句

   select * from employees where name > ‘a’ order by position;
   SELECT * FROM information_schema.OPTIMIZER_TRACE;

3. 将结果字段trace 拿出来分析

   { "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` 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 */ }, { "substitute_generated_columns": { } /* substitute_generated_columns */ }, { "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": [ /*预估表的访问成本*/ { "table": "`employees`", "range_analysis": { "table_scan": { /*全表扫描*/ "rows": 100036, /*扫描行数*/ "cost": 20362 /*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, /*不能使用*/ "cause": "not_applicable" /*不能使用的原因:不适用*/ }, { "index": "idx_name_age_position", /*辅助索引*/ "usable": true, /*可以适用*/ "key_parts": [ /*索引包含的列*/ "name", "age", "position", "id" ] /* key_parts */ } ] /* potential_range_indexes */, "setup_range_conditions": [ /*如果有可下推的条件，则带条件考虑范围查询*/ ] /* setup_range_conditions */, "group_index_range": { /*当使用了GROUP BY或DISTINCT时，是否有合适的索引可用。当未使用GROUP BY或DISTINCT时，会显示chosen=false, cause=not_group_by_or_distinct；如使用了GROUP BY或 DISTINCT，但是多表查询时，会显示chosen=false，cause =not_single_table。其他情况下会尝试 分析可用的索引（potential_group_range_indexes）并计算对应的扫描行数及其所需代价*/ "chosen": false, "cause": "not_group_by_or_distinct" } /* group_index_range */, "analyzing_range_alternatives": { /*分析各个适用的索引成本*/ "range_scan_alternatives": [ { "index": "idx_name_age_position", "ranges": [ "a < name" /*索引适用范围*/ ] /* ranges */, "index_dives_for_eq_ranges": true, "rowid_ordered": false, /*使用该索引获取的记录是否按照主键排序*/ "using_mrr": false, "index_only": false, /*是否能用覆盖索引*/ "rows": 50018, /*索引扫描行数*/ "cost": 60023, /*索引适用成本*/ "chosen": false, /*适用选择索引*/ "cause": "cost" /*不选择原因 cost 不是最小的*/ } ] /* 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": { /*最优访问路线*/ "considered_access_paths": [ /*最终选择的访问路径*/ { "rows_to_scan": 100036, "access_type": "scan", /*访问类型scan 为全表扫描 ref 为索引 具体可参考explain中的type字段*/ "resulting_rows": 100036, "cost": 20360, "chosen": true, /*确定选择*/ "use_tmp_table": true } ] /* considered_access_paths */ } /* best_access_path */, "condition_filtering_pct": 100, "rows_for_plan": 100036, "cost_for_plan": 20360, "sort_cost": 100036, "new_cost_for_plan": 120396, "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 */ }, { "reconsidering_access_paths_for_index_ordering": { "clause": "ORDER BY", "steps": [ ] /* steps */, "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 */ }, { "refine_plan": [ { "table": "`employees`" } ] /* refine_plan */ } ] /* 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": 100003, "examined_rows": 100003, "number_of_tmp_files": 30, "sort_buffer_size": 262056, "sort_mode": "<sort_key, packed_additional_fields>" } /* filesort_summary */ } ] /* steps */ } /* join_execution */ } ] /* steps */ }

   结论

分析trace结果 适用 全表扫描 cost=20362 适用索引 cost = 60023 所以该条语句选择全表扫描。 简单的一次trace工具使用及mysql对查询的最优路径选择就到此结束了。