Mysql之 optimizer_trace 相关总结
MySQL官网介绍：https://dev.mysql.com/doc/dev/mysql-server/latest/PAGE_OPT_TRACE.html
1. 简介
MySQL优化器可以生成Explain执行计划，通过执行计划查看sql是否使用了索引，使用了哪种索；
但是有些时候，你会发现为什么没想按照我们所想的思路执行：
为什么会使用这个索引 ？！
为什么没有使用添加的索引 ？！
于是，MySQL5.6版本之后开始引入 optimizer trace（优化器追踪），它可以查看优化器生成执行计划的整个过程，以及做出的各种决策，包括访问表的方法、各种开销计算、各种转换等等，帮助我们更好的去优化sql。
另外，optimizer_trace的开关默认是关闭的 ，开启trace工具会影响mysql性能，所以只适合临时分析sql使用，用完之后最好及时关闭。
2. 使用方法
1. 查看optimizer trace配置
show variables like '%optimizer_trace%';
查询结果：
查询结果字段说明：
- optimizer_trace： 主配置，enabled的on表示开启，off表示关闭，one_line表示是否展示成一行
- optimizer_trace_features： 表示优化器的可选特性，包括贪心搜索、范围优化等
- optimizer_trace_limit： 表示优化器追踪最大显示数目，默认是1条
- optimizer_trace_max_mem_size： 表示优化器追踪占用的最大容量
- optimizer_trace_offset： 表示显示的第一个优化器追踪的偏移量
2. 开启/关闭 optimizer trace
#开启trace
set session optimizer_trace="enabled=on",end_markers_in_json=on;
#关闭trace
set session optimizer_trace="enabled=off";
3. 执行需要进行分析的SQL语句
select * from test0816 where name > 'a' order by remark;
4. 使用optimizer trace查看优化器的选择过程
SELECT * FROM information_schema.OPTIMIZER_TRACE;
查询结果：
查询结果对应字段说明：
- QUERY： 表示我们执行的查询语句
- TRACE： 优化器生成执行计划的过程（重点关注）
- MISSING_BYTES_BEYOND_MAX_MEM_SIZE： 优化过程其余的信息会被显示在这一列
- INSUFFICIENT_PRIVILEGES： 表示是否有权限查看优化过程，0是，1否
5. 分析
trace的内容：
{
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `test0816`.`id` AS `id`,`test0816`.`name` AS `name`,`test0816`.`age` AS `age`,`test0816`.`remark` AS `remark`,`test0816`.`create_time` AS `create_time` from `test0816` where (`test0816`.`name` > 'a') order by `test0816`.`remark`"
}
] /* steps */
} /* join_preparation */
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": {
"condition": "WHERE",
"original_condition": "(`test0816`.`name` > 'a')",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "(`test0816`.`name` > 'a')"
},
{
"transformation": "constant_propagation",
"resulting_condition": "(`test0816`.`name` > 'a')"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "(`test0816`.`name` > 'a')"
}
] /* steps */
} /* condition_processing */
},
{
"substitute_generated_columns": {
} /* substitute_generated_columns */
},
{
"table_dependencies": [
{
"table": "`test0816`",
"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": "`test0816`",
"range_analysis": {
"table_scan": {
"rows": 3,
"cost": 2.65
} /* table_scan */,
"potential_range_indexes": [
{
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
"index": "idx_name_age_remark",
"usable": true,
"key_parts": [
"name",
"age",
"remark",
"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_remark",
"usable": false,
"cause": "query_references_nonkey_column"
}
] /* potential_skip_scan_indexes */
} /* skip_scan_range */,
"analyzing_range_alternatives": {
"range_scan_alternatives": [
{
"index": "idx_name_age_remark",
"ranges": [
"a < name"
] /* ranges */,
"index_dives_for_eq_ranges": true,
"rowid_ordered": false,
"using_mrr": false,
"index_only": false,
"in_memory": 1,
"rows": 3,
"cost": 1.31,
"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_remark",
"rows": 3,
"ranges": [
"a < name"
] /* ranges */
} /* range_access_plan */,
"rows_for_plan": 3,
"cost_for_plan": 1.31,
"chosen": true
} /* chosen_range_access_summary */
} /* range_analysis */
}
] /* rows_estimation */
},
{
"considered_execution_plans": [
{
"plan_prefix": [
] /* plan_prefix */,
"table": "`test0816`",
"best_access_path": {
"considered_access_paths": [
{
"rows_to_scan": 3,
"access_type": "range",
"range_details": {
"used_index": "idx_name_age_remark"
} /* range_details */,
"resulting_rows": 3,
"cost": 1.61,
"chosen": true,
"use_tmp_table": true
}
] /* considered_access_paths */
} /* best_access_path */,
"condition_filtering_pct": 100,
"rows_for_plan": 3,
"cost_for_plan": 1.61,
"sort_cost": 3,
"new_cost_for_plan": 4.61,
"chosen": true
}
] /* considered_execution_plans */
},
{
"attaching_conditions_to_tables": {
"original_condition": "(`test0816`.`name` > 'a')",
"attached_conditions_computation": [
] /* attached_conditions_computation */,
"attached_conditions_summary": [
{
"table": "`test0816`",
"attached": "(`test0816`.`name` > 'a')"
}
] /* attached_conditions_summary */
} /* attaching_conditions_to_tables */
},
{
"optimizing_distinct_group_by_order_by": {
"simplifying_order_by": {
"original_clause": "`test0816`.`remark`",
"items": [
{
"item": "`test0816`.`remark`"
}
] /* items */,
"resulting_clause_is_simple": true,
"resulting_clause": "`test0816`.`remark`"
} /* simplifying_order_by */
} /* optimizing_distinct_group_by_order_by */
},
{
"reconsidering_access_paths_for_index_ordering": {
"clause": "ORDER BY",
"steps": [
] /* steps */,
"index_order_summary": {
"table": "`test0816`",
"index_provides_order": false,
"order_direction": "undefined",
"index": "idx_name_age_remark",
"plan_changed": false
} /* index_order_summary */
} /* reconsidering_access_paths_for_index_ordering */
},
{
"finalizing_table_conditions": [
{
"table": "`test0816`",
"original_table_condition": "(`test0816`.`name` > 'a')",
"final_table_condition ": "(`test0816`.`name` > 'a')"
}
] /* finalizing_table_conditions */
},
{
"refine_plan": [
{
"table": "`test0816`",
"pushed_index_condition": "(`test0816`.`name` > 'a')",
"table_condition_attached": null
}
] /* refine_plan */
},
{
"considering_tmp_tables": [
{
"adding_sort_to_table": "test0816"
} /* filesort */
] /* considering_tmp_tables */
}
] /* steps */
} /* join_optimization */
},
{
"join_execution": {
"select#": 1,
"steps": [
{
"sorting_table": "test0816",
"filesort_information": [
{
"direction": "asc",
"expression": "`test0816`.`remark`"
}
] /* filesort_information */,
"filesort_priority_queue_optimization": {
"usable": false,
"cause": "not applicable (no LIMIT)"
} /* filesort_priority_queue_optimization */,
"filesort_execution": [
] /* filesort_execution */,
"filesort_summary": {
"memory_available": 262144,
"key_size": 400,
"row_size": 1091,
"max_rows_per_buffer": 15,
"num_rows_estimate": 15,
"num_rows_found": 3,
"num_initial_chunks_spilled_to_disk": 0,
"peak_memory_used": 32800,
"sort_algorithm": "std::sort",
"sort_mode": "<fixed_sort_key, packed_additional_fields>"
} /* filesort_summary */
}
] /* steps */
} /* join_execution */
}
] /* steps */
}
一共是3个阶段：
- join_preparation：sql准备阶段，sql格式化；
- join_optimization： sql分析优化阶段，是分析OPTIMIZER TRACE的重点。这段一般都比较长，分很多步，需要细看；
- join_execution： sql执行阶段；
其中的相关关键字解析：
//TODO
结论：全表扫描的成本低于索引扫描，所以MySQL最终选择全表扫描。
