mysql trace_MySQL SQL trace
MySQL SQL trace
从 MySQL 5.6 开始，可以使用 trace 查看优化器如何选择执行计划，通过SQL trace来跟踪sql语句的执行过程。通过trace文件能够进一步了解为什么优化器选择A计划, 而不是选择B计划。
打开trace，并设置格式为json
SET optimizer_trace="enabled=on",end_markers_in_json=on;
设置trace使用的内存大小,避免解析过程内存不足,文件显示不完整
set optimizer_trace_max_mem_size=1000000;
执行一条SQL语句，例如：
select * from t_user where id = '5' ；
然后，通过查询information_schema库的OPTIMIZER_TRACE表，就可以获取到SQL的执行计划跟踪信息。
select * from information_schema.optimizer_trace;
trace json文件分析
QUERY: select * from t_user where id = '5' --查询sql
TRACE: {
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `t_user`.`id` AS `id`,`t_user`.`username` AS `username`,`t_user`.`password` AS `password`,`t_user`.`name` AS `name` from `t_user` where (`t_user`.`id` = '5')"
}
] /* steps */
} /* join_preparation */
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": { --条件处理
"condition": "WHERE",
"original_condition": "(`t_user`.`id` = '5')",
"steps": [
{
"transformation": "equality_propagation", --等值优化
"resulting_condition": "(`t_user`.`id` = '5')"
},
{
"transformation": "constant_propagation", --常量优化
"resulting_condition": "(`t_user`.`id` = '5')"
},
{
"transformation": "trivial_condition_removal", --琐碎条件排除
"resulting_condition": "(`t_user`.`id` = '5')"
}
] /* steps */
} /* condition_processing */
},
{
"substitute_generated_columns": { -- 替换生成的列
} /* substitute_generated_columns */
},
{
"table_dependencies": [ -- 表依赖
{
"table": "`t_user`",
"row_may_be_null": false, --是否有null值
"map_bit": 0,
"depends_on_map_bits": [
] /* depends_on_map_bits */
}
] /* table_dependencies */
},
{
"ref_optimizer_key_uses": [ --相关索引优化使用
{
"table": "`t_user`",
"field": "id", --索引字段
"equals": "'5'",
"null_rejecting": false
}
] /* ref_optimizer_key_uses */
},
{
"rows_estimation": [ --行评估
{
"table": "`t_user`",
"rows": 1,
"cost": 1,
"table_type": "const",
"empty": false
}
] /* rows_estimation */
},
{
"condition_on_constant_tables": "('5' = '5')",
"condition_value": true
},
{
"attaching_conditions_to_tables": {
"original_condition": "('5' = '5')",
"attached_conditions_computation": [
] /* attached_conditions_computation */,
"attached_conditions_summary": [
] /* attached_conditions_summary */
} /* attaching_conditions_to_tables */
},
{
"refine_plan": [
] /* refine_plan */
}
] /* steps */
} /* join_optimization */
},
{
"join_execution": {
"select#": 1,
"steps": [
] /* steps */
} /* join_execution */
}
] /* steps */
}
MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0
INSUFFICIENT_PRIVILEGES: 0