mysql5.6提供了对sql的跟踪trace，通过trace文件能够进一步了解为什么优化其选择执行计划a而不选b执行计划，帮助我们更好的理解优化其的行为。

使用方式：首先打开trace，设置格式为json，设置trace最大能够使用的内存大小，避免接续过程中因为默认内存太小而不能够完整显示

set optimizer_trace="enabled=on",END_MARKERS_IN_JSON=on;

set optimizer_trace_max_mem_size=1000000;

执行sql语句

select * from xuehao;

最后检查infomation_schema.optimizer_trace就可以知道mysql是如何执行sql的

mysql> select * from information_schema.optimizer_trace\G
*************************** 1. row ***************************
QUERY: select * from xuehao
TRACE: {
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `xuehao`.`id` AS `id` from `xuehao`"
}
] /* steps */
} /* join_preparation */
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"table_dependencies": [
{
"table": "`xuehao`",
"row_may_be_null": false,
"map_bit": 0,
"depends_on_map_bits": [
] /* depends_on_map_bits */
}
] /* table_dependencies */
},
{
"rows_estimation": [
{
"table": "`xuehao`",
"table_scan": {
"rows": 8,
"cost": 2
} /* table_scan */
}
] /* rows_estimation */
},
{
"considered_execution_plans": [
{
"plan_prefix": [
] /* plan_prefix */,
"table": "`xuehao`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "scan",
"rows": 8,
"cost": 3.6137,
"chosen": true
}
] /* considered_access_paths */
} /* best_access_path */,
"cost_for_plan": 3.6137,
"rows_for_plan": 8,
"chosen": true
}
] /* considered_execution_plans */
},
{
"attaching_conditions_to_tables": {
"original_condition": null,
"attached_conditions_computation": [
] /* attached_conditions_computation */,
"attached_conditions_summary": [
{
"table": "`xuehao`",
"attached": null
}
] /* attached_conditions_summary */
} /* attaching_conditions_to_tables */
},
{
"refine_plan": [
{
"table": "`xuehao`",
"access_type": "table_scan"
}
] /* refine_plan */
}
] /* steps */
} /* join_optimization */
},
{
"join_execution": {
"select#": 1,
"steps": [
] /* steps */
} /* join_execution */
}
] /* steps */
}
MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0
INSUFFICIENT_PRIVILEGES: 0
1 row in set (0.01 sec)

通过trace分析优化其如何选择执行计划