Mysql trace命令介绍
trace作用：
- 对SQL的跟踪，可以知道SQL是如何执行的，比EXPLAIN还要强大。
注意点：
- 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。
开启trace：
set session optimizer_trace="enabled=on",end_markers_in_json=on; #开启
set session optimizer_trace="enabled=off"; #关闭
如果使用索引查询数据，但最终还是走了全表扫描，可能是全表扫描的成本低于索引扫描，所以mysql最终选择全表扫描。
具体根据trace命令就可以根据sql查询成本对比出来。
查看根据记录：
# 这里是需要执行的sql
SELECT * FROM information_schema.OPTIMIZER_TRACE;
出现的结果是json数据：
{
"steps": [
{
"join_preparation": { -- 第一阶段：SQL准备阶段
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `cr_shop_tables`.`id` AS `id`,`cr_shop_tables`.`title` AS `title`,`cr_shop_tables`.`num` AS `num`,`cr_shop_tables`.`qrcode_pic` AS `qrcode_pic`,`cr_shop_tables`.`sort` AS `sort`,`cr_shop_tables`.`create_time` AS `create_time`,`cr_shop_tables`.`modify_time` AS `modify_time`,`cr_shop_tables`.`shop_id` AS `shop_id`,`cr_shop_tables`.`status` AS `status` from `cr_shop_tables` where (`cr_shop_tables`.`title` > 'a') order by `cr_shop_tables`.`num`"
}
] /* 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": "(`cr_shop_tables`.`title` > 'a')",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "(`cr_shop_tables`.`title` > 'a')"
},
{
"transformation": "constant_propagation",
"resulting_condition": "(`cr_shop_tables`.`title` > 'a')"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "(`cr_shop_tables`.`title` > 'a')"
}
] /* steps */
} /* condition_processing */
},
{
"substitute_generated_columns": {
} /* substitute_generated_columns */
},
{
"table_dependencies": [ ‐‐表依赖详情
{
"table": "`cr_shop_tables`",
"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": "`cr_shop_tables`",
"table_scan": { ‐‐全表扫描情况
"rows": 207, ‐‐扫描行数
"cost": 4 ‐‐查询成本
} /* table_scan */
}
] /* rows_estimation */
},
{
"considered_execution_plans": [
{
"plan_prefix": [
] /* plan_prefix */,
"table": "`cr_shop_tables`",
"best_access_path": {
"considered_access_paths": [
{
"rows_to_scan": 207,
"access_type": "scan",
"resulting_rows": 207,
"cost": 45.4,
"chosen": true,
"use_tmp_table": true
}
] /* considered_access_paths */
} /* best_access_path */,
"condition_filtering_pct": 100,
"rows_for_plan": 207,
"cost_for_plan": 45.4,
"sort_cost": 207,
"new_cost_for_plan": 252.4,
"chosen": true
}
] /* considered_execution_plans */
},
{
"attaching_conditions_to_tables": {
"original_condition": "(`cr_shop_tables`.`title` > 'a')",
"attached_conditions_computation": [
] /* attached_conditions_computation */,
"attached_conditions_summary": [
{
"table": "`cr_shop_tables`",
"attached": "(`cr_shop_tables`.`title` > 'a')"
}
] /* attached_conditions_summary */
} /* attaching_conditions_to_tables */
},
{
"clause_processing": {
"clause": "ORDER BY",
"original_clause": "`cr_shop_tables`.`num`",
"items": [
{
"item": "`cr_shop_tables`.`num`"
}
] /* items */,
"resulting_clause_is_simple": true,
"resulting_clause": "`cr_shop_tables`.`num`"
} /* clause_processing */
},
{
"refine_plan": [
{
"table": "`cr_shop_tables`"
}
] /* refine_plan */
}
] /* steps */
} /* join_optimization */
},
{
"join_execution": {
"select#": 1,
"steps": [
{
"filesort_information": [
{
"direction": "asc",
"table": "`cr_shop_tables`",
"field": "num"
}
] /* filesort_information */,
"filesort_priority_queue_optimization": {
"usable": false,
"cause": "not applicable (no LIMIT)"
} /* filesort_priority_queue_optimization */,
"filesort_execution": [
] /* filesort_execution */,
"filesort_summary": {
"rows": 133,
"examined_rows": 207,
"number_of_tmp_files": 0,
"sort_buffer_size": 32136,
"sort_mode": ""
} /* filesort_summary */
}
] /* steps */
} /* join_execution */
}
] /* steps */
}