介绍
mysql从5.6的版本开始提供optimizer tracing功能，开发者可以通过该功能查看mysql的sql解释器的整个执行过程。
快速使用
# Turn tracing on (it's off by default):
SET optimizer_trace="enabled=on";
SELECT ...; # your query here
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
# possibly more queries...
# When done with tracing, disable it:
SET optimizer_trace="enabled=off";
示例
表结构
创建一个如下测试表，给name和age加上索引。
CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(35) NOT NULL,
`age` int(11) NOT NULL,
`city` varchar(100) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `name` (`name`) USING BTREE,
KEY `age` (`age`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;
explain
explain select * from user where name = 'tom' and age = 23 order by city;
+----+-------------+-------+------------+-------------+---------------+----------+---------+------+------+----------+--------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+----------+---------+------+------+----------+--------------------------------------------------------+
| 1 | SIMPLE | user | NULL | index_merge | name,age | name,age | 107,4 | NULL | 1 | 100 | Using intersect(name,age); Using where; Using filesort |
+----+-------------+-------+------------+-------------+---------------+----------+---------+------+------+----------+--------------------------------------------------------+
explain显示mysql同时使用了name和age两个索引（索引合并index_merge）。
optimizer trace
SET optimizer_trace="enabled=on";
select * from user where name = 'tom' and age = 23 order by city;
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
SET optimizer_trace="enabled=off";
{
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `user`.`id` AS `id`,`user`.`name` AS `name`,`user`.`age` AS `age`,`user`.`city` AS `city` from `user` where ((`user`.`name` = 'tom') and (`user`.`age` = 23)) order by `user`.`city`"
}
]
}
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": {
"condition": "WHERE",
"original_condition": "((`user`.`name` = 'tom') and (`user`.`age` = 23))",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "((`user`.`name` = 'tom') and multiple equal(23, `user`.`age`))"
},
{
"transformation": "constant_propagation",
"resulting_condition": "((`user`.`name` = 'tom') and multiple equal(23, `user`.`age`))"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "((`user`.`name` = 'tom') and multiple equal(23, `user`.`age`))"
}
]
}
},
{
"substitute_generated_columns": {}
},
{
"table_dependencies": [
{
"table": "`user`",
"row_may_be_null": false,
"map_bit": 0,
"depends_on_map_bits": []
}
]
},
{
"ref_optimizer_key_uses": [
{
"table": "`user`",
"field": "name",
"equals": "'tom'",
"null_rejecting": false
},
{
"table": "`user`",
"field": "age",
"equals": "23",
"null_rejecting": false
}
]
},
{
"rows_estimation": [ // 预估不同表访问方法的访问成本
{
"table": "`user`",
"range_analysis": {
"table_scan": { // 全表扫描的行数和成本
"rows": 5,
"cost": 4.1
},
"potential_range_indexes": [ // 分析可能使用的索引
{ // 主键索引不可用
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{ // name索引可用
"index": "name",
"usable": true,
"key_parts": [
"name",
"id"
]
},
{ // age索引可用
"index": "age",
"usable": true,
"key_parts": [
"age",
"id"
]
}
],
"setup_range_conditions": [],
"group_index_range": {
"chosen": false,
"cause": "not_group_by_or_distinct"
},
"analyzing_range_alternatives": { // 分析各种可能使用的索引的成本
"range_scan_alternatives": [
{
"index": "name",
"ranges": [
"tom <= name <= tom"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": true,
"using_mrr": false,
"index_only": false, // 是否索引覆盖
"rows": 2, // 使用该索引获取的记录条数
"cost": 3.41, // 使用该索引的成本
"chosen": true // 是否使用该索引
},
{
"index": "age",
"ranges": [
"23 <= age <= 23"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": true,
"using_mrr": false,
"index_only": false,
"rows": 2,
"cost": 3.41,
"chosen": false,
"cause": "cost"
}
],
"analyzing_roworder_intersect": { // 分析使用索引合并的成本
"intersecting_indexes": [
{
"index": "name",
"index_scan_cost": 1.0135,
"cumulated_index_scan_cost": 1.0135,
"disk_sweep_cost": 1.75,
"cumulated_total_cost": 2.7635,
"usable": true,
"matching_rows_now": 2,
"isect_covering_with_this_index": false,
"chosen": true
},
{
"index": "age",
"index_scan_cost": 1.001,
"cumulated_index_scan_cost": 2.0145,
"disk_sweep_cost": 0,
"cumulated_total_cost": 2.0145,
"usable": true,
"matching_rows_now": 0.8,
"isect_covering_with_this_index": false,
"chosen": true
}
],
"clustered_pk": {
"clustered_pk_added_to_intersect": false,
"cause": "no_clustered_pk_index"
},
"rows": 1,
"cost": 2.0145,
"covering": false,
"chosen": true
}
},
"chosen_range_access_summary": { // 最佳查询方案
"range_access_plan": {
"type": "index_roworder_intersect",
"rows": 1,
"cost": 2.0145,
"covering": false,
"clustered_pk_scan": false,
"intersect_of": [
{
"type": "range_scan",
"index": "name",
"rows": 2,
"ranges": [
"tom <= name <= tom"
]
},
{
"type": "range_scan",
"index": "age",
"rows": 2,
"ranges": [
"23 <= age <= 23"
]
}
]
},
"rows_for_plan": 1,
"cost_for_plan": 2.0145,
"chosen": true
}
}
}
]
},
{
"considered_execution_plans": [
{
"plan_prefix": [],
"table": "`user`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "ref",
"index": "name",
"rows": 2,
"cost": 2.4,
"chosen": true
},
{
"access_type": "ref",
"index": "age",
"rows": 2,
"cost": 2.4,
"chosen": false
},
{
"rows_to_scan": 1,
"access_type": "range",
"range_details": {
"used_index": "intersect(name,age)"
},
"resulting_rows": 1,
"cost": 2.2145,
"chosen": true,
"use_tmp_table": true
}
]
},
"condition_filtering_pct": 100,
"rows_for_plan": 1,
"cost_for_plan": 2.2145,
"sort_cost": 1,
"new_cost_for_plan": 3.2145,
"chosen": true
}
]
},
{
"attaching_conditions_to_tables": {
"original_condition": "((`user`.`age` = 23) and (`user`.`name` = 'tom'))",
"attached_conditions_computation": [],
"attached_conditions_summary": [
{
"table": "`user`",
"attached": "((`user`.`age` = 23) and (`user`.`name` = 'tom'))"
}
]
}
},
{
"clause_processing": {
"clause": "ORDER BY",
"original_clause": "`user`.`city`",
"items": [
{
"item": "`user`.`city`"
}
],
"resulting_clause_is_simple": true,
"resulting_clause": "`user`.`city`"
}
},
{
"refine_plan": [
{
"table": "`user`"
}
]
}
]
}
},
{
"join_execution": {
"select#": 1,
"steps": [
{
"filesort_information": [
{
"direction": "asc",
"table": "`user`",
"field": "city"
}
],
"filesort_priority_queue_optimization": {
"usable": false,
"cause": "not applicable (no LIMIT)"
},
"filesort_execution": [],
"filesort_summary": {
"rows": 1,
"examined_rows": 1,
"number_of_tmp_files": 0,
"sort_buffer_size": 261880,
"sort_mode": "<sort_key, packed_additional_fields>"
}
}
]
}
}
]
}
