MySQL优化器
优化器
MySQL存储引擎中存在了一个可插拔的优化器OPTIMIZER_TRACE，可以看到内部查询计划的TRACE信息，从而可以知道MySQL内部执行过程
查询优化器状态
show variables like 'optimizer_trace';
Variable_name Value
optimizer_trace enabled=off,one_line=off
开启优化器
set session optimizer_trace="enabled=on,one_line=on",end_markers_in_json=on;
查看优化器追踪内存大小
show variables like 'optimizer_trace_max_mem_size';
Variable_name Value
optimizer_trace_max_mem_size 16384
可以在information_schema数据库中的OPTIMIZER_TRACE查看sql执行情况
SELECT trace FROM information_schema.OPTIMIZER_TRACE;
示例：
explain select id,sum(cost) from jr_form where id in (
select max(id) from jr_form group by jr_code) group by id;
SELECT * FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
这里注意一下，我在Navicat中执行时如果两个语句分开执行会导致TRACE中内容为
错误的Trace操作
需要选中两条sql语句一块执行才可以
trace结果为
{
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"join_preparation": {
"select#": 2,
"steps": [
{
"expanded_query": "/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code`"
},
{
"transformation": {
"select#": 2,
"from": "IN (SELECT)",
"to": "semijoin",
"chosen": false
}
},
{
"transformation": {
"select#": 2,
"from": "IN (SELECT)",
"to": "EXISTS (CORRELATED SELECT)",
"chosen": true,
"evaluating_constant_having_conditions": []
}
}
]
}
},
{
"expanded_query": "/* select#1 */ select `jr_form`.`id` AS `id`,sum(`jr_form`.`cost`) AS `sum(cost)` from `jr_form` where (`jr_form`.`id`,(/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code` having ((`jr_form`.`id`) = (max(`jr_form`.`id`))))) group by `jr_form`.`id`"
}
]
}
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": {
"condition": "WHERE",
"original_condition": "(`jr_form`.`id`,(/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code` having ((`jr_form`.`id`) = (max(`jr_form`.`id`)))))" ,
"steps": [
{
"transformation": "equality_propagation",
"subselect_evaluation": [],
"resulting_condition": "(`jr_form`.`id`,(/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code` having ((`jr_form`.`id`) = (max(`jr_form`.`id`)))))"
},
{
"transformation": "constant_propagation",
"subselect_evaluation": [],
"resulting_condition": "(`jr_form`.`id`,(/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code` having ((`jr_form`.`id`) = (max(`jr_form`.`id`)))))"
},
{
"transformation": "trivial_condition_removal",
"subselect_evaluation": [],
"resulting_condition": "(`jr_form`.`id`,(/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code` having ((`jr_form`.`id`) = (max(`jr_form`.`id`)))))"
}
]
}
},
{
"substitute_generated_columns": {}
},
{
"table_dependencies": [
{
"table": "`jr_form`",
"row_may_be_null": false,
"map_bit": 0,
"depends_on_map_bits": []
}
]
},
{
"ref_optimizer_key_uses": []
},
{
"rows_estimation": [
{
"table": "`jr_form`",
"const_keys_added": {
"keys": [
"PRIMARY",
"jr_code"
],
"cause": "group_by"
},
"range_analysis": {
"table_scan": {
"rows": 184,
"cost": 44.9
},
"potential_range_indexes": [
{
"index": "PRIMARY",
"usable": true,
"key_parts": [
"id"
]
},
{
"index": "jr_code",
"usable": true,
"key_parts": [
"jr_code",
"id"
]
}
],
"setup_range_conditions": [],
"group_index_range": {
"chosen": false,
"cause": "not_applicable_aggregate_function"
}
}
}
]
},
{
"considered_execution_plans": [
{
"plan_prefix": [],
"table": "`jr_form`",
"best_access_path": {
"considered_access_paths": [
{
"rows_to_scan": 184,
"access_type": "scan",
"resulting_rows": 184,
"cost": 42.8,
"chosen": true,
"use_tmp_table": true
}
]
},
"condition_filtering_pct": 100,
"rows_for_plan": 184,
"cost_for_plan": 42.8,
"sort_cost": 184,
"new_cost_for_plan": 226.8,
"chosen": true
}
]
},
{
"attaching_conditions_to_tables": {
"original_condition": "(`jr_form`.`id`,(/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code` having ((`jr_form`.`id`) = (max(`jr_form`.`id`)))))" ,
"attached_conditions_computation": [],
"attached_conditions_summary": [
{
"table": "`jr_form`",
"attached": "(`jr_form`.`id`,(/* select#2 */ select max(`jr_form`.`id`) from `jr_form` group by `jr_form`.`jr_code` having ((`jr_form`.`id`) = (max(`jr_form`.`id`)))))"
}
]
}
},
{
"clause_processing": {
"clause": "GROUP BY",
"original_clause": "`jr_form`.`id`",
"items": [
{
"item": "`jr_form`.`id`"
}
],
"resulting_clause_is_simple": true,
"resulting_clause": "`jr_form`.`id`"
}
},
{
"reconsidering_access_paths_for_index_ordering": {
"clause": "GROUP BY",
"index_order_summary": {
"table": "`jr_form`",
"index_provides_order": true,
"order_direction": "asc",
"index": "PRIMARY",
"plan_changed": true,
"access_type": "index"
}
}
},
{
"refine_plan": [
{
"table": "`jr_form`"
}
]
}
]
}
},
{
"join_optimization": {
"select#": 2,
"steps": [
{
"condition_processing": {
"condition": "HAVING",
"original_condition": "((`jr_form`.`id`) = (max(`jr_form`.`id`)))" ,
"steps": [
{
"transformation": "constant_propagation",
"resulting_condition": "((`jr_form`.`id`) = (max(`jr_form`.`id`)))"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "((`jr_form`.`id`) = (max(`jr_form`.`id`)))"
}
]
}
},
{
"substitute_generated_columns": {}
},
{
"table_dependencies": [
{
"table": "`jr_form`",
"row_may_be_null": false,
"map_bit": 0,
"depends_on_map_bits": []
}
]
},
{
"rows_estimation": [
{
"table": "`jr_form`",
"const_keys_added": {
"keys": [
"jr_code"
],
"cause": "group_by"
},
"range_analysis": {
"table_scan": {
"rows": 184,
"cost": 44.9
},
"potential_range_indexes": [
{
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
"index": "jr_code",
"usable": true,
"key_parts": [
"jr_code",
"id"
]
}
],
"best_covering_index_scan": {
"index": "jr_code",
"cost": 41.319,
"chosen": true
},
"group_index_range": {
"potential_group_range_indexes": [
{
"index": "jr_code",
"covering": true,
"rows": 152,
"cost": 64.8
}
]
},
"best_group_range_summary": {
"type": "index_group",
"index": "jr_code",
"group_attribute": "id",
"min_aggregate": false,
"max_aggregate": true,
"distinct_aggregate": false,
"rows": 152,
"cost": 64.8,
"key_parts_used_for_access": [
"jr_code"
],
"ranges": [],
"chosen": false,
"cause": "cost"
}
}
}
]
},
{
"considered_execution_plans": [
{
"plan_prefix": [],
"table": "`jr_form`",
"best_access_path": {
"considered_access_paths": [
{
"rows_to_scan": 184,
"access_type": "scan",
"resulting_rows": 184,
"cost": 42.8,
"chosen": true,
"use_tmp_table": true
}
]
},
"condition_filtering_pct": 100,
"rows_for_plan": 184,
"cost_for_plan": 42.8,
"sort_cost": 184,
"new_cost_for_plan": 226.8,
"chosen": true
}
]
},
{
"transformation": {
"select#": 2,
"from": "IN (SELECT)",
"to": "materialization",
"has_nullable_expressions": true,
"treat_UNKNOWN_as_FALSE": true,
"possible": true
}
},
{
"execution_plan_for_potential_materialization": {
"surely_same_plan_as_EXISTS": true,
"cause": "EXISTS_did_not_change_WHERE",
"subq_mat_decision": {
"parent_fanouts": [
{
"select#": 1,
"subq_attached_to_table": true,
"table": "`jr_form`",
"fanout": 184,
"cacheable": true
}
],
"cost_to_create_and_fill_materialized_table": 265.6,
"cost_of_one_EXISTS": 226.8,
"number_of_subquery_evaluations": 184,
"cost_of_materialization": 302.4,
"cost_of_EXISTS": 41731,
"chosen": true
}
}
},
{
"transformation": {
"select#": 2,
"from": "IN (SELECT)",
"to": "materialization",
"chosen": true,
"unknown_key_1": {
"creating_tmp_table": {
"tmp_table_info": {
"row_length": 9,
"key_length": 9,
"unique_constraint": false,
"location": "memory (heap)",
"row_limit_estimate": 1864135
}
}
}
}
},
{
"attaching_conditions_to_tables": {
"original_condition": null,
"attached_conditions_computation": [],
"attached_conditions_summary": [
{
"table": "`jr_form`",
"attached": null
}
]
}
},
{
"clause_processing": {
"clause": "GROUP BY",
"original_clause": "`jr_form`.`jr_code`",
"items": [
{
"item": "`jr_form`.`jr_code`"
}
],
"resulting_clause_is_simple": true,
"resulting_clause": "`jr_form`.`jr_code`"
}
},
{
"reconsidering_access_paths_for_index_ordering": {
"clause": "GROUP BY",
"index_order_summary": {
"table": "`jr_form`",
"index_provides_order": true,
"order_direction": "asc",
"index": "jr_code",
"plan_changed": false
}
}
},
{
"refine_plan": [
{
"table": "`jr_form`"
}
]
}
]
}
},
{
"join_explain": {
"select#": 1,
"steps": [
{
"join_explain": {
"select#": 2,
"steps": []
}
}
]
}
}
]
}
内容很长，之后在详细说明
https://zhhll.icu/2021/数据库/关系型数据库/MySQL/进阶/25.优化器/
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