The
OPTIMIZER_TRACEtable provides information produced by the optimizer tracing capability for traced statements. To enable tracking, use theoptimizer_tracesystem variable. For details, see MySQL Internals: Tracing the Optimizer.
我以MySQL 8.0 的樣板庫為例演示如何開啟OPTIMIZER_TRACE.
- 開啟開關,只針對當前線程生效
SET optimizer_trace='enabled=on';
- 執行sql語句
select id,name,countryCode,district from city where countryCode = 'IND' order by name desc,district asc limit 5 ;
- 查看執行計劃
explain select id,name,countryCode,district from city where countryCode = 'IND' order by name desc,district asc limit 5 ;
image.png
Using filesort 說明本次查詢用到了文件排序, 雖然叫文件排序,但是不一定是在文件中執行. 這取決于排序所
需的內存和參數sort_buffer_size.
- 查看 OPTIMIZER_TRACE
SELECT * FROM `information_schema`.`OPTIMIZER_TRACE`;
結果如下:
{
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `city`.`ID` AS `id`,`city`.`Name` AS `name`,`city`.`CountryCode` AS `countryCode`,`city`.`District` AS `district` from `city` where (`city`.`CountryCode` = 'IND') order by `city`.`Name` desc,`city`.`District` limit 5"
}
]
}
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": {
"condition": "WHERE",
"original_condition": "(`city`.`CountryCode` = 'IND')",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "multiple equal('IND', `city`.`CountryCode`)"
},
{
"transformation": "constant_propagation",
"resulting_condition": "multiple equal('IND', `city`.`CountryCode`)"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "multiple equal('IND', `city`.`CountryCode`)"
}
]
}
},
{
"substitute_generated_columns": {
}
},
{
"table_dependencies": [
{
"table": "`city`",
"row_may_be_null": false,
"map_bit": 0,
"depends_on_map_bits": [
]
}
]
},
{
"ref_optimizer_key_uses": [
{
"table": "`city`",
"field": "CountryCode",
"equals": "'IND'",
"null_rejecting": false
}
]
},
{
"rows_estimation": [
{
"table": "`city`",
"range_analysis": {
"table_scan": {
"rows": 4188,
"cost": 441.99
},
"potential_range_indexes": [
{
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
"index": "CountryCode",
"usable": true,
"key_parts": [
"CountryCode",
"ID"
]
}
],
"setup_range_conditions": [
],
"group_index_range": {
"chosen": false,
"cause": "not_group_by_or_distinct"
},
"skip_scan_range": {
"potential_skip_scan_indexes": [
{
"index": "CountryCode",
"usable": false,
"cause": "query_references_nonkey_column"
}
]
},
"analyzing_range_alternatives": {
"range_scan_alternatives": [
{
"index": "CountryCode",
"ranges": [
"IND <= CountryCode <= IND"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": true,
"using_mrr": false,
"index_only": false,
"rows": 341,
"cost": 322.67,
"chosen": true
}
],
"analyzing_roworder_intersect": {
"usable": false,
"cause": "too_few_roworder_scans"
}
},
"chosen_range_access_summary": {
"range_access_plan": {
"type": "range_scan",
"index": "CountryCode",
"rows": 341,
"ranges": [
"IND <= CountryCode <= IND"
]
},
"rows_for_plan": 341,
"cost_for_plan": 322.67,
"chosen": true
}
}
}
]
},
{
"considered_execution_plans": [
{
"plan_prefix": [
],
"table": "`city`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "ref",
"index": "CountryCode",
"rows": 341,
"cost": 97.381,
"chosen": true
},
{
"access_type": "range",
"range_details": {
"used_index": "CountryCode"
},
"chosen": false,
"cause": "heuristic_index_cheaper"
}
]
},
"condition_filtering_pct": 100,
"rows_for_plan": 341,
"cost_for_plan": 97.381,
"chosen": true
}
]
},
{
"attaching_conditions_to_tables": {
"original_condition": "(`city`.`CountryCode` = 'IND')",
"attached_conditions_computation": [
],
"attached_conditions_summary": [
{
"table": "`city`",
"attached": "(`city`.`CountryCode` = 'IND')"
}
]
}
},
{
"optimizing_distinct_group_by_order_by": {
"simplifying_order_by": {
"original_clause": "`city`.`Name` desc,`city`.`District`",
"items": [
{
"item": "`city`.`Name`"
},
{
"item": "`city`.`District`"
}
],
"resulting_clause_is_simple": true,
"resulting_clause": "`city`.`Name` desc,`city`.`District`"
}
}
},
{
"finalizing_table_conditions": [
{
"table": "`city`",
"original_table_condition": "(`city`.`CountryCode` = 'IND')",
"final_table_condition ": null
}
]
},
{
"refine_plan": [
{
"table": "`city`"
}
]
},
{
"considering_tmp_tables": [
{
"adding_sort_to_table_in_plan_at_position": 0
}
]
}
]
}
},
{
"join_execution": {
"select#": 1,
"steps": [
{
"sorting_table_in_plan_at_position": 0,
"filesort_information": [
{
"direction": "desc",
"table": "`city`",
"field": "Name"
},
{
"direction": "asc",
"table": "`city`",
"field": "District"
}
],
"filesort_priority_queue_optimization": {
"limit": 5,
"chosen": true
},
"filesort_execution": [
],
"filesort_summary": {
"memory_available": 262144,
"key_size": 55,
"row_size": 120,
"number_of_tmp_files":2,
"max_rows_per_buffer": 6,
"num_rows_estimate": 9362,
"num_rows_found": 341,
"num_initial_chunks_spilled_to_disk": 0,
"peak_memory_used": 1188,
"sort_algorithm": "std::sort",
"unpacked_addon_fields": "using_priority_queue",
"sort_mode": "<fixed_sort_key, additional_fields>"
}
}
]
}
}
]
}
其中, filesort_summary中的number_of_tmp_files表示的是，排序過程中使用的臨時文件數。
select * from performance_schema.session_statu ;
部分結果如下:
image.png
