MySQL如何选择合适的索引
先来看一个栗子 EXPLAIN select * from employees where name 'a'; 如果用name索引查找数据需要遍历name字段联合索引树，然后根据遍历出来的主键值去主键索引树里再去查出最终数据，成本比全表扫
先来看一个栗子
EXPLAIN select * from employees where name > 'a';
如果用name索引查找数据需要遍历name字段联合索引树，然后根据遍历出来的主键值去主键索引树里再去查出最终数据，成本比全表扫描还高。
可以用覆盖索引优化，这样只需要遍历name字段的联合索引树就可以拿到所有的结果。
EXPLAIN select name,age,position from employees where name > 'a';
可以看到通过select出的字段是覆盖索引，MySQL底层使用了索引优化。在看另一个case：
EXPLAIN select * from employees where name > 'zzz';
对于上面的这两种 name>'a' 和 name>'zzz'的执行结果， mysql最终是否选择走索引或者一张表涉及多个索引， mysql最终如何选择索引，可以通过trace工具来一查究竟，开启trace工具会影响mysql性能，所以只能临时分析sql使用，用完之后需要立即关闭。
SET SESSION optimizer_trace="enabled=on",end_markers_in_json=on; --开启trace SELECT FROM employees WHERE name > 'a' ORDER BY position; SELECT FROM information_schema.OPTIMIZER_TRACE;
看trace字段：
{
"steps": [
{
"join_preparation": { --第一阶段：SQl准备阶段
"select#": 1,
"steps": [
{
"expanded_query": "/ select#1 / select employees.id AS id,employees.name AS name,employees.age AS age,employees.position AS position,employees.hire_time AS hire_time from employees where (employees.name > 'a') order by employees.position"
}
] / 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": "(employees.name > 'a')",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "(employees.name > 'a')"
},
{
"transformation": "constant_propagation",
"resulting_condition": "(employees.name > 'a')"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "(employees.name > 'a')"
}
] / steps /
} / condition_processing /
},
{
"table_dependencies": [ --表依赖详情
{
"table": "employees",
"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": [ --预估标的访问成本
{
"table": "employees",
"range_analysis": {
"table_scan": { --全表扫描情况
"rows": 3, --扫描行数
"cost": 3.7 --查询成本
} / table_scan /,
"potential_range_indices": [ --查询可能使用的索引
{
"index": "PRIMARY", --主键索引
"usable": false,
"cause": "not_applicable"
},
{
"index": "idx_name_age_position", --辅助索引
"usable": true,
"key_parts": [
"name",
"age",
"position",
"id"
] / key_parts /
},
{
"index": "idx_age",
"usable": false,
"cause": "not_applicable"
}
] / potential_range_indices /,
"setup_range_conditions": [
] / setup_range_conditions /,
"group_index_range": {
"chosen": false,
"cause": "not_group_by_or_distinct"
} / group_index_range /,
"analyzing_range_alternatives": { ‐‐分析各个索引使用成本
"range_scan_alternatives": [
{
"index": "idx_name_age_position",
"ranges": [
"a < name"
] / ranges /,
"index_dives_for_eq_ranges": true,
"rowid_ordered": false,
"using_mrr": false,
"index_only": false, ‐‐是否使用覆盖索引
"rows": 3, --‐‐索引扫描行数
"cost": 4.61, --索引使用成本
"chosen": false, ‐‐是否选择该索引
"cause": "cost"
}
] / range_scan_alternatives /,
"analyzing_roworder_intersect": {
"usable": false,
"cause": "too_few_roworder_scans"
} / analyzing_roworder_intersect /
} / analyzing_range_alternatives /
} / range_analysis /
}
] / rows_estimation /
},
{
"considered_execution_plans": [
{
"plan_prefix": [
] / plan_prefix /,
"table": "employees",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "scan",
"rows": 3,
"cost": 1.6,
"chosen": true,
"use_tmp_table": true
}
] / considered_access_paths /
} / best_access_path /,
"cost_for_plan": 1.6,
"rows_for_plan": 3,
"sort_cost": 3,
"new_cost_for_plan": 4.6,
"chosen": true
}
] / considered_execution_plans /
},
{
"attaching_conditions_to_tables": {
"original_condition": "(employees.name > 'a')",
"attached_conditions_computation": [
] / attached_conditions_computation /,
"attached_conditions_summary": [
{
"table": "employees",
"attached": "(employees.name > 'a')"
}
] / attached_conditions_summary /
} / attaching_conditions_to_tables /
},
{
"clause_processing": {
"clause": "ORDER BY",
"original_clause": "employees.position",
"items": [
{
"item": "employees.position"
}
] / items /,
"resulting_clause_is_simple": true,
"resulting_clause": "employees.position"
} / clause_processing /
},
{
"refine_plan": [
{
"table": "employees",
"access_type": "table_scan"
}
] / refine_plan /
},
{
"reconsidering_access_paths_for_index_ordering": {
"clause": "ORDER BY",
"index_order_summary": {
"table": "employees",
"index_provides_order": false,
"order_direction": "undefined",
"index": "unknown",
"plan_changed": false
} / index_order_summary /
} / reconsidering_access_paths_for_index_ordering /
}
] / steps /
} / join_optimization /
},
{
"join_execution": { --第三阶段：SQL执行阶段
"select#": 1,
"steps": [
{
"filesort_information": [
{
"direction": "asc",
"table": "employees",
"field": "position"
}
] / filesort_information /,
"filesort_priority_queue_optimization": {
"usable": false,
"cause": "not applicable (no LIMIT)"
} / filesort_priority_queue_optimization /,
"filesort_execution": [
] / filesort_execution /,
"filesort_summary": {
"rows": 3,
"examined_rows": 3,
"number_of_tmp_files": 0,
"sort_buffer_size": 200704,
"sort_mode": "
全表扫描的成本低于索引扫描， 索引MySQL最终会选择全表扫描。
SELECT FROM employees WHERE name > 'zzz' ORDER BY position; SELECT FROM information_schema.OPTIMIZER_TRACE;
{
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/ select#1 / select employees.id AS id,employees.name AS name,employees.age AS age,employees.position AS position,employees.hire_time AS hire_time from employees where (employees.name > 'zzz') order by employees.position"
}
] / steps /
} / join_preparation /
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": {
"condition": "WHERE",
"original_condition": "(employees.name > 'zzz')",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "(employees.name > 'zzz')"
},
{
"transformation": "constant_propagation",
"resulting_condition": "(employees.name > 'zzz')"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "(employees.name > 'zzz')"
}
] / steps /
} / condition_processing /
},
{
"table_dependencies": [
{
"table": "employees",
"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": [
{
"table": "employees",
"range_analysis": {
"table_scan": {
"rows": 3,
"cost": 3.7
} / table_scan /,
"potential_range_indices": [
{
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
"index": "idx_name_age_position",
"usable": true,
"key_parts": [
"name",
"age",
"position",
"id"
] / key_parts /
},
{
"index": "idx_age",
"usable": false,
"cause": "not_applicable"
}
] / potential_range_indices /,
"setup_range_conditions": [
] / setup_range_conditions /,
"group_index_range": {
"chosen": false,
"cause": "not_group_by_or_distinct"
} / group_index_range /,
"analyzing_range_alternatives": {
"range_scan_alternatives": [
{
"index": "idx_name_age_position",
"ranges": [
"zzz < name"
] / ranges /,
"index_dives_for_eq_ranges": true,
"rowid_ordered": false,
"using_mrr": false,
"index_only": false,
"rows": 1,
"cost": 2.21,
"chosen": true
}
] / range_scan_alternatives /,
"analyzing_roworder_intersect": {
"usable": false,
"cause": "too_few_roworder_scans"
} / analyzing_roworder_intersect /
} / analyzing_range_alternatives /,
"chosen_range_access_summary": {
"range_access_plan": {
"type": "range_scan",
"index": "idx_name_age_position",
"rows": 1,
"ranges": [
"zzz < name"
] / ranges /
} / range_access_plan /,
"rows_for_plan": 1,
"cost_for_plan": 2.21,
"chosen": true
} / chosen_range_access_summary /
} / range_analysis /
}
] / rows_estimation /
},
{
"considered_execution_plans": [
{
"plan_prefix": [
] / plan_prefix /,
"table": "employees",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "range",
"rows": 1,
"cost": 2.41,
"chosen": true,
"use_tmp_table": true
}
] / considered_access_paths /
} / best_access_path /,
"cost_for_plan": 2.41,
"rows_for_plan": 1,
"sort_cost": 1,
"new_cost_for_plan": 3.41,
"chosen": true
}
] / considered_execution_plans /
},
{
"attaching_conditions_to_tables": {
"original_condition": "(employees.name > 'zzz')",
"attached_conditions_computation": [
] / attached_conditions_computation /,
"attached_conditions_summary": [
{
"table": "employees",
"attached": "(employees.name > 'zzz')"
}
] / attached_conditions_summary /
} / attaching_conditions_to_tables /
},
{
"clause_processing": {
"clause": "ORDER BY",
"original_clause": "employees.position",
"items": [
{
"item": "employees.position"
}
] / items /,
"resulting_clause_is_simple": true,
"resulting_clause": "employees.position"
} / clause_processing /
},
{
"refine_plan": [
{
"table": "employees",
"pushed_index_condition": "(employees.name > 'zzz')",
"table_condition_attached": null,
"access_type": "range"
}
] / refine_plan /
},
{
"reconsidering_access_paths_for_index_ordering": {
"clause": "ORDER BY",
"index_order_summary": {
"table": "employees",
"index_provides_order": false,
"order_direction": "undefined",
"index": "idx_name_age_position",
"plan_changed": false
} / index_order_summary /
} / reconsidering_access_paths_for_index_ordering /
}
] / steps /
} / join_optimization /
},
{
"join_execution": {
"select#": 1,
"steps": [
{
"filesort_information": [
{
"direction": "asc",
"table": "employees",
"field": "position"
}
] / filesort_information /,
"filesort_priority_queue_optimization": {
"usable": false,
"cause": "not applicable (no LIMIT)"
} / filesort_priority_queue_optimization /,
"filesort_execution": [
] / filesort_execution /,
"filesort_summary": {
"rows": 0,
"examined_rows": 0,
"number_of_tmp_files": 0,
"sort_buffer_size": 200704,
"sort_mode": "
查看trace字段可知索引扫描的成本低于全表扫描的成本，所以MySQL最终选择索引扫描。
SET SESSION optimizer_trace="enabled=off"; -- 关闭trace
总结
以上所述是小编给大家介绍的MySQL如何选择合适的索引，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对每日运维网站的支持！如果你觉得本文对你有帮助，欢迎转载，烦请注明出处，谢谢！
