mysql优化器有没有可能选择一个低效索引，给数据库带来性能问题？
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mysql的优化器还比较二，所以不要给太复杂的事情让优化器做
可以通过explain查看执行计划。
更可以通过下面的方式去看 优化器的选择过程，明白了选择过程也就可以找到问题。
下面给一个简单的例子：
mysql> show variables like '%trace%';
+------------------------------+----------------------------------------------------------------------------+
| Variable_name | Value |
+------------------------------+----------------------------------------------------------------------------+
| optimizer_trace | enabled=off,one_line=off |
| optimizer_trace_features | greedy_search=on,range_optimizer=on,dynamic_range=on,repeated_subselect=on |
| optimizer_trace_limit | 1 |
| optimizer_trace_max_mem_size | 16384 |
| optimizer_trace_offset | -1 |
+------------------------------+----------------------------------------------------------------------------+
5 rows in set (0.00 sec)
mysql> set optimizer_trace = "enabled=on"; --打开跟踪， 只能跟踪自己session执行的，不能跟踪别人的
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test.ta where id=1;
+------+
| id |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql> select * from information_schema.optimizer_trace\G
*************************** 1. row ***************************
QUERY: select * from test.ta where id=1
TRACE: {
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `test`.`ta`.`id` AS `id` from `test`.`ta` where (`test`.`ta`.`id` = 1)"
}
]
}
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": {
"condition": "WHERE",
"original_condition": "(`test`.`ta`.`id` = 1)",
"steps": [
{
"transformation": "equality_propagation", --转换
"resulting_condition": "multiple equal(1, `test`.`ta`.`id`)"
},
{
"transformation": "constant_propagation",
"resulting_condition": "multiple equal(1, `test`.`ta`.`id`)"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "multiple equal(1, `test`.`ta`.`id`)"
}
]
}
},
{
"table_dependencies": [
{
"table": "`test`.`ta`",
"row_may_be_null": false,
"map_bit": 0,
"depends_on_map_bits": [
]
}
]
},
{
"ref_optimizer_key_uses": [
{
"table": "`test`.`ta`",
"field": "id",
"equals": "1",
"null_rejecting": false
}
]
},
{
"rows_estimation": [
{
"table": "`test`.`ta`",
"range_analysis": {
"table_scan": {
"rows": 9,
"cost": 4.9 --表扫描的cost
},
"potential_range_indices": [
{
"index": "idx_ta",
"usable": true,
"key_parts": [
"id"
]
}
],
"best_covering_index_scan": {
"index": "idx_ta",
"cost": 2.8107,
"chosen": true
},
"setup_range_conditions": [
],
"group_index_range": {
"chosen": false,
"cause": "not_group_by_or_distinct"
},
"analyzing_range_alternatives": {
"range_scan_alternatives": [
{
"index": "idx_ta",
"ranges": [
"1 <= id <= 1"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": true,
"using_mrr": false,
"index_only": true,
"rows": 1,
"cost": 2.21,
"chosen": true
}
],
"analyzing_roworder_intersect": {
"usable": false,
"cause": "too_few_roworder_scans"
}
},
"chosen_range_access_summary": {
"range_access_plan": {
"type": "range_scan",
"index": "idx_ta",
"rows": 1,
"ranges": [
"1 <= id <= 1"
]
},
"rows_for_plan": 1,
"cost_for_plan": 2.21,
"chosen": true
}
}
}
]
},
{
"considered_execution_plans": [
{
"plan_prefix": [
],
"table": "`test`.`ta`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "ref",
"index": "idx_ta",
"rows": 1,
"cost": 1.2,
"chosen": true --使用了这个执行计划
},
{
"access_type": "range",
"cause": "heuristic_index_cheaper",
"chosen": false
}
]
},
"cost_for_plan": 1.2,
"rows_for_plan": 1,
"chosen": true
}
]
},
{
"attaching_conditions_to_tables": {
"original_condition": "(`test`.`ta`.`id` = 1)",
"attached_conditions_computation": [
],
"attached_conditions_summary": [
{
"table": "`test`.`ta`",
"attached": null
}
]
}
},
{
"refine_plan": [
{
"table": "`test`.`ta`"
}
]
}
]
}
},
{
"join_execution": {
"select#": 1,
"steps": [
]
}
}
]
}
MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0
INSUFFICIENT_PRIVILEGES: 0
1 row in set (0.00 sec)
-- 里面包含条件的去除，查询的转换，选择最好的执行计划。
mysql> set optimizer_trace = "enabled=off"; --关闭跟踪
Query OK, 0 rows affected (0.00 sec)
mysql> show create table information_schema.optimizer_trace;
+-----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| OPTIMIZER_TRACE | CREATE TEMPORARY TABLE `OPTIMIZER_TRACE` ( --这是一个临时表，别的连接看不到本连接的内容，
`QUERY` longtext NOT NULL,
`TRACE` longtext NOT NULL,
`MISSING_BYTES_BEYOND_MAX_MEM_SIZE` int(20) NOT NULL DEFAULT '0',
`INSUFFICIENT_PRIVILEGES` tinyint(1) NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8 |
+-----------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
可以使用hint强制走某个执行索引，但不建议这样做，会有以下问题：
1 索引不存在时，sql会直接报错
2 随着数据分布的改变，也许以后这不是一个好的执行计划
所以，还是找到他为什么会选择错误了，才好根本上解决问题。
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