单表扫描,MySQL索引选择不正确 并 详细解析OPTIMIZER_TRACE格式
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单表扫描，mysql索引选择不正确 并 详细解析OPTIMIZER_TRACE格式
一 表结构如下:
MySQL 5.5.30 5.6.20 版本, 表大概有815万行
CREATE TABLE t_audit_operate_log (
Fid bigint(16) AUTO_INCREMENT,
Fcreate_time int(10) unsigned NOT NULL DEFAULT ‘0‘,
Fuser varchar(50) DEFAULT ‘‘,
Fip bigint(16) DEFAULT NULL,
Foperate_object_id bigint(20) DEFAULT ‘0‘,
PRIMARY KEY (Fid),
KEY indx_ctime (Fcreate_time),
KEY indx_user (Fuser),
KEY indx_objid (Foperate_object_id),
KEY indx_ip (Fip)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
执行查询:
mysql> explain select count(*) from t_audit_operate_log where Fuser=‘[email protected]‘ and Fcreate_time>=1407081600 and Fcreate_time<=1407427199\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t_audit_operate_log
type: ref
possible_keys: indx_ctime,indx_user
key: indx_user
key_len: 153
ref: const
rows: 2007326
Extra: Using where
发现,使用了一个不合适的索引, 不是很理想，于是改成指定索引：
mysql> explain select count(*) from t_audit_operate_log use index(indx_ctime) where Fuser=‘[email protected]‘ and Fcreate_time>=1407081600 and Fcreate_time<=1407427199\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t_audit_operate_log
type: range
possible_keys: indx_ctime
key: indx_ctime
key_len: 5
ref: NULL
rows: 670092
Extra: Using where
实际执行耗时，后者比前者快了接近10
问题: 很奇怪，优化器为何不选择使用 indx_ctime 索引，而选择了明显会扫描更多行的 indx_user 索引。
分析2个索引的数据量如下: 两个条件的唯一性对比：
select count(*) from t_audit_operate_log where Fuser=‘[email protected]‘;
+----------+
| count(*) |
+----------+
| 1238382 |
+----------+
select count(*) from t_audit_operate_log where Fcreate_time>=1407254400 and Fcreate_time<=1407427199;
+----------+
| count(*) |
+----------+
| 198920 |
+----------+
显然,使用索引indx_ctime好于indx_user,但MySQL却选择了indx_user. 为什么?
于是,使用 OPTIMIZER_TRACE进一步探索.
二 OPTIMIZER_TRACE的过程说明
以本处事例简要说明OPTIMIZER_TRACE的过程.
{\
"steps": [\
{\
"join_preparation": {\ ---优化准备工作
"select#": 1,\
"steps": [\
{\
"expanded_query": "/* select#1 */ select count(0) AS `count(*)` from `t_audit_operate_log` where ((`t_audit_operate_log`.`Fuser` = ‘[email protected]‘) and (`t_audit_operate_log`.`Fcreate_time` >= 1407081600) and (`t_audit_operate_log`.`Fcreate_time` <= 1407427199))"\
}\
] /* steps */\
} /* join_preparation */\
},\
{\
"join_optimization": {\ ---优化工作的主要阶段,包括逻辑优化和物理优化两个阶段
"select#": 1,\
"steps": [\ ---优化工作的主要阶段, 逻辑优化阶段
{\
"condition_processing": {\ ---逻辑优化,条件化简
"condition": "WHERE",\
"original_condition": "((`t_audit_operate_log`.`Fuser` = ‘[email protected]‘) and (`t_audit_operate_log`.`Fcreate_time` >= 1407081600) and (`t_audit_operate_log`.`Fcreate_time` <= 1407427199))",\
"steps": [\
{\
"transformation": "equality_propagation",\ ---逻辑优化,条件化简,等式处理
"resulting_condition": "((`t_audit_operate_log`.`Fuser` = ‘[email protected]‘) and (`t_audit_operate_log`.`Fcreate_time` >= 1407081600) and (`t_audit_operate_log`.`Fcreate_time` <= 1407427199))"\
},\
{\
"transformation": "constant_propagation",\ ---逻辑优化,条件化简,常量处理
"resulting_condition": "((`t_audit_operate_log`.`Fuser` = ‘[email protected]‘) and (`t_audit_operate_log`.`Fcreate_time` >= 1407081600) and (`t_audit_operate_log`.`Fcreate_time` <= 1407427199))"\
},\
{\
"transformation": "trivial_condition_removal",\ ---逻辑优化,条件化简,条件去除
"resulting_condition": "((`t_audit_operate_log`.`Fuser` = ‘[email protected]‘) and (`t_audit_operate_log`.`Fcreate_time` >= 1407081600) and (`t_audit_operate_log`.`Fcreate_time` <= 1407427199))"\
}\
] /* steps */\
} /* condition_processing */\
},\ ---逻辑优化,条件化简,结束
{\
"table_dependencies": [\ ---逻辑优化, 找出表之间的相互依赖关系. 非直接可用的优化方式.
{\
"table": "`t_audit_operate_log`",\
"row_may_be_null": false,\
"map_bit": 0,\
"depends_on_map_bits": [\
] /* depends_on_map_bits */\
}\
] /* table_dependencies */\
},\
{\
"ref_optimizer_key_uses": [\