单键索引是否会一个sql走多个？

当你的sql可以匹配到多个单间索引的时候，大家一般都认为只走一个索引。mysql> \s--------------mysql  Ver 14.14 Distrib 5.1.48, for Win32 (ia32)Connection id:          13Current database:       pk_testCurrent user:           root@localhostSSL:                    Not in useUsing delimiter:        ;Server version:         5.1.48-community MySQL Community Server (GPL)Protocol version:       10Connection:             localhost via TCP/IPServer characterset:    latin1Db     characterset:    utf8Client characterset:    latin1Conn.  characterset:    latin1TCP port:               3306Uptime:                 43 min 42 secThreads: 2  Questions: 2604  Slow queries: 0  Opens: 23  Flush tables: 1  Open tables: 2  Queries per second avg: 0.993--------------========================================首先下面两个表是在同一个库里。CREATE TABLE `t_carmodelparamvalue2` (  `ModelParamValueID` int(11) NOT NULL AUTO_INCREMENT,  `ParameterID` int(11) NOT NULL,  `ParamValueID` int(11) DEFAULT NULL,  `ModelID` int(11) NOT NULL,  `ValueText` varchar(512) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,  `SeriesID` int(11) DEFAULT NULL,  PRIMARY KEY (`ModelParamValueID`),  KEY `ModelID` (`ModelID`),  KEY `SeriesID` (`SeriesID`),  KEY `ak_paramvalue_parameterid` (`ParameterID`)) ENGINE=InnoDB AUTO_INCREMENT=1254098 DEFAULT CHARSET=gb2312;mysql> select count(*) from `t_carmodelparamvalue2`;+----------+| count(*) |+----------+|      500 |+----------+1 row in set (0.00 sec)mysql> explain select * from T_CarModelParamValue2 where modelid = 276666 and ParameterID = 180;+----+-------------+-----------------------+------+-----------------------------------+---------+---------+-------+------+-------------+| id | select_type | table                 | type | possible_keys                     | key     | key_len | ref   | rows | Extra       |+----+-------------+-----------------------+------+-----------------------------------+---------+---------+-------+------+-------------+|  1 | SIMPLE      | T_CarModelParamValue2 | ref  | ModelID,ak_paramvalue_parameterid | ModelID | 4       | const |    1 | Using where |+----+-------------+-----------------------+------+-----------------------------------+---------+---------+-------+------+-------------+1 row in set (0.00 sec)===================================================但是，当数据超过百万级的时候呢？CREATE TABLE `t_carmodelparamvalue` (  `ModelParamValueID` int(11) NOT NULL AUTO_INCREMENT,  `ParameterID` int(11) NOT NULL,  `ParamValueID` int(11) DEFAULT NULL,  `ModelID` int(11) NOT NULL,  `ValueText` varchar(512) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,  `SeriesID` int(11) DEFAULT NULL,  PRIMARY KEY (`ModelParamValueID`),  KEY `ModelID` (`ModelID`),  KEY `SeriesID` (`SeriesID`),  KEY `ak_paramvalue_parameterid` (`ParameterID`)) ENGINE=InnoDB AUTO_INCREMENT=1253591 DEFAULT CHARSET=gb2312;mysql> select count(*) from `t_carmodelparamvalue`;+----------+| count(*) |+----------+|  1189500 |+----------+1 row in set (2.73 sec)mysql> explain select * from T_CarModelParamValue where modelid = 276666 and ParameterID = 180;+----+-------------+----------------------+-------------+-----------------------------------+-----------------------------------+---------+------+------+-----------------------------------------------------------------+| id | select_type | table                | type        | possible_keys                     | key                               | key_len | ref  | rows | Extra                                                           |+----+-------------+----------------------+-------------+-----------------------------------+-----------------------------------+---------+------+------+-----------------------------------------------------------------+|  1 | SIMPLE      | T_CarModelParamValue | index_merge | ModelID,ak_paramvalue_parameterid | ModelID,ak_paramvalue_parameterid | 4,4     | NULL |1 | Using intersect(ModelID,ak_paramvalue_parameterid); Using where |+----+-------------+----------------------+-------------+-----------------------------------+-----------------------------------+---------+------+------+-----------------------------------------------------------------+1 row in set (0.00 sec)走的却是两个。

看表现的结论是，当数据量达到百万的时候就会走多个。

其实，是，当一个表做过大量的insert、select或者转换引擎等操作时，就会出现这个问题。

清空那个百万级的表之后还是会先走多个引擎。