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CREATETABLE`mem_test`(`id`int(10)unsignedNOTNULLDEFAULT'0',`name`varchar(10)DEFAULTNULL,`first`varchar(10)DEFAULTNULL,PRIMARYKEY(`id`),KEY`NewIndex1`(`name`,`first`))ENGINE=MEMORY;CREATETABLE`innodb_test`(`id`int(10)unsignedNOTNULLDEFAULT'0',`name`varchar(10)DEFAULTNULL,`first`varchar(10)DEFAULTNULL,PRIMARYKEY(`id`),KEY`NewIndex1`(`name`,`first`))ENGINE=InnoDB;

如：

1：在=或者<=>情况下，飞快，但是在如<或>情况下，他是不使用索引

mysql--chinastor.com-root@localhost:17db07:33:45>>explainselect*frommem_testwhereid>3;+----+-------------+----------+------+---------------+------+---------+------+------+-------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|+----+-------------+----------+------+---------------+------+---------+------+------+-------------+|1|SIMPLE|mem_test|ALL|PRIMARY|NULL|NULL|NULL|15|Usingwhere|+----+-------------+----------+------+---------------+------+---------+------+------+-------------+1rowinset(0.00sec)mysql--chinastor.com-root@localhost:17db07:33:49>>explainselect*frominnodb_testwhereid>3;+----+-------------+-------------+-------+---------------+---------+---------+------+------+-------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|+----+-------------+-------------+-------+---------------+---------+---------+------+------+-------------+|1|SIMPLE|innodb_test|range|PRIMARY|PRIMARY|4|NULL|7|Usingwhere|+----+-------------+-------------+-------+---------------+---------+---------+------+------+-------------+1rowinset(0.00sec)

2：不能用在orderby情况下来提高速度

mysql--chinastor.com-root@localhost:17db07:33:55>>explainselect*frominnodb_testorderbyid;+----+-------------+-------------+-------+---------------+---------+---------+------+------+-------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|+----+-------------+-------------+-------+---------------+---------+---------+------+------+-------+|1|SIMPLE|innodb_test|index|NULL|PRIMARY|4|NULL|15||+----+-------------+-------------+-------+---------------+---------+---------+------+------+-------+1rowinset(0.00sec)mysql--chinastor.com-root@localhost:17db07:34:27>>explainselect*frommem_testorderbyid;+----+-------------+----------+------+---------------+------+---------+------+------+----------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|+----+-------------+----------+------+---------------+------+---------+------+------+----------------+|1|SIMPLE|mem_test|ALL|NULL|NULL|NULL|NULL|15|Usingfilesort|+----+-------------+----------+------+---------------+------+---------+------+------+----------------+1rowinset(0.00sec)

MySQL内存表的弊端有什么

3：不能确定俩值之间有多少行

mysql--chinastor.com-root@localhost:17db07:37:14>>explainselectcount(1)frommem_testwhereid>3andid<6;+----+-------------+----------+------+---------------+------+---------+------+------+-------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|extra|+----+-------------+----------+------+---------------+------+---------+------+------+-------------+|1|simple|mem_test|all|primary|null|null|null|20|usingwhere|+----+-------------+----------+------+---------------+------+---------+------+------+-------------+1rowinset(0.00sec)mysql--chinastor.com-root@localhost:17db07:40:35>>explainselectcount(1)frominnodb_testwhereid>3andid<6;+----+-------------+-------------+-------+---------------+---------+---------+------+------+--------------------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|extra|+----+-------------+-------------+-------+---------------+---------+---------+------+------+--------------------------+|1|simple|innodb_test|range|primary|primary|4|null|1|usingwhere;usingindex|+----+-------------+-------------+-------+---------------+---------+---------+------+------+--------------------------+1rowinset(0.00sec) localhost:17db07:37:07="">>explainselect*frominnodb_testwherename='b';+----+-------------+-------------+------+---------------+-----------+---------+-------+------+--------------------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|+----+-------------+-------------+------+---------------+-----------+---------+-------+------+--------------------------+|1|SIMPLE|innodb_test|ref|NewIndex1|NewIndex1|33|const|8|Usingwhere;Usingindex|+----+-------------+-------------+------+---------------+-----------+---------+-------+------+--------------------------+1rowinset(0.00sec)mysql--chinastor.com-root@localhost:17db07:37:10>>explainselect*frommem_testwherename='b';+----+-------------+----------+------+---------------+------+---------+------+------+-------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|+----+-------------+----------+------+---------------+------+---------+------+------+-------------+|1|SIMPLE|mem_test|ALL|NewIndex1|NULL|NULL|NULL|20|Usingwhere|+----+-------------+----------+------+---------------+------+---------+------+------+-------------+

当然内存表也可以手动添加btree

CREATEINDEXBTREE_indexUSINGBTREEonmem_test(name,first)mysql--chinastor.com-root@localhost:17db03:36:41>>explainselect*frommem_testwherename='b';+----+-------------+----------+------+-----------------------+-------------+---------+-------+------+-------------+|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|+----+-------------+----------+------+-----------------------+-------------+---------+-------+------+-------------+|1|SIMPLE|mem_test|ref|NewIndex1,BTREE_index|BTREE_index|33|const|9|Usingwhere|+----+-------------+----------+------+-----------------------+-------------+---------+-------+------+-------------+1rowinset(0.00sec)

哈哈，它也用到索引了。

所以要选择合适的存储引擎至关重要。

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