MySQL内存表的弊端

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MySQL内存表的弊端
　MySQL 内存表使我们经常会用到的，但是 MySQL内存表的也不是提高读性能的万能工具，在有些情况下，MySQL内存表可能会比其实表类型的B-TREE更慢。
1. CREATE TABLE `mem_test` (
2. `id` int(10) unsigned NOT NULL DEFAULT 0,
3. `name` varchar(10) DEFAULT NULL,
4. `first` varchar(10) DEFAULT NULL,
5. PRIMARY KEY (`id`),
6. KEY `NewIndex1` (`name`,`first`)
7. ) ENGINE=MEMORY ;
9. CREATE TABLE `innodb_test` (
10. `id` int(10) unsigned NOT NULL DEFAULT 0,
11. `name` varchar(10) DEFAULT NULL,
12. `first` varchar(10) DEFAULT NULL,
13. PRIMARY KEY (`id`),
14. KEY `NewIndex1` (`name`,`first`)
15. ) 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
1. mysql--root@localhost:17db 07:33:45>>explain select * from mem_test where id>3;
2. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
5. | 1 | SIMPLE | mem_test | ALL | PRIMARY | NULL | NULL | NULL | 15 | Using where |
6. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
7. 1 row in set (0.00 sec)
9. mysql--root@localhost:17db 07:33:49>>explain select * from innodb_test where id>3;
10. +----+-------------+-------------+-------+---------------+---------+---------+------+------+-------------+
11. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
12. +----+-------------+-------------+-------+---------------+---------+---------+------+------+-------------+
13. | 1 | SIMPLE | innodb_test | range | PRIMARY | PRIMARY | 4 | NULL | 7 | Using where |
14. +----+-------------+-------------+-------+---------------+---------+---------+------+------+-------------+
15. 1 row in set (0.00 sec)
　　2：不能用在order by情况下来提高速度
1. mysql--root@localhost:17db 07:33:55>>explain select * from innodb_test order by id;
2. +----+-------------+-------------+-------+---------------+---------+---------+------+------+-------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+-------------+-------+---------------+---------+---------+------+------+-------+
5. | 1 | SIMPLE | innodb_test | index | NULL | PRIMARY | 4 | NULL | 15 | |
6. +----+-------------+-------------+-------+---------------+---------+---------+------+------+-------+
7. 1 row in set (0.00 sec)
9. mysql--root@localhost:17db 07:34:27>>explain select * from mem_test order by id;
10. +----+-------------+----------+------+---------------+------+---------+------+------+----------------+
11. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
12. +----+-------------+----------+------+---------------+------+---------+------+------+----------------+
13. | 1 | SIMPLE | mem_test | ALL | NULL | NULL | NULL | NULL | 15 | Using filesort |
14. +----+-------------+----------+------+---------------+------+---------+------+------+----------------+
15. 1 row in set (0.00 sec)
　　3：不能确定俩值之间有多少行
1. mysql--root@localhost:17db 07:37:14>>explain select count(1) from mem_test where id>3 and id<6;
2. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
5. | 1 | SIMPLE | mem_test | ALL | PRIMARY | NULL | NULL | NULL | 20 | Using where |
6. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
7. 1 row in set (0.00 sec)
9. mysql--root@localhost:17db 07:40:35>>explain select count(1) from innodb_test where id>3 and id<6;
10. +----+-------------+-------------+-------+---------------+---------+---------+------+------+--------------------------+
11. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
12. +----+-------------+-------------+-------+---------------+---------+---------+------+------+--------------------------+
13. | 1 | SIMPLE | innodb_test | range | PRIMARY | PRIMARY | 4 | NULL | 1 | Using where; Using index |
14. +----+-------------+-------------+-------+---------------+---------+---------+------+------+--------------------------+
15. 1 row in set (0.00 sec)
　　4：在多列索引的情况下，只有全部指定才能利用hash扫描，而B-tree确可以利用索引的最左端来查找
1. mysql--root@localhost:17db 07:37:07>>explain select * from innodb_test where name=b;
2. +----+-------------+-------------+------+---------------+-----------+---------+-------+------+--------------------------+
3. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
4. +----+-------------+-------------+------+---------------+-----------+---------+-------+------+--------------------------+
5. | 1 | SIMPLE | innodb_test | ref | NewIndex1 | NewIndex1 | 33 | const | 8 | Using where; Using index |
6. +----+-------------+-------------+------+---------------+-----------+---------+-------+------+--------------------------+
7. 1 row in set (0.00 sec)
9. mysql--root@localhost:17db 07:37:10>>explain select * from mem_test where name=b;
10. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
11. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
12. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
13. | 1 | SIMPLE | mem_test | ALL | NewIndex1 | NULL | NULL | NULL | 20 | Using where |
14. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
　　当然内存表也可以手动添加btree
1. CREATE INDEX BTREE_index USING BTREE on mem_test(name,first)
2. mysql--root@localhost:17db 03:36:41>>explain select * from mem_test where name=b;
3. +----+-------------+----------+------+-----------------------+-------------+---------+-------+------+-------------+
4. | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
5. +----+-------------+----------+------+-----------------------+-------------+---------+-------+------+-------------+
6. | 1 | SIMPLE | mem_test | ref | NewIndex1,BTREE_index | BTREE_index | 33 | const | 9 | Using where |
7. +----+-------------+----------+------+-----------------------+-------------+---------+-------+------+-------------+
8. 1 row in set (0.00 sec)
　　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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原文地址：https://www.cnblogs.com/balaamwe/p/2475641.html