MySQL内存表的弊端

2016/09/25 22:49
阅读数 16

MySQL内存表不是提高读性能的万能工具,它也是有着一定的弊端的,下文就为您分析MySQL内存表的不足,供您参考学习之用。

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 ;  
  8.  
  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;  
  16.  

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)  
  8.  
  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)  
  16.  

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)  
  8.  
  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)  
  16.  

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)  
  8.  
  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)  
  16.  

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)  
  8.  
  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. +----+-------------+----------+------+---------------+------+---------+------+------+-------------+  
  15.  

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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