mysql 存储引擎 memory

查看支持的引擎

mysql> show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                        | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| InnoDB             | DEFAULT | Supports transactions, row-level locking, and foreign keys     | YES          | YES  | YES        |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                          | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables      | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears) | NO           | NO   | NO         |
| MyISAM             | YES     | MyISAM storage engine                                          | NO           | NO   | NO         |
| CSV                | YES     | CSV storage engine                                             | NO           | NO   | NO         |
| ARCHIVE            | YES     | Archive storage engine                                         | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                             | NO           | NO   | NO         |
| FEDERATED          | NO      | Federated MySQL storage engine                                 | NULL         | NULL | NULL       |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.01 sec)

Memory表被存储在内存中。且默认使用哈希索引。这使得它们很快，而且对创建暂时表很实用。但是。当server关闭之时，全部存储在Memory表里的数据被丢失。

由于表的定义被存在磁盘上的.frm文件里，所以表自身继续存在。在server重新启动动时它们是空的。

要明白指出你想要一个Memory表。可使用ENGINE选项来指定：

CREATE TABLE t (i INT) ENGINE = MEMORY;

CREATE TABLE test ENGINE=MEMORY;
SELECT ip,SUM(downloads) AS down FROM log_table GROUP BY ip;
SELECT COUNT(ip),AVG(down) FROM test;
DROP TABLE test;