1、所有用户都有访问information_schema下的表权限(但只能看到这些表中用户具有访问权限的对象相对应的数据行)，但只能访问Server层的部分数据字典表.
2、Server层中的部分数据字典表以及InnoDB层的数据字典表需要额外授权才能访问，如果用户权限不足，当查询Server层数据字典表时将不会返回任何数据，或者某个列没有权限访问时，该列返回NULL值。
3、当查询InnoDB数据字典表时将直接拒绝访问(要访问这些表需要有process权限，注意不是select权限)
4、从information_schema中查询相关数据需要的权限也适用于SHOW语句。无论使用哪种查询方式，都必须拥有某个对象的权限才能看到相关的数据。
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process和show
mysql> select * from 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 (0.02 sec)
mysql>