我们可以从information_schema中获得信息，这是不言自明的，但我找不到有关此系统定义模式的文档。

SELECT * 
FROM information_schema.role_authorization_descriptors 
WHERE grantee = 'a_user';

更有用查询是检查用户是否有权访问表

SELECT *
FROM information_schema.table_privileges
WHERE table_schema = 'a_schema'
AND   table_name = 'a_table'
AND   privilege_type = 'SELECT'
AND   (grantee = 'a_user' OR grantee IN (SELECT role_name
                                      FROM information_schema.role_authorization_descriptors
                                      WHERE grantee = 'a_user'));