红移卸载中的问题

unload ('select * from tablename')
to 's3://bucketname/'
iam_role 'arn:aws:iam::123456789012:role/rolename';

ERROR:  User arn:aws:redshift:eu-west-1:123456789012:dbuser:dbaname/dbusername is not authorized to assume IAM Role arn:aws:iam::123456789012:role/rolename
DETAIL:
  -----------------------------------------------
  error:  User arn:aws:redshift:eu-west-1:123456789012:dbuser:dbname/dbusername is not authorized to assume IAM Role arn:aws:iam::123456789012:role/rolename
  code:      8001
  context:   IAM Role=arn:aws:iam::123456789012:role/rolename
  query:     0
  location:  xen_aws_credentials_mgr.cpp:238
  process:   padbmaster [pid=3203]
  -----------------------------------------------

我用rolename使用switch role从一个帐户连接到另一个帐户,这个角色在AWS上有完全的访问权限

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "*",
            "Resource": "*"
        }
    ]
}