10月8號 

A MySQL Server has been running an existing application successfully for six months. The
my.cnf is adjusted to contain this additional configuration:
The MySQL Server is restarted without error.
What effect will the new configuration have on existing account?

[mysqld]

default-authentication-plgin=sha256_password

A. They will have to change their password the next time they login to the server.
B. They are not affected by this configuration change.
C. They will have their passwords updated on start-up to sha256_password format.
D. They all connect via the secure sha256_password algorithm without any configuration change.

解析:

預設身份認證外掛。可接受的值 mysql_native_password（使用MySQL本地密碼）和sha256_password（用SHA-256 密碼）關於這些外掛見Section 6.3.7.1, “The Native Authentication Plugins”，和 Section 6.3.7.2, “The SHA-256 Authentication Plugin”。這個選項在MySQL5.6.6新增。

該選項的值影響服務操作的幾個方面：

1）對於沒有明確指名外掛，決定哪個外掛分配到新使用者。

2）在啟動時設定系統變數 old_passwords 的值和預設外掛所需的密碼雜湊格式發生衝突。這結果影響 PASSWORD() 函式使用的密碼雜湊方法。

3）下面任何建立新賬號的語句，伺服器將使用預設驗證插關聯帳戶和分配指定的密碼給帳戶，該雜湊值是old_passwords的值。CREATE USER ... IDENTIFIED BY 'cleartext password';
GRANT ...  IDENTIFIED BY 'cleartext password';

所以這種情況對老使用者沒有影響,選擇B