我尝试执行以下命令（PostgreSQL）：

ALTER TABLE authentication ADD CONSTRAINT overlapping_times EXCLUDE USING GIST
(method with =,
 authenticator with =,
 box(point(extract(epoch FROM validfrom at time zone 'UTC'),extract(epoch FROM validfrom at time zone 'UTC') ),
     point(extract(epoch FROM validuntil at time zone 'UTC'), extract(epoch FROM validuntil at time zone 'UTC'))) WITH &&
)

我得到了以下错误消息：

ERROR:  data type character varying has no default operator class for access method "gist"
HINT:  You must specify an operator class for the index or define a default operator class for the data type.

我做了相当广泛的谷歌搜索，但我仍然无法翻译成简单的英语。我应该怎么做才能执行上面的命令？“方法”的类型是字符变化的，“验证器”是文本，“有效期”，“有效期”是日期。