postgresql解析失败：v_date_desc::date - max(access_time)::date

[Galthen]

select v_date_desc::date - max(access_time)::date from table1;
解析错误：syntax error, error in :'- max(access_time)::date from table1, pos 45, line 1, column 44, token ::

select v_date_desc::date - (max(access_time))::date from table1;
解析成功。