Which permissions does a Linux executable file has when it is executed by root?

I am using CentOS 6.8.

I have an executable called railgun, whose user and group are both set to root.

I then use the root user to start it as a service, as below:

service railgun start

However, it produces a permission denied error:

/tmp/memcached.sock: connect: permission denied

I check the user & group for /tmp/memcached.sock and find both are "memcached". So I just wonder why railgun cannot access /tmp/memcached.sock since it is executed by root? Which permissions does it have when it is launched by root?

In my opinion, root can access anything. So the executable started by root also can access anything. Is that correct?