Will get_current_user_id() in wordpress fire a database request?

I am using get_current_user_id() and wp_get_current_user() multiple times in my plugin. Are these functions going to send a database request everytime I use them or is the user object of the current user always available (cause wordpress requests it anyway on initializing)?

Would it be better to declare a global var at the start of my script with the current user info?

<?php
global $currentUser;
$currentUser = wp_get_current_user();

function function1() {
    global $currentUser;
    echo $currentUser->ID;
}
?>