The following query does not always use index of primary key on user_group table:

SELECT ug.*
FROM ugroup_user AS ug_u
       INNER JOIN user USING (user_id)
       INNER JOIN ugroup AS ug USING (ugroup_id)
       INNER JOIN groups AS g USING (group_id)
       LEFT JOIN user_group USING (group_id, user_id)
       INNER JOIN forgeconfig ON (forgeconfig.name = 'access_mode')
WHERE ug_u.user_id = '10430' AND (
    (
        forgeconfig.value = 'restricted'
          AND
        (
            user_group.group_id IS NOT NULL
              OR
            user.status = 'A' AND g.access IN ('public', 'unrestricted')
              OR
            user.status = 'R' AND g.access = 'unrestricted'
            )
        )
      OR
    (
        forgeconfig.value <> 'restricted'AND
        (
            user_group.group_id IS NOT NULL OR g.access <> ('private')
            )
        )
    );