Note sure I understand well the question, but there is what you can do with a database:
1) Play database like a rosbag (though it is recommended to use directly a rosbag if it is what you want to do):
$ rosrun rtabmap_ros data_player _database:=~/my_map.db --clock
2) Start rtabmap with the database as input, then subscribe to /rtabmap/cloud_map and call service /rtabmap/publish_map to get the map:
$ roslaunch rtabmap_ros rtabmap.launch database_path:=~/my_map.db
$ rostopic echo /rtabmap/cloud_map
$ rosservice call /rtabmap/publish_map 1 1 0
EDIT
3) Without rtabmap.launch:
$ roscore
$ export ROS_NAMESPACE=rtabmap && rosrun rtabmap_ros rtabmap _database_path:=~/my_map.db
$ rostopic echo /rtabmap/cloud_map
$ rosservice call /rtabmap/publish_map 1 1 0
4) For RVIZ with rtabmap_ros/MapCloud display, launch rtabmap node in rtabmap namespace, then in RVIZ's MapCloud plugin should be able to click on Download Map:
$ roscore
$ export ROS_NAMESPACE=rtabmap && rosrun rtabmap_ros rtabmap _database_path:=~/my_map.db
$ rviz
5) For rtabmapviz, we can do something similar:
$ roscore
$ export ROS_NAMESPACE=rtabmap && rosrun rtabmap_ros rtabmap _database_path:=~/my_map.db
$ export ROS_NAMESPACE=rtabmap && rosrun rtabmap_ros rtabmapviz
