I'm developing a nodejs api server for a single board computer with 2 GB ram and 16 GB SD card (ODROID-HC2). The OS is Ubuntu Linux. I need a database mainly for admin purposes. Data collected from sensors will be save to a hard drive. I wish to use a database that uses minimal ram so that all of my time-critical processes have sufficient ram. I've considered mysql, mongodb, and postgresql. Which one of these is likely to have the least demand on ram?