No idea what DH2 uses, apparently H2N is also by default using their own bundled database server.
You'll have to ask the devs in their support threads in the software section on 2+2.
Muh poker friend says that hand2note also uses postgres and by default it sends it to a secure server but can be changed to make your own postgresql server. shrugs. I'm def interested in h2note. I downloaded h2n briefly , thought it was very good, annoyed that it wasn't keeping track of tourney results correctly on betonline but otherwise working perfectly out of box, then my pc got hacked and I formatted everything, and haven't tried it again since i'm used to poker tracker 4 and not very knowledgeable about these things; i'm a live poker player.