For anyone curious. This function called from the gateway scope will "restart" the history provider:
from com.inductiveautomation.ignition.gateway import IgnitionGateway
def restartHist(prov):
context = IgnitionGateway.get()
tagHistMan = context.getTagHistoryManager()
tagHistProv = tagHistMan.getTagHistoryProvider(prov)
tagHistProv.shutdown()
tagHistProv.startup()
(Be aware that messing with the historian DB tables or SDK can seriously mess up your historian or gateway.) Use at your own risk. Don't use in production.