Your greenhouse should be bolted to a foundation, much like a house foundation. The foundation should be deep enough to reach below the freeze/frost line (at my house, that's 18 inches). The outside of the foundation should be insulated to keep the cold from coming in. Then the temperature of the floor will be determined by the temperature of the air inside the greenhouse. It will still feel cold but not as cold as outside.
The floor in my current greenhouse is concrete. But my old greenhouse had a paver floor. Its was essentially built like a patio inside a greenhouse. Whatever you use: gravel, sand, pavers, concrete, (not wood), it should be level or your benches and pots will sit wonky and it should drain exceedingly well. Don't put anything under the floor that will impede the flow of water downwards. My concrete floor is slightly sloped toward drains on the edge of the wall with a French drain outside to carry water to the street.
