I believe how current endgame databases work is that they store every possible position by the pieces currently on the board. They then calculate the shortest possible path from the current position to mate move by move, because the best move may change in the situation that your opponent doesn't play perfect chess. I don't believe that they have the best move stored for every position, though. I assume that if Chess could be "solved" they would simply extend this principle to all the pieces on the board instead of limiting it to 6 like it is now.