What is the legal test for a preliminary issue?

The Court of Appeal addressed the preliminary trial recently in Sware v. Welda Estate, 1999 ABCA 308 at para.2: The decision whether to try a preliminary issue always involves risks and estimates of the future. It is rare, if ever, that such a thing can be guaranteed to produce a good result no matter what the result of the trial. Therefore, it is something of a risk. It seems to us that what one must do is see whether one is able to estimate the likely beneficial results and the likely detrimental results of either course of action, and see if one can say anything about the odds. In many cases it is very difficult to see those things at all, and in other cases a comparison of the likely risks and benefits is not very encouraging. That is doubtless why the cases say that trying a preliminary issue requires an exceptional case.