Class-action defendants often ask a judge to "bifurcate" discovery so that it's undertaken in two stages: first, discovery pertinent to class certification issues; second, discovery pertinent to the merits of the classwide case. The idea is that proceeding in this way will be less burdensome for the defendant.
In my experience, plaintiffs are wise to oppose a motion to bifurcate discovery. Most subjects of discovery will fall into both categories, meaning it won't be easy to figure out what is and isn't a proper subject of discovery. Certainly, there's no bright line and lots of overlap. As a practical matter, this means that the parties will become involved in disputes as to the scope of allowable discovery, which will slow the case to a crawl and prevent the plaintiff from developing the facts he needs to support class certification and defend against a motion for summary judgment.
If the judge deciding the bifurcation issue is swayed by defendant's claim of burdensomeness, remind the judge that as in every case, the defendant will have an opportunity to object to any specific discovery request for the reason that it is overbroad or burdensome. Bifurcation simply isn't necessary.