"select" Isn't Broken
Remember, if you see hoof prints, think horses—not zebras. The OS is probably not broken. And the database is probably just fine. It is rare to find a bug in the OS or the compiler, or even a third-party product or library. The bug is most likely in the application.
If you "changed only one thing" and the system stopped working, that one thing was likely to be responsible, directly or indirectly, no matter how farfetched it seems. Sometimes the thing that changed is outside of your control: new versions of the OS, compiler, database, or other third-party software can wreak havoc with previously correct code. New bugs might show up. Bugs for which you had a work-around get fixed, breaking the work-around. APIs change, functionality changes; in short, it's a whole new ball game, and you must retest the system under these new conditions. So keep a close eye on the schedule when considering an upgrade; you may want to wait until after the next release.
If, however, you have no obvious place to start looking, you can always rely on a good old-fashioned binary search. See if the symptoms are present at either of two far away spots in the code. Then look in the middle. If the problem is present, then the bug lies between the start and the middle point; otherwise, it is between the middle point and the end. You can continue in this fashion until you narrow down the spot sufficiently to identify the problem.