Exclusion Constraints are generalized SQL UNIQUE

Say you are writing an online reservation system. The first requirement you’ll encounter is that no two reservations may overlap (i.e. no schedule conflicts). But how do you prevent that?

It’s worth thinking about your solution carefully. My claim is that no existing SQL DBMS has a good solution to this problem before PostgreSQL 9.0, which has just been released. This new release includes a feature called Exclusion Constraints (authored by me), which offers a good solution to a class of problems that includes the “schedule conflict” problem.

I previously wrote a two part series (Part 1 and Part 2) on this topic. Chances are that you’ve run into a problem similar to this at one time or another, and these articles will show you the various solutions that people usually employ in the real world, and the serious problems and limitations of those approaches.

The rest of this article will be a brief introduction to Exclusion Constraints to get you started using a much better approach.

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