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happen the second time around. Recursion, unlike nesting, is a database-level option and can be set
using the sp_dboption system sproc.
The danger in recursive triggers is that you’ll get into some form of unintended loop. As such, you’ll
need to make sure that you get some form of recursion check in place to stop the process if necessary.
Triggers Don’t Prevent Architectural Changes
This is a classic good news/bad news story.
Using triggers is positively great in terms of making it easy to make architecture changes. Indeed, I often
use triggers for referential integrity early in the development cycle (when I’m more likely to be making
lots of changes to the design of the database), and then change to DRI late in the cycle when I’m close to
production.
When you want to drop a table and re-create it using DRI, you must first drop all the constraints before
dropping the table. This can create quite a maze in terms of dropping multiple constraints, making your
changes, and then adding the constraints again. It can be quite a wild ride trying to make sure that
everything drops that is supposed to so that your changed scripts will run. Then it’s just as wild a ride to
make sure that you’ve got everything back on that needs to be. Triggers take care of all this because they
don’t care that anything has changed until they actually run. There’s the rub, though — when they run.
You see, it means that you may change architecture and break several triggers without even realizing that
you’ve done it. It won’t be until the first time that those triggers try to address the object(s) in question
that you find the error of your ways. By that time, you may find together exactly what you did and why.
Both sides have their hassles — just keep the hassles in mind no matter which method you’re employing.
Triggers Can Be Turned Off Without Being Removed
Sometimes, just like with CHECK constraints, you want to turn off the integrity feature so you can perform
some valid action that will violate the constraint. (Importation of data is probably the most common of
these.)
Another common reason for doing this is when you are performing some sort of bulk insert (importation
again), but you are already 100 percent certain the data is valid. In this case, you may want to turn off
the triggers to eliminate their overhead and speed up the insert process.
You can turn a trigger off and on by using an ALTER TABLE statement. The syntax looks like this:
ALTER TABLE
TRIGGER
Chapter 15: Triggers
As you might expect, my biggest words of caution in this area are, don’t forget to re-enable your triggers!
One last thing. If you’re turning them off to do some form of mass importation of data, I highly recommend
that you kick out all your users and go either to single-user mode, dbo-only mode, or both. This
will ensure that no one sneaks in behind you while you have the triggers turned off.
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