[baroffoos]

I have seen it done in a small company to check if a query will run too slow in production. Take a copy of the biggest database. Run query, see what happens, delete copy.

[fooey]

It's probably more often that the query is just run against production in the first place.

Making a copy is probably more effort than most developers out in the wild are going to make.

[sroussey]

Not true. If you were to throw up a slow locking query in production, you could take down the site. Restoring a backup should be fast.