I sometimes think in that direction as well... but in the case of degraded query performance it could be simple a DOS attack, where they found some combination

I sometimes think in that direction as well... but in the case of degraded query performance it could be simple a DOS attack, where they found some combination of operations that brings the db down... and they exploit it because they don't expect it to do anything more than this. If you want your enemy to fear the repercussions of attacking you, you need him to understand the strength you have to fight back. And in the case of cyberwarfare the only way to show strength is by hacking stuff... so I imagine there's some value in smaller not-so-destructive attacks.