By mixer I mean a service that has coins and will send coins from its wallet when it receives coins, ensuring they are not the same.

On AML laws, are you seriously going to go in front of a judge and try to argue that Bitcoin is not a currency and users of a mixer/mixnet are not "customers" in some way? I would be interested to see the reaction to such an argument given that the system is called BitCoin and the front page title is "Bitcoin P2P Virtual Currency".

For the obfuscation yes, I think I didn't explain it well. The columns are not clear text. Instead of a key you store a hash and instead of the value, you store the value XORd with a different hash of the key. It means you can't see what keys are in the table, and you can only read the value when you present a key that is in there.