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I have a users table, the user ID is public. But I want to obfuscate the number of registered user and trends of the project, so I don't want to have public incrementing IDs. When a new user is created I want to find a random integer number that is greater than a certain number and that is not yet in the database. Naive code: <?php $found = false; while(!$found) { $uid = rand(1000000000,4294967295) // find random number betwen minimum and maximum $dbh->beginTransaction(); // check if user id is in use, and if not insert it if($dbh->query("SELECT * FROM users WHERE uid = $uid")) { $dbh->exec("INSERT INTO users (uid) VALUES ($uid)"); $found = true; } $dbh->commit(); } // we just got our new uid ... ?> This will work it however may become inefficient. True that there is a big range and the probability of hitting an unused uid is high. But what if I want to use a smaller range, because I don't want to have so long userids? Example of my concerns: 60% of all user ids are in use the chance of hitting an unused uid are 0.4 the first attempt has 0.4% success rate if 1st not successful the second attempt has 0.6*0.4 probability so with a maximum of two tries i have 0.4 + 0.6*0.4 proability (is that right??) So one method to optimize is that came to my mind is the following: find a random number, check if its free, if not, increment it by 1 and try again and so on if the maximum number is hit, continue with the minimum number That should give me a number with a maximum runtime of O(range) That sounds pretty bad but I think it is not, because I submit random numbers to the database and that they are all at the beginnig is very unlikely. So how good/bad is it really? I think this would work just fine but I

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