best way to generate a random unique id

[Peter O'Malley]

I have a database with a table of users, and would like to assign a random, unique id to each user. I will need to tell each user what their id is, but I don't want to disclose any information to users about the number of users in the table, which is why the id has to be random. What is the best way to generate a random and unique id, with minimal cost to performance?

I've seen comments about the built-in UUID() function, and have heard that using it to fill a column every time a row is inserted can lead to performance issues, or collation and indexing issues. I've also seen comments saying that the function shouldn't be used because it's old or outdated. Is there a better alternative to UUID()? How does the performance cost compare?

Thanks for any help.