Use email address as primary key?
I will also point out that email is a bad choice to make a unique field, there are people and even small businesses that share an email address. And like phone numbers, emails can get re-used. [email protected] can easily belong to John Smith one year and Julia Smith two years later.
Another problem with emails is that they change frequently. If you are joining to other tables with that as the key, then you will have to update the other tables as well which can be quite a performance hit when an entire client company changes their emails (which I have seen happen.)
the primary key should be unique and constant
email addresses change like the seasons. Useful as a secondary key for lookup, but a poor choice for the primary key.
Disadvantages of using an email address as a primary key:
Slower when doing joins.
Any other record with a posted foreign key now has a larger value, taking up more disk space. (Given the cost of disk space today, this is probably a trivial issue, except to the extent that the record now takes longer to read. See #1.)
An email address could change, which forces all records using this as a foreign key to be updated. As email address don't change all that often, the performance problem is probably minor. The bigger problem is that you have to make sure to provide for it. If you have to write the code, this is more work and introduces the possibility of bugs. If your database engine supports "on update cascade", it's a minor issue.
Advantages of using email address as a primary key:
You may be able to completely eliminate some joins. If all you need from the "master record" is the email address, then with an abstract integer key you would have to do a join to retrieve it. If the key is the email address, then you already have it and the join is unnecessary. Whether this helps you any depends on how often this situation comes up.
When you are doing ad hoc queries, it's easy for a human being to see what master record is being referenced. This can be a big help when trying to track down data problems.
You almost certainly will need an index on the email address anyway, so making it the primary key eliminates one index, thus improving the performance of inserts as they now have only one index to update instead of two.
In my humble opinion, it's not a slam-dunk either way. I tend to prefer to use natural keys when a practical one is available because they're just easier to work with, and the disadvantages tend to not really matter much in most cases.
String comparison is slower than int comparison. However, this does not matter if you simply retrieve a user from the database using the e-mail address. It does matter if you have complex queries with multiple joins.
If you store information about users in multiple tables, the foreign keys to the users table will be the e-mail address. That means that you store the e-mail address multiple times.