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For example, I always generate an auto-increment field for the users table, but I also specify a UNIQUE index on their usernames. There are situations that I first need to get the userId for a given username and then execute the desired query, or use a JOIN in the desired query. It’s 2 trips to the database or a JOIN vs. a varchar index.
Should I use integer primary IDs?
Is there a real performance benefit on INT over small VARCHAR indexes?
There are several advantages of having a surrogate primary key, including:
When you have a foreign key in another table, if it is an integer it takes up only a few bytes extra space and can be joined quickly. If you use the username as the primary key it will have to be stored in both tables – taking up more space and it takes longer to compare when you need to join.
If a user wishes to change their username, you will have big problems if you have used it as a primary key. While it is possible to update a primary key, it is very unwise to do so and can cause all sorts of problems as this key might have been sent out to all sorts of other systems, used in links, saved in backups, logs that have been archived, etc. You can’t easily update all these places.