Which DB design is faster: a unique index and INSERT IGNORE, or using SELECT to find existing records?

Question

I have a table with just one column: userid.

When a user accesses a certain page, his userid is being inserted to the table. Userids are unique, so there shouldn\'t

Answer 1

what about doing update, e.g.

UPDATE xxx SET x=x+1 WHERE userid=y

and if that fails (e.g. no matched rows), then do an insert for a new user?