Server backed sessions (where the browser is given a random cookie value which is then associated with a larger chunk of serialized data on the server) are a very poor fit for relational databases. They are often created for every visitor, even those who stumble in from Google and then leave, never to return again. They then hang around for weeks taking up valuable database space. They are never queried by anything other than their primary key.

Database sessions also force an additional SQL statement to be executed for every page view to read that user's session, even if only to update a "Logged in as X" bar at the top of the page.

A fast key-value store like Redis is a much better fit for session data. The per-page overhead is far, far smaller than a round-trip to a regular database.