From https://phabricator.wikimedia.org/T148988#2782161 :
we don't need the key (i.e. the normalized title) as a string at all. We just need a hash of the normalized title. A 64 bit hash can be represented as an integer. You may still want to have a table for titles, so you don't have to store the same title 20 times if the title exists on 20 wikis. If you do this, I suggest to again use the (unnormalized) title's hash as the numeric representation of the title. This ensures consistency between wikis, and reduces the need for lookups.