Right now it does nothing, but I guess we would have to code a cost which is higher than an index but lower than a normal column, although to be fair I've never ventured in these specifics of sqlite. Well what we use right now seems correct; I'm not saying that there aren't bad usages (of index, additional and requred), but I suspect that the most common situation is that there's no constraint optimization implemented in the table logic. Or when it's implemented, that column is actually an index or changes all the results coming from the table, so it's additional.