So looking into this with a postgresql databse to work with: the above query does not work. You have to specify either the column name or the constraint name that you expect to be violated in order for the update to take place.

With a map like this one you're using

map {
    pattern = shared/last-login/$user/$domain
    table = last_login
    value_field = last_login
    value_type = uint

    fields {
        username = $user
        domain = $domain
    }
}

there's no field name that is obviously the primary key. I've reworked the patch to use the postgres default primary key constraint name (tablename_pkey).

The attached fix should work in that case, although I feel it's not general enough.