This index is only on a single column, `relative_position`. However,
when we query issues by anything other than ID, we pretty much always do
that with a filter on `project_id`. (The exception is an admin user
viewing a dashboard, which is a rare case, and there aren't
instance-level boards anyway.)

We already have an index that's perfect for filtering by `project_id`
and ordering by `relative_position`, because it's on these columns:

    (project_id, relative_position, state_id, id DESC)

But sometimes the planner won't use it because of this index, which can
lead to poor query performance.