Constant LINK_FOR_HANDLE 

pub const LINK_FOR_HANDLE: &str = "\
SELECT link_id FROM bindings \
WHERE collection = :collection AND source = :source AND handle = :handle";

The link id one source’s handle is bound to, for a batch that drops a placement: a drop names a handle, and the hub is keyed by link id.

Served by the bindings_by_handle index, so resolving it is a seek rather than a scan over every item.