Constant LIVE_BYTES 

pub const LIVE_BYTES: &str = "\
SELECT coalesce(sum(size), 0) FROM objects WHERE hash IN \
(SELECT object_hash FROM items WHERE object_hash IS NOT NULL AND retained_at IS NULL)";

The bytes held by objects at least one live item binds. An object a live and a retained item share counts here, since purging the retained one frees nothing.