The CHECK constraints are load-bearing, not decoration: every compare-and-set
below is stated as WHERE ... AND state = '<literal>', which is only a TOTAL
decision while the column cannot hold a fourth value. A row hand-edited into
an unknown state would otherwise be a reservation no CAS can advance and no
purge can reap — a permanently wedged key.
The reservation schema.
The CHECK constraints are load-bearing, not decoration: every compare-and-set below is stated as
WHERE ... AND state = '<literal>', which is only a TOTAL decision while the column cannot hold a fourth value. A row hand-edited into an unknown state would otherwise be a reservation no CAS can advance and no purge can reap — a permanently wedged key.