Ch8 Sol2a: booking races, then the room-row lock Diagram

Check-then-insert doubles

Both checks run against an empty table, both INSERTs land
[9,11) and [10,12) overlap: one room sold twice
Wrapping the same pair in READ COMMITTED changes nothing

Room-row lock serializes

Lock the STABLE room row in rooms, not the reservation rows
The loser blocks, then re-checks against the winner's row
One winner, zero overlaps, refusal is explicit

Empty locks lock nothing

SELECT FOR UPDATE on an EMPTY reservation result locks nothing
Future overlapping ranges are phantoms to a row lock
Lock something that exists, or constrain the write itself