I have heard of concurrency problems like that in MySQL before. Not so in Postgres.
Built-in row-level locks in the default
READ COMMITTED transaction isolation level are enough.
I suggest a single statement with a data-modifying CTE (something that MySQL also doesn't have) because it's convenient to pass values from one table to the other directly (if you should need that). If you don't need anything from the
coupon table you can use a transaction with separate
INSERT statements just as well.
WITH upd AS (
SET used = true
WHERE coupon_id = 123
AND NOT used
RETURNING coupon_id, other_column
INSERT INTO log (coupon_id, other_column)
SELECT coupon_id, other_column FROM upd;
It should be a rare thing that more than one transaction tries to redeem the same coupon. They have a unique number, don't they? More than one transaction trying at the same moment in time should be much rarer, yet. (Maybe an application bug or somebody trying to game the system?)
Be that as it may, the
UPDATE only succeeds for exactly one transaction, no matter what. An
UPDATE acquires a row level lock on each target row before updating. If a concurrent transaction tries to
UPDATE the same row, it will see the lock on the row and wait till the blocking transaction is finished (
COMMIT), then being the first in the lock queue:
If committed, recheck the condition. If it's still
NOT used, lock the row and proceed. Else the
UPDATE now finds no qualifying row and does nothing, returning no row, so the
INSERT also does nothing.
If rolled back, lock the row and proceed.
There is no potential for a race condition.
There is no potential for a deadlock unless you put more writes into the same transaction or otherwise lock more rows than just the one.
INSERT is care-free. If, by some mistake the
coupon_id already is in the
log table (and you have a UNIQUE or PK constraint on
log.coupon_id), the whole transaction will be rolled back after a unique violation. Would indicate an illegal state in your DB. If the above statement is the only way to write to the
log table, that should never occur.