Will a constraint be applied between two statements during a transaction? - sql

Question

I have a configuration that a thing can have multiple properties and a property can belong to multiple things:
CREATE TABLE thing (
id integer NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE property (
id integer NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE thingproperty (
thing integer NOT NULL,
property integer NOT NULL
);
ALTER TABLE thingproperty
ADD CONSTRAINT tp_thing
FOREIGN KEY (thing) REFERENCES thing(id)
ON UPDATE CASCADE ON DELETE CASCADE;
ALTER TABLE thingproperty
ADD CONSTRAINT tp_property
FOREIGN KEY (property) REFERENCES property(id)
ON UPDATE CASCADE ON DELETE CASCADE;
I'd like to ensure that a property can only exist if it belongs to at least one thing, and for this I wrote this transaction that removes things (and then also properties if necessary) but I don't know whether it's correct:
START TRANSACTION;
DELETE FROM thing ... ;
DELETE FROM property
WHERE id NOT IN (
SELECT property
FROM thingproperty
);
COMMIT TRANSACTION;
So I basically trusted the engine that after the DELETE FROM thing ... query had run, it immediately applies the tp_thing constraint and removes the record(s) of thingproperty belonging to the deleted thing(s) before DELETE FROM property ... is executed.
Is this a safe way to do it?

Answer 1

By default constraints are applied immediately after every command in a transaction. You can (in this case you do not want to) change this behaviour declaring a constraint as deferrable. Read more about deferrable constraints in the documentation.
DEFERRABLE
NOT DEFERRABLE
This controls whether the constraint can be deferred. A constraint that is not deferrable will be checked immediately after every command. (...)