On 26.02.25 06:15, Paul Jungwirth wrote:
I agree with that last principle: it shouldn't matter how the primary
keys are split up. But it seems to me that "matches" in the standard
should include the period. It does for NO ACTION, so why not RESTRICT?
That's why your example of expanding the referenced range succeeds. None
of the referenced moments were changed, so there are no referencing
moments to match.
> I'm not sure what other behavior of RESTRICT there might be that is
internally consistent and is
> meaningfully different from NO ACTION.
The difference between RESTRICT and NO ACTION for temporal foreign keys
is the same as the difference for ordinary foreign keys: we perform the
check prior to applying any "action" or allowing any other commands to
provide substitutes for the lost references. There are tests in sql/
without_overlaps.sql showing how their behavior differs.
Also you haven't yet explained why anyone would *want* to use RESTRICT
as you've described it, since the temporal part of the key is just
ignored, and they could just define a non-temporal foreign key instead.
Or to be precise, it fails *more* than a non-temporal foreign key,
because changing the period can violate the constraint, even though we
ignore the period when looking for matches.
This is not what I'm aiming for. (Maybe my patches were wrong about that.)
In the theory of the SQL standard, executing referential actions and
checking the foreign-key constraint are two separate steps. So it kind
of goes like this:
1. run command
2. run any referential actions
3. check that foreign key is still satisfied
This is why the default referential action is called "NO ACTION": It
just skips the step 2. But it still does step 3.
This means that under RESTRICT and with my interpretation, the check for
a RESTRICT violation in step 2 can "ignore" the period part, but the
step 3 still has to observe the period part.
In the implementation, these steps are mostly combined into one trigger
function, so it might be a bit tricky to untangle them.
But since we don't agree on the behavior, it seems best to me to wait to
implement RESTRICT. Not much is lost, since NO ACTION is so similar. We
can wait for the SQL committee to clarify things, or see what another
RDBMS vendor does.
I'm fine with that.
