Re: SQL:2011 application time - Mailing list pgsql-hackers

On 17.02.25 07:42, Paul Jungwirth wrote:
>> After staring at this a bit more, I think my interpretation above was 
>> not correct.  This seems better:
>>
>> The clause "Execution of referential actions" in the SQL standard only
>> talks about referenced and referencing columns, not periods.  The 
>> RESTRICT error is raised when a "matching row" exists in the 
>> referencing table.  The "matching row" is determined purely by looking 
>> at the "normal" columns of the key, not the period columns.
>>
>> So in our implementation in ri_restrict(), ISTM, we just need to 
>> ignore the period/range columns when doing the RESTRICT check.
>>
>> Attached is a quick patch that demonstrates how this could work.  I 
>> think the semantics of this are right and make sense.
> 
> I can see how this is plausible given a very strict reading of the 
> standard, but I don't think it makes sense practically. And perhaps an 
> ever stricter reading will take us back to a more practical understanding.
> 
> Starting with the practical argument: let's say the referenced table has 
> two rows, with (id, valid_at) of (1, '[2000-01-01,2001-01-01)') and (1, 
> '[2010-01-01,2011-01-01)'), and the referencing table has a row with 
> (id, valid_at) of (1, '[2010-03-01,2010-04-01)'), and we have 
> `referencing (id, PERIOD valid_at) REFERENCES referenced (id, PERIOD 
> valid_at)`. then deleting *either* referenced row would cause a RESTRICT 
> key to fail? If that is what the user wants, why not just make a non- 
> temporal foreign key? If I create a temporal foreign key, it would be 
> very surprising for it simply to ignore its temporal parts.

I think maybe we have a different idea of what RESTRICT should do in the 
first place.  Because all the different behavior options come from the 
same underlying difference.

Consider a related example.  What if you have in the referenced table 
just one row:

(1, '[2000-01-01,2015-01-01)')

and in the referencing row as above

(1, '[2010-03-01,2010-04-01)')

with ON UPDATE RESTRICT and ON DELETE RESTRICT.  And then you run

UPDATE pk SET valid_at = '[2000-01-01,2021-01-01)' WHERE id = 1;

So this extends the valid_at of the primary key row, which is completely 
harmless for the referential integrity.  But I argue that this is an 
error under ON UPDATE RESTRICT.  Because that's the whole point of 
RESTRICT over NO ACTION: Even harmless changes to the primary key row 
are disallowed if the row is referenced.

If we accept that this is an error, then the rest follows.  If the 
primary row is split into two:

(1, '[2000-01-01,2011-01-01)')
(1, '[2011-01-01,2015-01-01)')

then the command that extends the validity

UPDATE pk SET valid_at = '[2011-01-01,2021-01-01)'
   WHERE id = 1 AND valid_at = '[2011-01-01,2015-01-01)';

must also be an error, even though the row it is updating is not 
actually the one that is referenced.  If this were allowed, then the 
behavior would be different depending on in which way the primary key 
ranges are split up, which is not what we want.

And then, if that UPDATE is disallowed, then the analogous DELETE

DELETE FROM pk
   WHERE id = 1 AND valid_at = '[2011-01-01,2015-01-01)';

must also be disallowed.  Which would be my answer to your above question.

I'm not sure what other behavior of RESTRICT there might be that is 
internally consistent and is meaningfully different from NO ACTION.