Re: Checking number of entries

At 11:25 30/09/00 -0700, Stephan Szabo wrote:
>
>On Sat, 30 Sep 2000, Philip Warner wrote:
>
>>
>> alter table zzz add constraint
>>      check( (select sum(zzzz.f1*zzz1.f1_1) from zzzz,zzz1
>>                 where zzzz.f2=zzz1.f1_1) + f1 > 0);
>>
>> Any updates to the referenced tables cause it to run a query that *seems*
>> to be:
>>
>>      select * from ZZZ where not ( <constraint-condition> )
>>
>
>Well, I'm not sure how easy/hard it is to get all referenced tables right
>now, since you'd have to decend into subqueries.   But, yeah, that seems
>like it would probably work since you probably couldn't currently put
>anything in the check constraint that would work there but not in a
>subquery context (although i could be wrong about that).

Yes; we'd need to generate a plan for the constraint, and find all the
tables it references. Is that a hard thing to do?


>Can you do arbitrary user functions in Dec RDB that access tables and put
>those in constraints? I'm not sure what to do about the fact that we can't
>actually get the referenced tables inside functions for some cases.

RDB has two kinds of functions: external & SQL. External functions can't
make data changes, or  even easily read the database, and SQL functions are
just pieces of (complex multi-line) SQL, that can be parsed like anything
else. As a result, when you call a function in a constraint, it plans the
function, and gets the list of tables.

Also, RDB does allow non-deterministic constraints, but if someone is silly
enough to use one, then so be it...


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