Re: Inheritance

Jan Wieck wrote:
>Oliver Elphick wrote:
>> Jan Wieck wrote:
>> >> Yes, but what about referential integrity? Can I have a table column
>> >> reference a column from Entities*? In my experimentation, this is not
> the
>> >> case.
>> >
>> > Referential integrity doesn't work with inheritance, and will
>> > not in 7.1.
>> >
>> > It isn't possible to put a unique constraint on a column of
>> > Entities* (AFAIK). So that a SELECT pkey FROM Entities will
>> > never return any duplicates? The RI implementation of
>> > PostgreSQL doesn't insist on such a unique constraint to
>> > exist up to now, but it is required by the SQL specs and thus
>> > we'll do so someday.
>>
>> The corollary of using RI on an inheritance tree is that there should be a
>> unique index on the primary keys of the whole tree. If constraints could
>be
>> inherited, this would become available.
>
> The UNIQUE constraint is implemented as a UNIQUE INDEX in
> PostgreSQL. And exactly what you say: "ONE" index on the
> whole tree would be required to do it.
>
> What you're asking for is index inheritance instead of RI
> trigger inheritance first - right? So that an index will
> contain tuples for all the keys present in it's tables
> subclasses, for beeing able to detect a dupkey.

Yes, at least as an option. In many cases of inheritance the id should be
unique in the whole tree, and so a unique index on the whole tree is
needed. If this index is present, the tree can be available for RI use.
There may be cases where a unique index is not wanted, but in that case the
cost is that RI becomes unavailable except on individual tables.

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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