jlrobins@[EMAIL PROTECTED]
(James Robinson) writes:
> Academic question here:
>
> Given a table with a pair of any sort of line-segment-esqe
> range delimiter columns, is it possible to build a unique index to
> enforce non-overlapping ranges? Such as:
>
> create table test
> (
> id int not null primary key,
> low_value int not null,
> high_value int not null
> );
>
> Can one build an index to enforce a rule such that no
> (low_value, high_value) range is identical or overlaps with another
> (low_value, high_value) range described by the table? And, more
> interestingly, what about for ranges of dates / timestamps as opposed
> to simple integers?
>
> I can see how a trigger on insert or update could enforce such
> a constraint [ probe the table for an existing overlapping row, and
> raise exception one exists ], but can such an activity be performed
> with fewer lines using some sort of r-tree index?
Aside: A constraint won't do what you want, because a CHECK constraint
can only reference the columns of the current row.
This is a useful sort of thing to have in a tem****al database, where
you might want to add tem****al constraints to what was originally a
stateful table which had an ordinary primary key.
Thus, we might start with table:
create table state_of_something (
id serial primary key,
att1 text,
att2 text,
att3 text
);
and want to make it tem****al. The tem****al equivalent (well, *one*
tem****al equivalent; there's several ways to treat this) would be:
create table tem****al_state_of_something (
id serial,
att1 text,
att2 text,
att3 text,
from_date timestamptz,
to_date timestamptz
);
where the requirement, for any given id value, is for there to be a
series of non-overlapping (from_date,to_date) intervals.
A *vague* approximation that is easy to do is to have a new primary
key:
create table tem****al_state_of_something (
id serial,
att1 text,
att2 text,
att3 text,
from_date timestamptz,
to_date timestamptz,
primary key (id, from_date),
constraint from_to check (to_date >= from_date)
);
That doesn't prevent there from being overlapping ranges, such as
(id,from_date_to_date) tuples like:
(151,'2007-01-01','2009-01-01') and (151 '2008-01-01','2008-02-02')
I'm not sure that this is *so* different from a regular foreign key
constraint that it wouldn't be reasonable to handle it via a trigger.
It's certainly a more complex trigger function, but the pattern will
be pretty common.
--
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" "linuxfinances.info")
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