I sort of think that this kind of stuff belongs in a separate table
somehow. For example in this case I would want to:
CREATE TABLE attack_probability (
attacker_type_id integer not null references animal_type (id),
victim_type_id integer not null references animal_type (id),
percent_chance integer not null,
primary key (attacker_type_id, victim_type_id)
...and then declare that the CONSTRAINT on the "maulings" table is that
if I look up the types of the attacker and victim, that pair of types
must be present in the attack_probability table with percent_chance > 0.
I guess my point here is that I think in your proposal you are letting
domain-specific data creep into the schema. It's the job of the schema
to enforce integrity constraints, but not to know specifically how
things work. The fact (if it is a fact) that the chance of one type of
animal attacking another can be captured as a probability (rather than,
say, one probability for the day time and another probability for the
night time, or one probability for each specific animal rather than each
animal type, or I don't know what's going on and want to infer the
probabilities from the data after I've gathered it) is domain-specific.
I don't really want the information about attack probabilities (or
whatever) to be something that's hardcoded in my schema; I want it to be
part of the data in the schema, with the schema enforcing such
constraints on that data as I may see fit to define. I don't want to
have to select things out of system tables to find out attack
probabilities. Also, as a practical matter, I suspect that such a setup
would result in an absurdly complex constraint language.
From: Alban Hertroys [mailto:alban(at)magproductions(dot)nl]
Sent: Monday, February 26, 2007 4:15 AM
To: Robert Haas
Cc: David Fetter; pgsql-general(at)postgresql(dot)org
Subject: Re: [GENERAL] complex referential integrity constraints
Robert Haas wrote:
> I don't understand what a weighted constraint would mean. Either the
> attacker_id can be a wolf, or it can't. Knowing that it is only 1%
> likely over the long haul is insufficient to disallow any particular
Basically I suggested to combine the constraint with a probability. If
the probability of one animal attacking another is 0% it can't attack
the other animal - that's a strict constraint. The other way around it
can, and you'll also immediately know how likely that is to happen.
An added bonus is that you can generalize certain constraints. If
certain animals are less than - say 25% - likely to attack other certain
other animals you could determine that the attacked animal is not in
fact prey. An example would probably be wolves attacking other wolves
(or predators in general). For relations that depend on an animal being
prey, a constraint would be that this number be <25%.
In this discussion it is also not entirely defined what attacking means.
Is a ram defending his horde from wolves attacking (wolves)?
I realise this all started from an analogy to a real problem, so most of
this is probably not very relevant to your actual problem. No less
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