Vik Reykja <> wrote:
> Kevin Grittner <>wrote:
>> One of the problems that Florian was trying to address is that
>> people often have a need to enforce something with a lot of
>> similarity to a foreign key, but with more subtle logic than
>> declarative foreign keys support.  One example would be the case
>> Robert has used in some presentations, where the manager column
>> in each row in a project table must contain the id of a row in a
>> person table *which has the project_manager boolean column set to
>> TRUE*.  Short of using the new serializable transaction isolation
>> level in all related transactions, hand-coding enforcement of
>> this useful invariant through trigger code (or application code
>> enforced through some framework) is very tricky.  The change to
>> SELECT FOR UPDATE that Florian was working on would make it
>> pretty straightforward.
> I'm not sure what Florian's patch does, but I've been trying to
> advocate syntax like the following for this exact scenario:
> foreign key (manager_id, true) references person (id, is_manager)
> Basically, allow us to use constants instead of field names as
> part of foreign keys.
Interesting.  IMV, a declarative approach like that is almost always
better than the alternatives, so something like this (possibly with
different syntax) would be another step in the right direction.  I
suspect that there will always be a few corner cases where the
business logic required is too esoteric to be handled by a
generalized declarative construct, so I think Florian's idea still
has merit -- especially if we want to ease the transition to
PostgreSQL for large shops using other products.
> I have no idea what the implementation aspect of this is,
> but I need the user aspect of it and don't know the best way to
> get it.
There are those in the community who make their livings by helping
people get the features they want.  If you have some money to fund
development, I would bet you could get this addressed -- it sure
sounds reasonable to me.

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