Re: Proposed feature: Selective Foreign Keys

Hi Laurenz!

On 2 Dec 2013, at 19:27, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:
> What strikes me is that since foreign key constraints are implemented
> as triggers in PostgreSQL, this solution would probably not have many
> performance benefits over a self-written trigger that implements the
> same functionality.  Since you need two triggers for your example,
> the performance might even be worse than a single self-written trigger.

Well, the main cost on insert in the FK table should be looking for matching rows in the referenced tables, which the
patchavoids for non-matching rows. So while you’ll get the overhead of N triggers firing, you only pay the expected
querycost (which will even use a partial index if you’ve got one set up). Each of the referenced tables is only
involvedin one FK, so there’s no difference in cost there. 

> Now performance isn't everything, but that would mean that the benefit
> of your proposal is entirely on the usability side.

Well, I don’t think there’s much of a performance hit, and I don’t think any of the alternatives would perform much
betterin practice, but certainly performance wasn’t  a motivating factor for this feature, it was a) correctness and b)
avoidingthe ugliness of the existing solutions.  

> I personally don't think that it is so difficult to write a trigger
> for that functionality yourself, but I guess that the argument for
> this feature rests on how coveted such a functionality would be
> (to justify the trade-off in code complexity).

The patch is pretty small so far - and more than half of it is regression tests. So there’s not much extra code
complexityIMO. I wouldn’t want to touch the FK system with anything but the lightest touch. 

Cheers

Tom