Re: pre-commit triggers

On 11/19/2013 03:54 PM, Andres Freund wrote:
> On 2013-11-19 12:45:27 -0800, Josh Berkus wrote:
>> On 11/19/2013 08:42 AM, Andres Freund wrote:
>>> Could you explain a bit what the use case of this is and why it's not
>>> sufficient to allow constraint triggers to work on a statement level?
>>> "Just" that there would be multiple ones fired?
>> The main reason is to enforce arbitrary assertions which need
>> enforcement at the end of a transaction and not before.  For example:
>> [...]
>> You can't enforce this at the statement level because the
>> update/insert/deletes can happen in any order on the various tables.
> That's why I suggested adding statement level constraint triggers
> (should be a farily small patch), which can be deferred till commit. The
> problem there is that they can be triggered several times, but that can
> relatively easily accounted for in user code.
>
> I can't really say why, but commit time even triggers make me nervous...


This feature is really extremely close to being a deferred constraint 
trigger that is called once. The code that calls these event triggers 
runs right before the code that runs the deferred triggers. That spot in 
the code was chosen with some care, to try to reduce any risk from the 
feature.

Putting the onus on the user to detect multiple invocations of the 
trigger would make for MORE fragility, not less.


cheers

andrew