Re: NOT ENFORCED constraint feature

On 03.02.25 13:19, Alvaro Herrera wrote:
> On 2025-Feb-03, Ashutosh Bapat wrote:
> 
>> ```
>>        If the
>>        constraint is <literal>NOT ENFORCED</literal>, the database system will
>>        not check the constraint.  It is then up to the application code to
>>        ensure that the constraints are satisfied.  The database system might
>>        still assume that the data actually satisfies the constraint for
>>        optimization decisions where this does not affect the correctness of the
>>        result.
>> ```
> 
> IMO the third sentence should be removed because it is bogus.  There's
> no situation in which a not-enforced constraint can be used for any
> query optimizations -- you cannot know if a constraint remains valid
> after it's been turned NOT ENFORCED, because anyone could insert data
> that violates it milliseconds after it stops being enforced.  I think
> the expectation that the application is going to correctly enforce the
> constraint after it's told the database server not to enforce it, is
> going to be problematic.  As I recall, we already do this in FDWs for
> instance and it's already a problem.

The database system could use the presence of a not enforced constraint 
for selectivity estimation, for example.