On 24 June 2014 23:52, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> On 24 June 2014 23:44, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Simon Riggs <simon@2ndQuadrant.com> writes:
>>>> Having said that, any join plan that relies upon a constraint will
>>>> still be valid even if we drop a constraint while the plan executes
>>>> because any new writes will not be visible to the executing join plan.
>
>>> mumble ... EvalPlanQual ?
>
>> As long as we are relaxing a constraint, we are OK if an earlier
>> snapshot thinks its dealing with a tighter constraint whereas the new
>> reality is a relaxed constraint.
>
> I guess I should have been more explicit: EvalPlanQual processing could
> see newer versions of tuples that might not satisfy the constraints the
> plan was designed against. Now, this is true only for the tuple that's
> the target of the UPDATE/DELETE, so it's possible you could prove that
> there's no problem --- but it would take careful analysis of the specific
> semantics of the constraints in question. I don't believe the argument
> you've made here holds up.
OK, thanks for raising that. You're better at seeing these things than I.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
