Thanks Tom, I did solve the problem by adding the null constraint for now, it's a quick solution, and I look forward to the future where this case is handled appropriately.
I wrote:
> Try it like
> EXPLAIN SELECT COUNT(*) FROM "user_event" WHERE ((parameters ->>
> 'suggestion_id'::text) = '26') AND what = 'suggestion_notification';
> I don't think we assume that CoerceViaIO is strict, and without that
> the deduction that the value couldn't be null doesn't hold.
Hmm ... on closer inspection, we do assume that CoerceViaIO is strict,
but not everyplace knows that, so there's something that could be
improved here. In the meantime, try it as stated above.
regards, tom lane