You might consider whether you can write 'spa-000'::uid explicitly in your query; that results in immediate application of the domain coercion, so that the planner no longer sees that as a run-time operation it has to avoid.
I should have mentioned that I tried an explicit cast and found that 'spa-000' and 'spa-000'::uid produce identical results. As far as I can tell, there is *no* way to get the planner to constant-fold in this case without using prepared statements.
It's tempting to wonder whether we could somehow constant-fold CoerceToDomain, at least in the estimation case, but I'm afraid that would lead to domain constraint failures that would not necessarily occur at runtime. Or we could skip the domain check for estimation purposes, but then we're possibly feeding a value that fails the domain check to a function that might not work right under such conditions. So on the whole I'm afraid to monkey with that decision.
OK, I can see how that makes sense. But shouldn't an explicit cast still work?
