On 2018-02-09 09:42:41 -0500, Tom Lane wrote:
> It doesn't look significantly different to me than the restriction
> that you can't have sub-selects in CHECK expressions, index
> expressions, etc. Obviously we need a clean failure like you get for
> those cases. But otherwise it's an OK restriction that stems from
> exactly the same cause: we do not want to invoke the full planner in
> this context (and even if we did, we don't want to use the full
> executor to execute the result).