I've just started looking at this patch today, but I was wondering if
you might include a test case which minimally reproduces the original
problem you had.
The only plan diff I see is in updatable_views.sql, and it doesn't
illustrate the problem as well as a more straightforward SELECT query
with EXISTS sublink might.
After putting in some logging, I see that there are only a
few non-catalog queries which exercise this code path.
This query from groupingsets.sql is an example of one such query:
select ten, sum(distinct four) from onek a
group by grouping sets((ten,four),(ten))
having exists (select 1 from onek b where sum(distinct a.four) = b.four);
But, the chosen plan for this query stays the same.
It would be helpful to see a query where a different plan is chosen
because of this change that is not from updatable_views.sql.