Re: Can we rely on the ordering of paths in pathlist?

I'm not sure about this conclusion.  Surely we can depend on that the
partial_pathlist is kept sorted by total_cost ASC.  This is emphasized
in the comment of add_partial_path, and also leveraged in practice, such
as in many places we just use linitial(rel->partial_pathlist) as the
cheapest partial path.

But get_cheapest_parallel_safe_total_inner works on pathlist not
partial_pathlist.  And for pathlist, I'm not sure if it's a good
practice to depend on its ordering.  Because

1) the comment of add_path only mentions that add_path_precheck relies
on this ordering, but it does not mention that other functions also do;

2) other than add_path_precheck, I haven't observed any other functions
that rely on this ordering.  The only exception as far as I notice is
get_cheapest_parallel_safe_total_inner.

On the other hand, if we declare that we can rely on the pathlist being
sorted in ascending order by total_cost, we should update the comment
for add_path to highlight this aspect.  We should also include a comment
for get_cheapest_parallel_safe_total_inner to clarify why an early exit
is possible, similar to what we do for add_path_precheck.  Additionally,
in several places, we can optimize our code by taking advantage of this
fact and terminate the iteration through the pathlist early when looking
for the cheapest path of a certain kind.

Thanks
Richard