Re: why the big difference on this explain analyze?

"Dan Langille" <dan@langille.org> writes:
> The difference between the two SQL statements is the inclusion of 
> Ports_Check in the WHERE EXISTS select clause.

Which caused it to do the wrong thing.  I think you'll find this
command set found_in_ports = TRUE in *every* row of Ports_Check.

Reason: the EXISTS clause no longer depends on the outer update,
but evaluates to a constant TRUE.  (Or possibly a constant FALSE,
but that seems unlikely.)
        regards, tom lane