seq scan woes

A production system has had a query recently degrade in performance.
What once took < 1s now takes over 1s.  I have tracked down the
problem to a working example.

Compare  http://rafb.net/paste/results/itZIx891.html
with   http://rafb.net/paste/results/fbUTNF95.html

The first shows the query as is, without much change (actually, this
query is nested within a larger query, but it demonstrates the
problem).  The query time is about 1 second.

In the second URL, a "SET ENABLE_SEQSCAN TO OFF;" is done, and the
time drops to 151ms, which is acceptable.

What I don't understand is why the ports table is scanned in the
first place.  Clues please?
--
Dan Langille : http://www.langille.org/
BSDCan - http://www.bsdcan.org/