I wrote:
> ... What seems to be happening is that Informix is willing to
> flatten the sub-SELECT into an IN join even though the sub-SELECT is
> correlated to the outer query (that is, it contains outer references).
I did some googling this morning and found confirmation that recent
versions of Informix have pretty extensive support for optimizing
correlated subqueries:
http://www.iiug.org/waiug/archive/iugnew83/FeaturesIDS73.htm
This is something we've not really spent much time on for Postgres,
but it might be interesting to look at someday. Given that the problem
with your query was really a mistake anyway, I'm not sure that your
example is compelling evidence for making it a high priority.
regards, tom lane