Re: queries with lots of UNIONed relations

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Jan 13, 2011 at 3:12 PM, Jon Nelson <jnelson+pgsql@jamponi.net> wrote:
>> I still think that having UNION do de-duplication of each contributory
>> relation is a beneficial thing to consider -- especially if postgresql
>> thinks the uniqueness is not very high.

> This might be worth a TODO.

I don't believe there is any case where hashing each individual relation
is a win compared to hashing them all together.  If the optimizer were
smart enough to be considering the situation as a whole, it would always
do the latter.

            regards, tom lane