I wrote:
> Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> writes:
>> Unique indexes can sort the tuples in corresponding tables
>> prefectly. So this query might can use index.
BTW, how much of any of this is correct if the "unique" index contains
multiple NULL values? We might need to restrict the optimization(s)
to cases where the unique-ified columns are all marked NOT NULL.
regards, tom lane