Re: self join revisited

Tom Lane wrote:
> Rikard Pavelic <rikard(dot)pavelic(at)zg(dot)htnet(dot)hr> writes:
>
>> It would be great if Postgres could rewrite this query
>>
>
> AFAICS those queries are not going to produce the same results,
> so Postgres would be 100% incorrect to rewrite it like that for you.
>
> (If they do produce the same results, it would depend on a bunch
> of assumptions you have not stated.)
>
> regards, tom lane
>

Can I try again? :)

How hard would it be to teach the planner about preserving uniqueness of
relations in subqueries?
And using that information to remove unnecessary self joins on unique sets?

I can try to rewrite some queries to test it on real data for how much
gain it would provide.

Regards,
Rikard