Re: Multiple Uniques

Markus Schaber <schabios@logi-track.com> writes:
> Today, we stumbled about the following query plan on PostGreSQL 7.4.1:

> logigis=# explain select count(id) from (select distinct id from (select distinct ref_in_id as id from streets union
selectdistinct nref_in_id as id from streets) as blubb) as blabb;  

> I was somehow irritated by the fact that the Query Plan contains 4 Uniques.

Well, if you write a silly query, you can get a silly plan ...

As you appear to have realized later, given the definition of UNION,
all three of the explicit DISTINCTs are redundant.

> So, now my question is, why does the query optimizer not recognize that
> it can throw away those "non-unique" Sort/Unique passes?

Because the issue doesn't come up often enough to justify expending
cycles to check for it.

            regards, tom lane