Re: subquery/alias question

On Sep 25, 2007, at 17:30 , Alvaro Herrera wrote:

> Michael Glaesemann wrote:
>>
>> select dom_id,
>>        dom_name,
>>        usr_count
>>   from domains
>>   natural join (select usr_dom_id as dom_id,
>>                        count(usr_dom_id) as usr_count
>>                   from users) u
>>   where usr_count > 0
>>   order by dom_name;
>
> Maybe the usr_count should be tested in a HAVING clause instead of
> WHERE?  And put the count(*) in the result list instead of a
> subselect.
> That feels more natural to me anyway.

I believe you'd have to write it like

select dom_id, dom_name, count(usr_dom_id) as usr_count
   from domains
   join users on (usr_dom_id = dom_id)
   having count(usr_dom_id) > 0
   order by dom_name;

I don't know how the performance would compare. I think the backend
is smart enough to know it doesn't need to perform two seq scans to
calculate count(usr_dom_id), but I wasn't sure.

Madison, how do the two queries compare with explain analyze?

Michael Glaesemann
grzm seespotcode net