> > > > SELECT controller_id FROM control
> > > > WHERE controller_id NOT IN
> > > > (SELECT DISTINCT controller_id FROM datapack);
> > > The DISTINCT is not necessary. I have heard with Oracle that DISTINCT
> > is a
> > > huge performance problem. Is that true on PostgreSQL also?
> >
> > From my experience, it does not preform as well as the standard group by
> > clause. I noticed a ~20%
> > increase in query run times.
>
>
>
> So in that case this would be better:
>
> SELECT controller_id FROM control
> WHERE controller_id NOT IN
> (SELECT controller_id FROM datapack);
>
> or
>
> SELECT controller_id FROM control
> WHERE controller_id NOT IN
> (SELECT controller_id FROM datapack GROUP BY controller_id);
Well in this case, the group by or distinct is simple not needed for the query to preform
correctly. The additional group by clause in the second query could cause it to preform
additional processing which "may" cause it to preform slower.
Regards,
Richard Broersma Jr.