Re: optimizing select ... not in (select ...)

Laurent Martelli <laurent(at)aopsys(dot)com> writes:

> I have this query :
>
> select distinct on (Pictures.PictureID) * from Pictures where Pictures.PictureID not in (select distinct PictureID from Keywords);
>
> and I find it a bit slow. Does anybody have suggestions to run this
> faster ? (I have indexes on PictureID on both Pictures and Keywords)

Try this instead:

select distinct on (Pictures.PictureID) *
from Pictures
where not exists (select distinct PictureID from Keywords where Pictures.PictureID=Keywords.PictureID);

I've found that it's usually faster, probably because it can use
indices better.

Tomas