Re: set difference

Tom Lane wrote:
> > I would like to make symmetrical(set) difference in a query.
> > But the simpliest way I could find is
>
> > select id from a
> > except
> > select id from b
> > union
> > select id from b
> > except
> > select id from a
>
> > Is there any better solution for this problem?
>
> One thing you should definitely do is change "union" to "union all".
> "union" implies a pass of duplicate removal, which shouldn't be
> necessary here (unless a or b individually contain duplicates and
> you want to get rid of those too).
>
> Another thing to try is
> (a union b) except (a intersect b)
> (Again, you might be able to say union all instead of union.)
> Not sure which will be faster.

It's likely faster but my problem is I have to run queries twice. I've looked at
setOp executor and it seems symmetrical differencial can run as time as an EXCEPT.
I would write it, because it speeds up my query very much, but I don't know if it is worth. So, do you think a DIFFERENCE or EXCEPT SYMMETRIC clause will be reasonable
feauture for postgresql?

Laca