Re: target list evaluation wrt sequences

On Fri, Apr 30, 2004 at 01:01:25 -0400,
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Bruno Wolff III <bruno(at)wolff(dot)to> writes:
> > No. You can do effectively this by joining a select nextval to whatever
> > you main select is. Something like:
> > insert into ...
> > select a.n as a, a.n as b, ....
> > from (select nextval('n') as n) as a, lalala
>
> Urgh ... I'd not want to promise that nextval() will always be evaluated
> just once in the above example ... this really seems *much* more fragile
> than assuming left-to-right targetlist evaluation :-(
>

Thanks for the heads up. I have so far only used that technique to speed up
some queries with respect to using subselects, where the subquery would
always evaluate to the same value anyway.
If I need a single value from a volatile calculation to be used more than
once, I will remember to use a separate query to save the value in a table and
then refer to that value later.