Strange Execution-Plan for NOT EXISTS

Hello all,

I tried (just for academical fun) to resolve ID´s which are not used
anymore, e.g.:

ID NAME
-- + ------------------
1 | NOBODY
2 | ANYBODY
4 | EVERYBODY

Now I want to get "3" as the next usable ID. I really know that this is
kind of bad style but a friend of mine asked me if I had a solution for
this problem.

My solution would be the following statement:

--------------------------------
select (min(id) + 1)
from t_dummy d1
where not exists (
select id
from t_dummy d2
where d2.id = (d1.id + 1)
);
--------------------------------

The explain plan for this statement, tested against a real table with
about 8,000 records in a freshly vacuumed database, looks like the
following:

Aggregate (cost=2924207.88..2924207.88 rows=1 width=12)
-> Seq Scan on t_dummy d1 (cost=0.00..2924207.88 rows=1 width=12)
SubPlan
-> Seq Scan on t_dummy d2 (cost=0.00..331.36 rows=1 width=12
)

Why that? Wouldn´t it be possible to simple use the primary key index in
the sub-query and exit from the outer query at the first occurence of an
ID which has no following entry? Any ideas from anyone? Am I wrong?

Best regards, Jens

=============================================
Jens Hartwig
---------------------------------------------
debis Systemhaus GEI mbH
10875 Berlin
Tel. : +49 (0)30 2554-3282
Fax : +49 (0)30 2554-3187
Mobil : +49 (0)170 167-2648
E-Mail : jhartwig(at)debis(dot)com
=============================================