NOT IN clause performing badly

I have a problem with NOT IN queries
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Has anyone experinced the same problem, or even found a solution to it ?

-- simple NOT IN clause does not perform
-- create a table with an integer primary key and any number and type  of attributes.
-- load approx. 10'000 records and vacuum;
-- then select a subset of say 8000 id's into a seperate table.
-- now try to find out the Id's of the remaining 2000 records

-- I would use a NOT IN clause
-- but this is what happens :

drop table tempx1;

select mytable.id      into table tempx1      from mytable, othertable      where mytable.id = othertable.id      and
othertable.someattribute= "Somevalue";
 

-- this will take only a few seconds
-- on platform : PG6.4 SusE 6.0 PII/300mhz
-- rdbms startup : su - postgres -c with parameters :
-- "/sbin/startproc -l $LOGFILE $H -B 256 -i -o -F -D$DATADIR"

-- select count(*) from tempx1 : now returns 8000 rows

select id from mytable      where id NOT IN (      select id from tempx1      );

-- this will take some 100 minutes at least !!!
-- but it does finish ! no crash or hangup.
-- NOTICE:  QUERY PLAN:
-- Seq Scan on mytable  (cost=958.89 size=10000 width=4)
--   SubPlan
--     ->  Seq Scan on tempx1  (cost=0.00 size=0 width=4)
--
-- an index on mytbale(id) exists.

Questions
---------
Is this a known problem ?
What is causing the problem ?
Is it a problem of RDBMS parameters ?
Is there an alternative to the NOT IN clause ?


I'd appreciate any help in this matter.
ThanX
Chris
-- 
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Christian Rudow                 E-Mail: Christian.Rudow@thinx.ch
ThinX networked business services    Stahlrain 10, CH-5200 Brugg
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