'order by' and 'desc' not working in subquery using 'not in'

Rahul Gade (Rahul_g(at)ip(dot)eth(dot)net) reports a bug with a severity of 1
The lower the number the more severe it is.

Short Description
'order by' and 'desc' not working in subquery using 'not in'

Long Description
Hello,
I am facing two big problems,they may not be really big but in my case these are big :
1) temporary tables can not be created inside function using PL/pgsql
2) order by and desc keywords are not functioning inside subquery

in the following query i am trying to delete all the rows except the top 20 rows from reporttable.
How i can do this.
i have done this in MSSQL using same syntax

Out of these two 2nd problem is important for me,
plase tell me what to do,
--- Thanks for response ----

Sample Code
delete from reporttable where (srvServerid=serverid) and
(rptreportid not in(select rptreportid from reporttable
where (srvserverid=serverid order by rpttimestamp desc)));

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