NOT IN query issues

I'm trying to run a DELETE query using NOT IN and it's causing me some
problems.

Here is the query that I want to run:

delete from sharinfo where file__no not in (select file__no from
allforms);

When I explain this I get:

Seq Scan on sharinfo (cost=0.00..2225204.88 rows=5239 width=6)
SubPlan
-> Seq Scan on allforms (cost=0.00..424.69 rows=8669 width=12)

Which while a very large number does seem to work. But when I run the query
nothing appears to happen, I've left this query running for hours with no
response back. Is there a better way to run a query like this (deleting all
rows that don't appear in a second table)?

Thanks,
Rob