Is IDLE session really idle?

Hello everybody!

I have many app servers using connection pooling. At any time, there are
about 1000 total connection to the database from all the app servers;
however, only few random connections are active.
The application often executes relatively big sorts. The work_mem size is
32MB, and eventually many sessions have a chance to run a sort and allocate
a big sort area. I see hundreds of postgres processes with DATA segment >
15MB.
Eventually, it consumes all the available memory. Most of this memory is
allocated to the sessions that are idle. I cannot change the connection
pooling on the application side, and the big sorts cannot be eliminated. I
need a solution on the DB side.

I see two ways to solve the problem.:
1. Reducing the work_mem to something like 10MB will cap the total memory
consumption. However, the memory will still be allocated to the mostly idle
sessions. Also, reduced work_mem will cause increased disk I/O, which is
already high.
2. We can kill the idle sessions periodically. This will free up a big chunk
of memory already allocated to the sessions. The application will gradually
reestablish the connections, and the new sessions will start with small
memory foot-print.

The question is, how safe it is to kill an idle session? If a session just
became idle in pg_stats_activity, is it possible that it is still returning
data to the client, or doing some other useful work?

In case it makes a difference: The Postgres version is 8.2.4 on Linux Suse
10.

Thank you.