Backends hanging around (was Re: Number of open files)

I wrote:
> "Mark Alliban" <MarkA@idnltd.com> writes:
>> Also backends seem to hang around for about a minute after I close
>> the ODBC connections. Is this normal?

> Seems odd to me too.

How reproducible is that behavior --- does it happen for all
connections, or only a few?  Is the time before disconnection
consistent?

I've just noticed and repaired a bug that might explain this
misbehavior, but only if the behavior is not as consistent as you imply.
The bug I found is that if a connection request is completed and a
backend is forked off while other connection request(s) are in progress,
the new child backend has an open file descriptor for the other client
connection(s) as well as its own.  It will never touch the other
clients' descriptors, but simply having them might affect the kernel's
behavior.  In particular, if another client performs its tasks and exits
without sending a disconnect message --- which ODBC doesn't send ---
then I think the backend spawned for the other client wouldn't be told
the connection is closed until the first one exits and its descriptor
for the connection is discarded.

You could check into this possibility with a tool like lsof: when you
see a backend hanging around with no client, look to see if any other
backends have open file descriptors for the same connection.

            regards, tom lane