Don't understand error?

Hi everyone,

I'm working with a large spatial dataset at the moment. About 109m
rows. What I do is join some other tables to it, do some maths and
spatial joins, and then put the results into a new table (which is
therefore 109m rows too).

When I run my SQL for 50 people it's fine. 1 person is fine. A
thousand people it's fine. But when I try and run it for the whole
dataset I'm given this error:

ERROR: value out of range: underflow
****************Error *******************
Error: Value out of range: underflow
SQL state: 22003

I've looked this up, but I don't really get what it is saying. I'd
appreciate some advice? I was going to post my actual SQL code but
it's about 150 lines long and I wasn't sure it would be useful.

Thanks

James