Big copy slowdown

[Brian Hurt <bhurt(at)janestcapital(dot)com>]

I'm not sure if this is the right forum for this question, so I'll start 
here and escalate as necessary.

One of the things I do a lot is copy large tables from one database to 
another.  I'd started life using pgdump, but this requires I save the 
whole table into an intermediate file (or play games with pipes).  
Rather than doing either, I instead wrote a quick script which creates a 
cursor at the source, and starts schlepping data from one to other.  For 
"small" tables (say, a million rows or less), this works just fine, but 
when I try to copy a bigger table (tens of millions of rows),  the 
performance continually drops off, with the copy becoming slower and slower.

On 8.3-beta1, I see the memory utilization skyrocketing, specifically 
the non-shared residential space of the postmaster daemon, driving the 
whole system deep into swap (sooner or later), and bringing things to a 
screeching halt.  But on 8.1.4, I don't see memory utilization problem- 
the memory utilization of a daemon pegs out at just a little larger than 
the shared buffers size (i.e. what I'd expect)- but I still see the 
slowdown.

It is definately the copy that is the problem- I've tried a variation of 
my script with the copy commented out (just throwing the data away) to 
test if it's the cursor- and I get 20K+ rows/second.from the cursor.  
I've also tried breaking the copy up- finishing the current copy command 
and starting a new one every million rows or so, and that doesn't help.  
I've also fiddled with transactions, and rate-limiting the copy, and 
neither of those help.

Is this a bug in postgres?  If not, as I'm assume, what should I be 
doing to make this work fast? 

Brian