Sequential scan evaluating function for each row, seemingly needlessly

On psql 8.3.9, I ran a limited query limited to 5 results. There was a
moderately expensive function call
which I expected to be called 5 times, but was apparently called for
each row of the sequential scan. Why?

preproduction=> explain analyze select url(context_key) from extractq
order by add_date desc limit 5;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Limit (cost=19654.53..19654.54 rows=5 width=12) (actual
time=10001.976..10001.990 rows=5 loops=1)
-> Sort (cost=19654.53..19826.16 rows=68651 width=12) (actual
time=10001.972..10001.976 rows=5 loops=1)
Sort Key: add_date
Sort Method: top-N heapsort Memory: 25kB
-> Seq Scan on extractq (cost=0.00..18514.26 rows=68651
width=12) (actual time=19.145..9770.689 rows=73550 loops=1)
Total runtime: 10002.150 ms
(6 rows)

preproduction=> explain analyze select context_key from extractq order
by add_date desc limit 5;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
Limit (cost=2491.78..2491.79 rows=5 width=12) (actual
time=250.188..250.203 rows=5 loops=1)
-> Sort (cost=2491.78..2663.41 rows=68651 width=12) (actual
time=250.184..250.188 rows=5 loops=1)
Sort Key: add_date
Sort Method: top-N heapsort Memory: 25kB
-> Seq Scan on extractq (cost=0.00..1351.51 rows=68651
width=12) (actual time=0.015..145.432 rows=73557 loops=1)
Total runtime: 250.450 ms
(6 rows)

preproduction=> select version();
version
------------------------------------------------------------------------------------------------
PostgreSQL 8.3.9 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.3.real
(Debian 4.3.2-1.1) 4.3.2