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