update slows down in pl/pgsql function

I have stored procedure written in pl/pgsql which takes about 13 seconds
to finish. I was able to identify that the slowness is caused by one
update SQL:

UPDATE shopping_cart SET sc_sub_total=sc_subtotal, sc_date=now()
WHERE sc_id=sc_id;

If I comment this sql out, the stored procedure returns within 1 second.

What puzzles me is that if I execute the same update SQL in psql
interface, it returns very fast. The following is the explain analyze
output for that SQL.

#>explain analyze UPDATE shopping_cart SET sc_sub_total=1, sc_date=now()
where sc_id=260706;
QUERY
PLAN
----------------------------------------------------------------------------------------------------------------------------------
Index Scan using shopping_cart_pkey on shopping_cart (cost=0.00..5.01
rows=1 width=144) (actual time=0.22..0.37 rows=1 loops=1)
Index Cond: (sc_id = 260706::numeric)
Total runtime: 1.87 msec
(3 rows)

Is it true that using pl/pgsql increases the overhead that much?

TIA,
Jenny
--
Jenny Zhang
Open Source Development Lab
12725 SW Millikan Way, Suite 400
Beaverton, OR 97005
(503)626-2455 ext 31