Slow update SQL

I'm experiencing a very slow query. The table contains 611,564 rows of
data. I vaccumed the table:

VACUUM ANALYZE ncccr10;

SELECT count(*) FROM ncccr10;
count
--------
611564
(1 row)

When I try to analyze the query plan with:

EXPLAIN ANALYZE
UPDATE ncccr10
SET key = facilityno||'-'||
lastname||'-'||
sex||'-'||
ssno||'-'||
birthdate||'-'||
primarysit||'-'||
dxdate||'-'||
morphology3
WHERE date_part('year',dxdate) > '2000';

The query just never finishes (even 1 hour later). The colum key100 is
indexed, and I'm setting the value of this
column from other columns. Why is this so slow?