SELECT is immediate but the UPDATE takes forever

Hi,

I want to understand why one of my postgresql functions takes an eternity to finish.

Here's an example:

UPDATE comptes SET belongs_to_compte_id=42009 WHERE (codi_compte LIKE '10000%' AND empresa_id=2 AND nivell=11); // takes forever to finish

QUERY PLAN
--------------------------------------------------------------------------------------------
Seq Scan on comptes (cost=0.00..6559.28 rows=18 width=81)
Filter: (((codi_compte)::text ~~ '10000%'::text) AND (empresa_id = 2) AND (nivell = 11))
(2 rows)

but the same SELECT count, it's immediate:

SELECT count(id) FROM comptes WHERE codi_compte LIKE '10000%' AND empresa_id=2 AND nivell=11;

what I'm doing wrong ?

thanks,

regards,

r.