Query optimisation

Hi!
I first apologize for my poor english.

We are working with linux-apache-postgresql-php3.
One among our queries is structured as follow :

$result = pg_exec ($conn, "SELECT produits.id_produit, produits.id_proprio,
articles.categ, groupes.nom, produits.pvttc_prod, articles.titre,
articles.type_1, articles.obs_art
FROM produits, articles, groupes
WHERE $conditions $relations;");

$conditions may be

1/ ...AND groupes.nom = '$something' AND....
or
2/ ...AND groupes.nom ~* '$something' AND....

In the first case, the query is reasonnably fast (0.30 s for 4 items on
15000)
In the second case, the query becomes very slow (more than 31 s for the same
result)

to give example, in the first case $something = "Beatles"
in the second case $something = "beatl"

How to optimise speed ? I tried to type EXPLAIN but I do not understand the
result :

For 1/
Nested loop (cost=0.00 size=1 width=86)
-> Merge Join (cost=0.00 size=1 width=72)
-> Seq Scan (cost=0.00 size=0 width=0)
-> Sort (cost=0.00 size=0 width=0)
-> Seq Scan on produits (cost=0.00 size=0 width=16)
-> Seq Scan (cost=0.00 size=0 width=0)
-> Sort (cost=0.00 size=0 width=0)
-> Index Scan using type_1.idx on articles (cost=0.00 size=0
width=56)
-> Index Scan using nom_groupe.idx on groupes (cost=0.00 size=0 width=16)

For 2/
Nested loop (cost=0.00 size=1 width=86)
-> Merge Join (cost=0.00 size=1 width=72)
-> Seq Scan (cost=0.00 size=0 width=0)
-> Sort (cost=0.00 size=0 width=0)
-> Seq Scan on produits (cost=0.00 size=0 width=16)
-> Seq Scan (cost=0.00 size=0 width=0)
-> Sort (cost=0.00 size=0 width=0)
-> Index Scan using type_1.idx on articles (cost=0.00 size=0
width=56)
-> Seq Scan on groupes (cost=0.00 size=0 width=16)

The only difference seems to be the use (or not) of index in the last line.
Do you think it causes such a decrease of speed ? How to work around ?