> might it be possible that "id" is not type integer/int4, e.g. numeric or
int8?
>
> try this query:
> rs2.executeUpdate("UPDATE my_table SET my_date=now() WHERE id='"+i+"'");

Yes !
you've got it... Now, postgres uses "Index Scan".

"id" is an int8. I didn't knew that it has to be quoted in order to be
interpreted into something else than an int4.

Thanks to everybody.

--
Xavier Bugaud