Bug with large IN query

dear all thanks for your reply
i just follow the "bug" i found
this code was for previously viewd items on an ecommerce website
we never excpected a user to see more than 1000 produtcs in a session
but botnet do
i'll manage to recode this stuff or simply limit the previously view items to 100 or so

question, in term on performence, what would be better ?
100 lines like this:
$this->db->or_where_in('id_prod', $chunk);
or 100 Ids in a IN clause ?