You could also use query composition, pushing out the client_id filtering to a separate function.. should look something like this:

def filter_policy_client_id(query, nil), do: query
def filter_policy_client_id(query, client_id) do
  query |> where([p], p.client_id in ^client_id)
end

# ...

page =
Policy
|> join(:left, [p], c in Client, p.client_id == c.id)
|> where([p, c], c.user_id == ^conn.assigns.current_user.id)
|> filter_policy_client_id(params["policy"]["client_id"])
|> preload([p, c], [client: c])
|> Repo.paginate(query, params)

Is client_id supposed to be a list? If it’s not, I suspect you should use equality comparison instead of in.