my @nodes = map {
    $_->[0]
} sort {
    # What goes here??
} map {
    [ $_->{node_id}, $_->{depth}, $_->{sibling_order} ]
} $sth->fetchall_hashref();

##</code><code>##

    $a->{depth} <=> $b->{depth}
        ||
    $a->{sibling_order} <=> $b->{sibling_order}