my $sth = $sth->prepare ("select field from table");
    $sth->execute;
    $href = $sth->fetchrow_hashref;   # slowest
    @arr  = $sth->fetchrow_array;
    $aref = $sth->fetchrow_arrayref;
    my $field;
    $sth->bind_columns (\$field);
    $sth->fetch;                      # fastest