**Reported in Launchpad by Daniel Nichter last update 02-08-2013 19:14:12

If there are only non-unique index, pt-table-checksum should choose the one with the highest cardinality, but it doesn't. In NIbbleIterator.pm:

if ( !$best_index && @possible_indexes ) {
PTDEBUG && _d('No PRIMARY or unique indexes;',
'will use index with highest cardinality');
foreach my $index ( @possible_indexes ) {
$indexes->{$index}->{cardinality} = _get_index_cardinality(
%args,
index => $index,
);
}
@possible_indexes = sort {

1. Prefer the index with the highest cardinality.
   my $cmp
   = $indexes->{$b}>{cardinality} <=> $indexes>{$b}->{cardinality};
   if ( $cmp == 0 ) {

2. Indexes have the same cardinality; prefer the one with

3. more columns.
   $cmp = scalar @{$indexes->{$b}->{cols}}
   <=> scalar @{$indexes->{$a}->{cols}};
   }
   $cmp;
   } @possible_indexes;
   $best_index = $possible_indexes[0];
   }

We need to test that block.