diff --git a/bin/pt-table-checksum b/bin/pt-table-checksum
index bcd54383..6be21987 100755
--- a/bin/pt-table-checksum
+++ b/bin/pt-table-checksum
@@ -6560,7 +6560,7 @@ sub _find_best_index {
       }
    }
    
-   if (!$best_index && !$want_index) {
+   if (!$best_index) {
       PTDEBUG && _d('Auto-selecting best index');
       foreach my $index ( $tp->sort_indexes($tbl_struct) ) {
          if ( $index eq 'PRIMARY' || $indexes->{$index}->{is_unique} ) {
diff --git a/lib/NibbleIterator.pm b/lib/NibbleIterator.pm
index 3491d600..ea5f08c5 100644
--- a/lib/NibbleIterator.pm
+++ b/lib/NibbleIterator.pm
@@ -540,7 +540,8 @@ sub _find_best_index {
       }
    }
    
-   # still no best index? select amongst all candidates.
+   # still no best index?
+   # prefer unique index. otherwise put in candidates array. 
    if (!$best_index) {
       PTDEBUG && _d('Auto-selecting best index');
       foreach my $index ( $tp->sort_indexes($tbl_struct) ) {
@@ -554,6 +555,7 @@ sub _find_best_index {
       }
    }
 
+   # choose the one with best cardinality 
    if ( !$best_index && @possible_indexes ) {
       PTDEBUG && _d('No PRIMARY or unique indexes;',
          'will use index with highest cardinality');