What is cardinality with respect to an indexed column? Well cardinality is just a measure of how unique the information is in a column. For most index types, unique values are good and repeated values are bad.

For example, with a telephone directory, the more unique a name is the faster you can identify the person you are looking for. A name like “John Smith” is likely to have many entries and thus require verification of unindexed data to resolve the correct entry for your search, whereas “Zaphod Beeblebrox”, would be relative simple to identify.

Thus having a high cardinality or relatively unique column values, is typically a positive thing, in terms of index efficiency anyway.