An index can help even on low cardinality fields if: When one of possible values is very infrequent compared to the other values and you search for it. For instance, there are very few color blind women, so this query: SELECT * FROM color_blind_people WHERE gender=”F” would most probably benefit from an index on gender. … Read more
A source of confusion may be the use of the word in two different contexts – data modelling and database query optimization. In data modelling terms, cardinality is how one table relates to another. 1-1 (one row in table A relates to one row in tableB) 1-Many (one row in table A relates to many … Read more
Not directly, but you could use the following trick: enum class Example { A, B, C, D, E, Count }; Then the cardinality is available as static_cast<int>(Example::Count). Of course, this only works nicely if you let values of the enum be automatically assigned, starting from 0. If that’s not the case, you can manually assign … Read more