The memory overhead on this is large, it does a `SELECT * `, and serializes all fields into ActiveRecord instances just to get  the `payout_class` field. 

```ruby
PaymentEngine.pluck(:payout_class).uniq
```

will do a `SELECT payout_class `, but will still have to iterate over all returned records and uniq them

```
PaymentEngine.group(:payout_class).count.keys
```
will do the uniq in SQL, generating the fewest objects and be more efficient, but will sacrifice a bit of readability