Cassandra doesn't have capped collections (or row size limits), but one way of simulating it is to use an offline mapreduce job clean up extra entries. Would it be better to have a second table that stores row counts for primary keys in another table? The downside is that you have to scan through the entire row_count table since counters aren't indexable. Or would it be faster to just scan over the backing table with the real data?
Or is there another technique I should look into?
Edit: I found this Columns count vs counter column performance. Row counts go over all the data, so I'm leaning away from that.