Why does BigTable have column families?

Question

Why is BigTable structured as a two-level hierarchy of "family:qualifier"? Specifically, why is this enforced rather than just having columns and, say, recommending that users name their qualifiers "vertical:column"?

I am interested in whether or not enforcing this enables some engineering optimizations or if this is strictly a design thing.

Answer 1

There are a couple of advantages to family groups:

1. queries become easier by getting a group of column qualifiers in a single column family

2. Bigtable has a notion of "locality groups". Locality groups allow a family to be written to a separate file, which helps in situations where some column families are accessed less frequently than others. You can see some information about locality groups in this analysis of HBase vs. Bigtable.