How a B-Tree index on a columnar table would look like?

Greenplum database supports B-tree index on append-optimized columnar tables which allows UPDATE operations as well . Even though it's not a recommended practice to have index on such tables(probably because they are intended for append-only and do fast sequential scans) for update operation, an index on distributing column reduces execution time drastically. While traditionally a B-tree index on rowstore table holds the pointer to heap with offset value, how this will be implemented on a columnar table? If the table has N columns does each entry in index contain total N-1 pointers to each column blocks?