> Wouldn't that be two columns? Columnar databases can also have indices. If there is an index by date (or time) then you DB will know row range from the index

> Wouldn't that be two columns? Columnar databases can also have indices. If there is an index by date (or time) then you DB will know row range from the index and will read only price column within given row range. If there is no index by data it would be two columns, but it is still much less than a full row with many columns.