It is bad because in a column store each column will be stored in a different location, resulting in seeks on disk or cache misses in memory. In a row store the

It is bad because in a column store each column will be stored in a different location, resulting in seeks on disk or cache misses in memory. In a row store the complete row will often be in a contiguous block of memory. It gets worse when the column store uses dictionary compression and the dictionary is not already cached, since you need one more memory access to a random location. For wide tables the overhead is noticeable and the application should really only retrieve what is necessary.