Traditional relational and document databases are optimized for read performance. Writing data to a relational database will typically involve making in - place updates to complicated data structures on disk, in order to maintain a data structure that can be read efficiently and flexibly. Updating these data structures is a very expensive operation from a standpoint of disk I/O, which is often the limiting factor for database performance. Since writes are more expensive than reads, you'll typically avoid any unnecessary updates to a relational database, even at the expense of extra read operations.
Cassandra, on the other hand, is highly optimized for write throughput and, in fact, never modifies data on disk; it only appends ...
