I find CockroachDB one of the most interesting systems adding distributed transactions, joins and SQL support on top of a highly scalable key value store engine:

https://www.cockroachlabs.com/docs/stable/architecture/overv...

For example:

http://info.snowflake.net/rs/252-RFO-227/images/Snowflake_SI...

https://www.snowflake.com/wp-content/uploads/2014/10/A-Detai...