Comment by atombender

There are definitely hybrid OLTP-OLAP databases, or HTAP (Hybrid Transaction and Analytical Processing). Microsoft and, I believe, Oracle both have HTAP tech.

The most novel design, I think, is CedarDB (developed by Thomas Neumann's database group at TUM), which adaptively stores both row and column versions of the data [1], where some data is permanently compressed to columnar format and hot rows are converted "just in time" to columnar data as needed.

[1] https://cedardb.com/blog/colibri/