Materialize https://materialize.io/
Incremental update/materialization of database views with joins and aggregates is super interesting. It enables listening to data changes, not just on a row level, but on a view level. It's an approach that may completely solve the problem of cache invalidation of relational data. Imagine a memcache server, except it now also guaranties consistency. In addition, being able to listen to changes could make live-data applications trivial to make, even with filters, joins, whatever.
Similarly, someone is developing a patch for postgres that implements incrementally updating/materializing views[1]. I haven't tried it so I can't speak of its performance or the state of the project, but according to the postgres wiki page on the subject [2] it seems to support some joins and aggregates, but probably not something that would be recommended for production use.
[1] https://www.postgresql-archive.org/Implementing-Incremental-... [2] https://wiki.postgresql.org/wiki/Incremental_View_Maintenanc...