Ask HN: Databases that support arbitrary real-time queries

I'm working on a side project that will involve millions of rows of data. The dashboard for this project will display small snapshots of this data along with several aggregate queries, such as the total number of data points, the total number of data points grouped by day, the average value for the current day, etc. I want the client to update immediately when a new row is inserted, which could be several times per second.

What are my options when it comes to databases that support real-time queries? Here's what I've tried so far:

Firebase:

- Easy to use real-time database but has poor support for queries

- Doesn't support aggregate queries at all

RethinkDB:

- Excellent real-time database that supports changefeeds on most queries

- Horizon offers a client side library that is comparable to Firebase

- As of version 2.3, RethinkDB does not support changefeeds on aggregate queries

PostgreSQL:

- Supports pub/sub via LISTEN and NOTIFY

- Requires a lot more work to set up since each query has to be written as a TRIGGER

I've also looked at PipelineDB[1], which seems to match my needs perfectly. Can anyone recommend anything else?

[1]: https://www.pipelinedb.com/