Tools to preprocess a big data for dashboards?
We have dashboards that show information about some processes, which have billions of rows in the database. It's not queried directly though, but instead from pre-aggregated data.

We have automated scripts running in database that populate the aggregated data tables specifically for dashboards. In most extreme cases the raw data is so large and comes in at such a high pace that there's a two-tier aggregation in place. First tier will de-normalize the data and 2nd tier will do the actual sum/count type of aggregation.

So you don't need two tools since you can do aggregation purely in database.

edit (answering question from comment): Our dashboards are in Tableau. Data is all in PostgreSQL databases. Automation is done with unix cronjob executing a database function. The function in turn queries which tasks it has to run and runs them. There is a few moving pieces to the whole system, but the architecture isn't too complicated. Write down your ideas and go talk to some architects / engineers - they will know the best way to approach this.