Spice supports what DataFusion supports, which is generally yes but there is still work to do to push down more queries to TableProviders. For example, joins within a single source are not yet pushed down to the underlying provider.

You can write a single query across many data sources which is what we show in the demo on the Git repo.

There is an effort within DataFusion to support pushing down joins across tables from the same remote provider that we will likely contribute to as well: https://github.com/datafusion-contrib/datafusion-federation