I have a post showing how to control predicate pushdown in Polybase:
As a reminder, in order to allow predicate pushdown to occur, we need to hit a Hadoop cluster; we can’t use predicate pushdown on other systems like Azure Blob Storage. Second, we need to have a resource manager link set up in our external data source. Third, we need to make sure that everything is configured correctly on the Polybase side. But once you have those items in place, it’s possible to use the FORCE EXTERNALPUSHDOWN command like so:
There’s also discussion of preventing MapReduce job creation as well as a pushdown-related error I had received in the past.