But MySQL is only doing full table scans on the cust_order table because we don't have suitable indexes, and I do not do a full table scan of order_line because I have an index I can use to avoid that ...

That's the ScarlettDME I found, but it's largely a collection of opaque binary blobs, with some form of interpreter for those blobs. The interpreter doesn't seem to restrict what the blobs can do, ...