Before I start, let’s talk about predicate pushdown for a moment. The gist of it is that when you have data in two sources, you have two options for combining the data:
1. Bring the data in its entirety from your remote source to your local target and work as though everything were in the local target to begin with. I’ll call this the streaming approach.
2. Send as much of your query’s filters, projections, and pre-conditions to the remote source, have the remote source perform some of the work, and then have the remote source send its post-operative data to the local target. Then, the local target once more treats this as though it were simply local data. This is the pushdown approach because you push down those predicates (that is, filters, projections, and pre-conditions).
Click through for the unfortunate finding and also vote up my UserVoice feature request if you want to see string columns as filters.