Yes, indices are updated incrementally. You can currently only evaluate an index for a given row based only on that row, not on other rows. The reasons for this

Yes, indices are updated incrementally. You can currently only evaluate an index for a given row based only on that row, not on other rows. The reasons for this are pretty deep -- if you start evaluating indexes on a row based on other rows, the result becomes non-deterministic and can't easily be replicated across the cluster. Once we introduce value-based replication (as opposed to evaluation-based replication we have now) this will become possible.