SQL
Beginning in MarkLogic 9, an updated version of SQL using the triple index is introduced. The existing version (pre-MarkLogic 9) will continue to work in a mixed cluster, and after the cluster has been upgraded to 9.0-x or 10.0-x. The updated version of SQL will not work in a mixed cluster. You will need to upgrade to the newer version of MarkLogic and commit the upgrade before those features are available.
The earlier version of SQL based on range indexes will work in the mixed cluster (prior to committing the upgrade), and it will also work with MarkLogic 9 and later.