We currently don't support tuples in DistSQL streams. We don't have a ColumnType_Kind for it and no way to encode it over the wire. This makes certain queries non-distributed.

For example: SELECT (k, v) FROM t.

Another example is more subtle: SELECT k, v FROM t ORDER BY (k, v). Here the extra parens cause a tuple to be created for each row and used as the sort key. So this query is not supported by DistSQL, as opposed to the equivalent SELECT k, v FROM t ORDER BY k, v. Granted, this case should be probably converted to latter version before distsql planning.

CC @asubiotto