In the LT/ST-J2 class unindexed join, no combination of the join columns of the small tables comprises any index of the large table.
Reasonable Unindexed Joins, LT/ST-J2a
This subclass consists of those LT/ST-J2 joins in which the cardinality of the Cartesian product of the small tables is much smaller than the cardinality of the large table.
The difference between the cardinalities of the small tables Cartesian product and the cardinality of the large table is much larger for LT/ST-J2a joins than for LT/ST-J1a joins, though it cannot be defined rigorously.
In special cases, the sum of the sizes of the small tables can fit in memory, so the Optimizer can use a single-step, n-way join.
Unreasonable Unindexed Joins, LT/ST-J2b
This subclass consists of all LT/ST-J2 joins that are not of subclass LT/ST-J2a.