Cost of Disk Resources
A join index table has the same properties as a base table except that it cannot be queried directly.
Because it is essentially a base table, the cardinality of a simple join index is generally of the same order as its component join table with the highest cardinality, with adjustments being necessary for row compression and other miscellaneous issues. The cardinality of an aggregate join index is typically much smaller than that of any of its component join tables.
The degree of either type of join index table is entirely dependent on how it is defined.
Irrespective of any other factors, join index tables extract a cost in terms of increased disk space requirements. If a join index table is fallback-protected, then it exerts twice again the burden in disk space.
The disk resources required for a join index are described in “Join Index Storage” on page 549.