If a many-to-many relation does not have a deployment table, it's data stores in the links table along with the data from all the other many-to-many relations that don't have a deployment table. Of course, you will get serious performance problems.
When you can’t use a deployment table?
When Item Types already has a deployment
inherited from a supertype. Technically it's possible but is not recommended
because you will get performance issues because the supertype requires a UNION
clause with a new table.
Why do you need to use a deployment table?
All items that don't have a deployment table
are stored in the genericitems table.
It makes the genericitems table extremely
large. This makes the database work inefficiently causing performance issues.
Need to define a deployment table for all item
types that extend GenericItem except abstract item type.