4.12. Dependency Tracking

When you create complex database structures involving many tables with foreign key constraints, views, triggers, functions, etc. you will implicitly create a net of dependencies between the objects. For instance, a table with a foreign key constraint depends on the table it references.

To ensure the integrity of the entire database structure, EnterpriseDB makes sure that you cannot drop objects that other objects still depend on. For example, attempting to drop the dept table we had considered in Section 4.4.5, with the emp table depending on it, would result in an error message such as this:

DROP TABLE dept;

NOTICE:  constraint emp_ref_dept_fk on table emp depends on table dept
ERROR:  cannot drop table emp because other objects depend on it
HINT:  Use DROP ... CASCADE to drop the dependent objects too.

The error message contains a useful hint: if you do not want to bother deleting all the dependent objects individually, you can run

DROP TABLE dept CASCADE;

and all the dependent objects will be removed.

All drop commands in EnterpriseDB support specifying CASCADE CONSTRAINT. Of course, the nature of the possible dependencies varies with the type of the object. You can also write RESTRICT instead of CASCADE to get the default behavior, which is to prevent drops of objects that other objects depend on.

Note: According to the SQL standard, specifying either RESTRICT or CASCADE is required. No database system actually implements it that way, but whether the default behavior is RESTRICT or CASCADE varies across systems.