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,
PostgreSQL makes sure that you cannot
drop objects that other objects still depend on. For example,
attempting to drop the products table we had considered in Section 2.4.5, with the orders table depending on
it, would result in an error message such as this:
DROP TABLE products;
NOTICE: constraint $1 on table orders depends on table products
ERROR: Cannot drop table products because other objects depend on it
Use DROP ... CASCADE to drop the dependent objects too
The error message contains a useful hint: If you don't want to
bother deleting all the dependent objects individually, you can run
DROP TABLE products CASCADE;
and all the dependent objects will be removed. In this case, it
doesn't remove the orders table, it only removes the foreign key
constraint. (If you want to check what DROP ... CASCADE will do,
run DROP without CASCADE and read the NOTICE messages.)
All drop commands in PostgreSQL support
specifying CASCADE. 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
restrict 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.
Note: Foreign key constraint dependencies and serial column dependencies
from PostgreSQL versions prior to 7.3
are not maintained or created during the
upgrade process. All other dependency types will be properly
created during an upgrade.