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 5.3.5, “Foreign Keys”, with the orders table depending on it, would result in an error message such as this:
DROP TABLE products; NOTICE: constraint orders_product_no_fkey on table orders depends on table products ERROR: cannot drop table products 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 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
prevent drops of objects that other objects depend on.
According to the SQL standard, specifying either
RESTRICT
or CASCADE
is
required. No database system actually enforces that rule, but
whether the default behavior is RESTRICT
or
CASCADE
varies across systems.
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 from a pre-7.3 database.