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When you create complex database structures involving many tables with foreign key constraints, views, triggers, functions, etc. you 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 considered inSection 5.5.5, with the orders table depending on it, would result in an error message like this:

DROP TABLE products;ERROR:  cannot drop table products because other objects depend on itDETAIL:  constraint orders_product_no_fkey on table orders depends on table productsHINT:  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, as will any objects that depend on them, recursively. In this case, it doesn't remove the orders table, it only removes the foreign key constraint. It stops there because nothing depends on the foreign key constraint. (If you want to check whatDROP ... CASCADE will do, runDROP withoutCASCADE and read theDETAIL output.)

Almost allDROP commands inPostgreSQL support specifyingCASCADE. Of course, the nature of the possible dependencies varies with the type of the object. You can also writeRESTRICT instead ofCASCADE to get the default behavior, which is to prevent dropping objects that any other objects depend on.

If aDROP command lists multiple objects,CASCADE is only required when there are dependencies outside the specified group. For example, when sayingDROP TABLE tab1, tab2 the existence of a foreign key referencingtab1 fromtab2 would not mean thatCASCADE is needed to succeed.

For a user-defined function or procedure whose body is defined as a string literal,PostgreSQL tracks dependencies associated with the function's externally-visible properties, such as its argument and result types, butnot dependencies that could only be known by examining the function body. As an example, consider this situation:

CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow',                             'green', 'blue', 'purple');CREATE TABLE my_colors (color rainbow, note text);CREATE FUNCTION get_color_note (rainbow) RETURNS text AS  'SELECT note FROM my_colors WHERE color = $1'  LANGUAGE SQL;

(SeeSection 36.5 for an explanation of SQL-language functions.)PostgreSQL will be aware that theget_color_note function depends on therainbow type: dropping the type would force dropping the function, because its argument type would no longer be defined. ButPostgreSQL will not considerget_color_note to depend on themy_colors table, and so will not drop the function if the table is dropped. While there are disadvantages to this approach, there are also benefits. The function is still valid in some sense if the table is missing, though executing it would cause an error; creating a new table of the same name would allow the function to work again.

On the other hand, for an SQL-language function or procedure whose body is written in SQL-standard style, the body is parsed at function definition time and all dependencies recognized by the parser are stored. Thus, if we write the function above as

CREATE FUNCTION get_color_note (rainbow) RETURNS textBEGIN ATOMIC  SELECT note FROM my_colors WHERE color = $1;END;

then the function's dependency on themy_colors table will be known and enforced byDROP.