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><A
NAME="DDL-DEPEND"
>5.12. Dependency Tracking</A
></H1
><P
> 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.
</P
><P
> To ensure the integrity of the entire database structure,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> makes sure that you cannot
drop objects that other objects still depend on. For example,
attempting to drop the products table we considered in <A
HREF="ddl-constraints.html#DDL-CONSTRAINTS-FK"
>Section 5.3.5</A
>, with the orders table depending on
it, would result in an error message like this:
</P><PRE
CLASS="SCREEN"
>DROP TABLE products;
ERROR: cannot drop table products because other objects depend on it
DETAIL: constraint orders_product_no_fkey on table orders depends on table products
HINT: Use DROP ... CASCADE to drop the dependent objects too.</PRE
><P>
The error message contains a useful hint: if you do not want to
bother deleting all the dependent objects individually, you can run:
</P><PRE
CLASS="SCREEN"
>DROP TABLE products CASCADE;</PRE
><P>
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 <TT
CLASS="COMMAND"
>DROP ... CASCADE</TT
> will do,
run <TT
CLASS="COMMAND"
>DROP</TT
> without <TT
CLASS="LITERAL"
>CASCADE</TT
> and read the
<TT
CLASS="LITERAL"
>DETAIL</TT
> output.)
</P
><P
> All <TT
CLASS="COMMAND"
>DROP</TT
> commands in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> support
specifying <TT
CLASS="LITERAL"
>CASCADE</TT
>. Of course, the nature of
the possible dependencies varies with the type of the object. You
can also write <TT
CLASS="LITERAL"
>RESTRICT</TT
> instead of
<TT
CLASS="LITERAL"
>CASCADE</TT
> to get the default behavior, which is to
prevent the dropping of objects that other objects depend on.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> According to the SQL standard, specifying either
<TT
CLASS="LITERAL"
>RESTRICT</TT
> or <TT
CLASS="LITERAL"
>CASCADE</TT
> is
required in a <TT
CLASS="COMMAND"
>DROP</TT
> command. No database system actually
enforces that rule, but whether the default behavior
is <TT
CLASS="LITERAL"
>RESTRICT</TT
> or <TT
CLASS="LITERAL"
>CASCADE</TT
> varies
across systems.
</P
></BLOCKQUOTE
></DIV
><P
> For user-defined functions, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> tracks
dependencies associated with a function's externally-visible properties,
such as its argument and result types, but <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> dependencies
that could only be known by examining the function body. As an example,
consider this situation:
</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
(See <A
HREF="xfunc-sql.html"
>Section 35.4</A
> for an explanation of SQL-language
functions.) <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will be aware that
the <CODE
CLASS="FUNCTION"
>get_color_note</CODE
> function depends on the <TT
CLASS="TYPE"
>rainbow</TT
>
type: dropping the type would force dropping the function, because its
argument type would no longer be defined. But <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
will not consider <CODE
CLASS="FUNCTION"
>get_color_note</CODE
> to depend on
the <TT
CLASS="STRUCTNAME"
>my_colors</TT
> 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.
</P
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