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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>5.13. Dependency Tracking</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="ddl-others.html" title="5.12. Other Database Objects" /><link rel="next" href="dml.html" title="Chapter 6. Data Manipulation" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.13. Dependency Tracking</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-others.html" title="5.12. Other Database Objects">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="dml.html" title="Chapter 6. Data Manipulation">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="DDL-DEPEND"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.13. Dependency Tracking</h2></div></div></div><a id="id-1.5.4.15.2" class="indexterm"></a><a id="id-1.5.4.15.3" class="indexterm"></a><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 class="xref" href="ddl-constraints.html#DDL-CONSTRAINTS-FK" title="5.3.5. Foreign Keys">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, 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 what <code class="command">DROP ... CASCADE</code> will do,
run <code class="command">DROP</code> without <code class="literal">CASCADE</code> and read the
<code class="literal">DETAIL</code> output.)
</p><p>
Almost all <code class="command">DROP</code> commands in <span class="productname">PostgreSQL</span> support
specifying <code class="literal">CASCADE</code>. Of course, the nature of
the possible dependencies varies with the type of the object. You
can also write <code class="literal">RESTRICT</code> instead of
<code class="literal">CASCADE</code> to get the default behavior, which is to
prevent dropping objects that any other objects depend on.
</p><div class="note"><h3 class="title">Note</h3><p>
According to the SQL standard, specifying either
<code class="literal">RESTRICT</code> or <code class="literal">CASCADE</code> is
required in a <code class="command">DROP</code> command. No database system actually
enforces that rule, but whether the default behavior
is <code class="literal">RESTRICT</code> or <code class="literal">CASCADE</code> varies
across systems.
</p></div><p>
If a <code class="command">DROP</code> command lists multiple
objects, <code class="literal">CASCADE</code> is only required when there are
dependencies outside the specified group. For example, when saying
<code class="literal">DROP TABLE tab1, tab2</code> the existence of a foreign
key referencing <code class="literal">tab1</code> from <code class="literal">tab2</code> would not mean
that <code class="literal">CASCADE</code> is needed to succeed.
</p><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"><em>not</em></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 class="xref" href="xfunc-sql.html" title="38.5. Query Language (SQL) Functions">Section 38.5</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 <code class="type">rainbow</code>
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 <code class="structname">my_colors</code> 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></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-others.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="dml.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.12. Other Database Objects </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 6. Data Manipulation</td></tr></table></div></body></html>
