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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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>5.14. 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="5.13. Other Database Objects" /><link rel="next" href="dml.html" title="Chapter 6. Data Manipulation" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.14. Dependency Tracking</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-others.html" title="5.13. 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 16.3 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 /></div><div class="sect1" id="DDL-DEPEND"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.14. Dependency Tracking <a href="#DDL-DEPEND" class="id_link">#</a></h2></div></div></div><a id="id-1.5.4.16.2" class="indexterm"></a><a id="id-1.5.4.16.3" class="indexterm"></a><p>3   When you create complex database structures involving many tables4   with foreign key constraints, views, triggers, functions, etc. you5   implicitly create a net of dependencies between the objects.6   For instance, a table with a foreign key constraint depends on the7   table it references.8  </p><p>9   To ensure the integrity of the entire database structure,10   <span class="productname">PostgreSQL</span> makes sure that you cannot11   drop objects that other objects still depend on.  For example,12   attempting to drop the products table we considered in <a class="xref" href="ddl-constraints.html#DDL-CONSTRAINTS-FK" title="5.4.5. Foreign Keys">Section 5.4.5</a>, with the orders table depending on13   it, would result in an error message like this:14</p><pre class="screen">15DROP TABLE products;16 17ERROR:  cannot drop table products because other objects depend on it18DETAIL:  constraint orders_product_no_fkey on table orders depends on table products19HINT:  Use DROP ... CASCADE to drop the dependent objects too.20</pre><p>21   The error message contains a useful hint: if you do not want to22   bother deleting all the dependent objects individually, you can run:23</p><pre class="screen">24DROP TABLE products CASCADE;25</pre><p>26   and all the dependent objects will be removed, as will any objects27   that depend on them, recursively.  In this case, it doesn't remove28   the orders table, it only removes the foreign key constraint.29   It stops there because nothing depends on the foreign key constraint.30   (If you want to check what <code class="command">DROP ... CASCADE</code> will do,31   run <code class="command">DROP</code> without <code class="literal">CASCADE</code> and read the32   <code class="literal">DETAIL</code> output.)33  </p><p>34   Almost all <code class="command">DROP</code> commands in <span class="productname">PostgreSQL</span> support35   specifying <code class="literal">CASCADE</code>.  Of course, the nature of36   the possible dependencies varies with the type of the object.  You37   can also write <code class="literal">RESTRICT</code> instead of38   <code class="literal">CASCADE</code> to get the default behavior, which is to39   prevent dropping objects that any other objects depend on.40  </p><div class="note"><h3 class="title">Note</h3><p>41    According to the SQL standard, specifying either42    <code class="literal">RESTRICT</code> or <code class="literal">CASCADE</code> is43    required in a <code class="command">DROP</code> command.  No database system actually44    enforces that rule, but whether the default behavior45    is <code class="literal">RESTRICT</code> or <code class="literal">CASCADE</code> varies46    across systems.47   </p></div><p>48   If a <code class="command">DROP</code> command lists multiple49   objects, <code class="literal">CASCADE</code> is only required when there are50   dependencies outside the specified group.  For example, when saying51   <code class="literal">DROP TABLE tab1, tab2</code> the existence of a foreign52   key referencing <code class="literal">tab1</code> from <code class="literal">tab2</code> would not mean53   that <code class="literal">CASCADE</code> is needed to succeed.54  </p><p>55   For a user-defined function or procedure whose body is defined as a string56   literal, <span class="productname">PostgreSQL</span> tracks57   dependencies associated with the function's externally-visible properties,58   such as its argument and result types, but <span class="emphasis"><em>not</em></span> dependencies59   that could only be known by examining the function body.  As an example,60   consider this situation:61 62</p><pre class="programlisting">63CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow',64                             'green', 'blue', 'purple');65 66CREATE TABLE my_colors (color rainbow, note text);67 68CREATE FUNCTION get_color_note (rainbow) RETURNS text AS69  'SELECT note FROM my_colors WHERE color = $1'70  LANGUAGE SQL;71</pre><p>72 73   (See <a class="xref" href="xfunc-sql.html" title="38.5. Query Language (SQL) Functions">Section 38.5</a> for an explanation of SQL-language74   functions.)  <span class="productname">PostgreSQL</span> will be aware that75   the <code class="function">get_color_note</code> function depends on the <code class="type">rainbow</code>76   type: dropping the type would force dropping the function, because its77   argument type would no longer be defined.  But <span class="productname">PostgreSQL</span>78   will not consider <code class="function">get_color_note</code> to depend on79   the <code class="structname">my_colors</code> table, and so will not drop the function if80   the table is dropped.  While there are disadvantages to this approach,81   there are also benefits.  The function is still valid in some sense if the82   table is missing, though executing it would cause an error; creating a new83   table of the same name would allow the function to work again.84  </p><p>85   On the other hand, for a SQL-language function or procedure whose body86   is written in SQL-standard style, the body is parsed at function87   definition time and all dependencies recognized by the parser are88   stored.  Thus, if we write the function above as89 90</p><pre class="programlisting">91CREATE FUNCTION get_color_note (rainbow) RETURNS text92BEGIN ATOMIC93  SELECT note FROM my_colors WHERE color = $1;94END;95</pre><p>96 97   then the function's dependency on the <code class="structname">my_colors</code>98   table will be known and enforced by <code class="command">DROP</code>.99  </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" title="5.13. Other Database Objects">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="dml.html" title="Chapter 6. Data Manipulation">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.13. Other Database Objects </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 6. Data Manipulation</td></tr></table></div></body></html>