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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/ddl-depend.html

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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 V1.79.1" /><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><div xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 12.4 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">5.14. Dependency Tracking</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 tables

  4.    with foreign key constraints, views, triggers, functions, etc. you

  5.    implicitly create a net of dependencies between the objects.

  6.    For instance, a table with a foreign key constraint depends on the

  7.    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 cannot

  11.    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 on

  13.    it, would result in an error message like this:

  14. </p><pre class="screen">

  15. DROP TABLE products;

  16. 

  17. ERROR:  cannot drop table products because other objects depend on it

  18. DETAIL:  constraint orders_product_no_fkey on table orders depends on table products

  19. HINT:  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 to

  22.    bother deleting all the dependent objects individually, you can run:

  23. </p><pre class="screen">

  24. DROP TABLE products CASCADE;

  25. </pre><p>

  26.    and all the dependent objects will be removed, as will any objects

  27.    that depend on them, recursively.  In this case, it doesn't remove

  28.    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 the

  32.    <code class="literal">DETAIL</code> output.)

  33.   </p><p>

  34.    Almost all <code class="command">DROP</code> commands in <span class="productname">PostgreSQL</span> support

  35.    specifying <code class="literal">CASCADE</code>.  Of course, the nature of

  36.    the possible dependencies varies with the type of the object.  You

  37.    can also write <code class="literal">RESTRICT</code> instead of

  38.    <code class="literal">CASCADE</code> to get the default behavior, which is to

  39.    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 either

  42.     <code class="literal">RESTRICT</code> or <code class="literal">CASCADE</code> is

  43.     required in a <code class="command">DROP</code> command.  No database system actually

  44.     enforces that rule, but whether the default behavior

  45.     is <code class="literal">RESTRICT</code> or <code class="literal">CASCADE</code> varies

  46.     across systems.

  47.    </p></div><p>

  48.    If a <code class="command">DROP</code> command lists multiple

  49.    objects, <code class="literal">CASCADE</code> is only required when there are

  50.    dependencies outside the specified group.  For example, when saying

  51.    <code class="literal">DROP TABLE tab1, tab2</code> the existence of a foreign

  52.    key referencing <code class="literal">tab1</code> from <code class="literal">tab2</code> would not mean

  53.    that <code class="literal">CASCADE</code> is needed to succeed.

  54.   </p><p>

  55.    For user-defined functions, <span class="productname">PostgreSQL</span> tracks

  56.    dependencies associated with a function's externally-visible properties,

  57.    such as its argument and result types, but <span class="emphasis"><em>not</em></span> dependencies

  58.    that could only be known by examining the function body.  As an example,

  59.    consider this situation:

  60. 

  61. </p><pre class="programlisting">

  62. CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow',

  63.                              'green', 'blue', 'purple');

  64. 

  65. CREATE TABLE my_colors (color rainbow, note text);

  66. 

  67. CREATE FUNCTION get_color_note (rainbow) RETURNS text AS

  68.   'SELECT note FROM my_colors WHERE color = $1'

  69.   LANGUAGE SQL;

  70. </pre><p>

  71. 

  72.    (See <a class="xref" href="xfunc-sql.html" title="37.5. Query Language (SQL) Functions">Section 37.5</a> for an explanation of SQL-language

  73.    functions.)  <span class="productname">PostgreSQL</span> will be aware that

  74.    the <code class="function">get_color_note</code> function depends on the <code class="type">rainbow</code>

  75.    type: dropping the type would force dropping the function, because its

  76.    argument type would no longer be defined.  But <span class="productname">PostgreSQL</span>

  77.    will not consider <code class="function">get_color_note</code> to depend on

  78.    the <code class="structname">my_colors</code> table, and so will not drop the function if

  79.    the table is dropped.  While there are disadvantages to this approach,

  80.    there are also benefits.  The function is still valid in some sense if the

  81.    table is missing, though executing it would cause an error; creating a new

  82.    table of the same name would allow the function to work again.

  83.   </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">5.13. 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>
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