<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Dependency Tracking</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.5 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Data Definition" HREF="ddl.html"><LINK REL="PREVIOUS" TITLE="Other Database Objects" HREF="ddl-others.html"><LINK REL="NEXT" TITLE="Data Manipulation" HREF="dml.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2013-10-08T03:47:12"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.5 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Other Database Objects" HREF="ddl-others.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="ddl.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 5. Data Definition</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Data Manipulation" HREF="dml.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><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 had 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 such as this: </P><PRE CLASS="SCREEN" >DROP TABLE products; NOTICE: constraint orders_product_no_fkey on table orders depends on table products ERROR: cannot drop table products because other objects depend on it 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" >NOTICE</TT > messages.) </P ><P > All drop 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. 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 ><DIV CLASS="NOTE" ><BLOCKQUOTE CLASS="NOTE" ><P ><B >Note: </B > Foreign key constraint dependencies and serial column dependencies from <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > versions prior to 7.3 are <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >not</I ></SPAN > maintained or created during the upgrade process. All other dependency types will be properly created during an upgrade from a pre-7.3 database. </P ></BLOCKQUOTE ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="ddl-others.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="dml.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Other Database Objects</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="ddl.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Data Manipulation</TD ></TR ></TABLE ></DIV ></BODY ></HTML >