postgresql/doc/src/sgml/advanced.sgml

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 <chapter id="tutorial-advanced">
  <title>Advanced Features</title>

  <sect1 id="tutorial-advanced-intro">
   <title>Introduction</title>

   <para>
    In the previous chapter we have covered the basics of using
    <acronym>SQL</acronym> to store and access your data in
    <productname>PostgreSQL</productname>.  We will now discuss some
    more advanced features of <acronym>SQL</acronym> that simplify
    management and prevent loss or corruption of your data.  Finally,
    we will look at some <productname>PostgreSQL</productname>
    extensions.
   </para>

   <para>
    This chapter will on occasion refer to examples found in <xref
    linkend="tutorial-sql"> to change or improve them, so it will be
    of advantage if you have read that chapter.  Some examples from
    this chapter can also be found in
    <filename>advanced.sql</filename> in the tutorial directory.  This
    file also contains some example data to load, which is not
    repeated here.  (Refer to <xref linkend="tutorial-sql-intro"> for
    how to use the file.)
   </para>
  </sect1>


  <sect1 id="tutorial-views">
   <title>Views</title>

   <indexterm zone="tutorial-views">
    <primary>view</primary>
   </indexterm>

   <para>
    Refer back to the queries in <xref linkend="tutorial-join">.
    Suppose the combined listing of weather records and city location
    is of particular interest to your application, but you don't want
    to type the query each time you need it.  You can create a
    <firstterm>view</firstterm> over the query, which gives a name to
    the query that you can refer to like an ordinary table.

<programlisting>
CREATE VIEW myview AS
    SELECT city, temp_lo, temp_hi, prcp, date, location
        FROM weather, cities
        WHERE city = name;

SELECT * FROM myview;
</programlisting>
   </para>

   <para>
    Making liberal use of views is a key aspect of good SQL database
    design.  Views allow you to encapsulate the details of the
    structure of your tables, which may change as your application
    evolves, behind consistent interfaces.
   </para>

   <para>
    Views can be used in almost any place a real table can be used.
    Building views upon other views is not uncommon.
   </para>
  </sect1>


  <sect1 id="tutorial-fk">
   <title>Foreign Keys</title>

   <indexterm zone="tutorial-fk">
    <primary>foreign key</primary>
   </indexterm>

   <indexterm zone="tutorial-fk">
    <primary>referential integrity</primary>
   </indexterm>

   <para>
    Recall the <classname>weather</classname> and
    <classname>cities</classname> tables from <xref
    linkend="tutorial-sql">.  Consider the following problem:  You
    want to make sure that no one can insert rows in the
    <classname>weather</classname> table that do not have a matching
    entry in the <classname>cities</classname> table.  This is called
    maintaining the <firstterm>referential integrity</firstterm> of
    your data.  In simplistic database systems this would be
    implemented (if at all) by first looking at the
    <classname>cities</classname> table to check if a matching record
    exists, and then inserting or rejecting the new
    <classname>weather</classname> records.  This approach has a
    number of problems and is very inconvenient, so
    <productname>PostgreSQL</productname> can do this for you.
   </para>

   <para>
    The new declaration of the tables would look like this:

<programlisting>
CREATE TABLE cities (
	name		varchar(80) primary key,
	location	point
);

CREATE TABLE weather (
	city		varchar(80) references weather,
	temp_lo		int,
	temp_hi		int,
	prcp		real,
	date		date
);

</programlisting>

    Now try inserting an invalid record:

<programlisting>
INSERT INTO weather VALUES ('Berkeley', 45, 53, 0.0, '1994-11-28');
</programlisting>

<screen>
ERROR:  &lt;unnamed&gt; referential integrity violation - key referenced from weather not found in cities
</screen>

   </para>

   <para>
    The behavior of foreign keys can be finely tuned to your
    application.  We will not go beyond this simple example in this
    tutorial and refer you to the <citetitle>Reference
    Manual</citetitle> for more information.  Making correct use of
    foreign keys will definitely improve the quality of your database
    applications, so you are strongly encouraged to learn about them.
   </para>
  </sect1>


  <sect1 id="tutorial-transactions">
   <title>Transactions</title>

   <comment>This section needs to be written.</comment>

   <para>

   </para>
  </sect1>


  <sect1 id="tutorial-inheritance">
   <title>Inheritance</title>

   <indexterm zone="tutorial-inheritance">
    <primary>inheritance</primary>
   </indexterm>

   <para>
    Inheritance is a concept from object-oriented databases.  It opens
    up interesting new possibilities of database design.
   </para>

   <para>
    Let's create two tables:  A table <classname>cities</classname>
    and a table <classname>capitals</classname>.  Naturally, capitals
    are also cities, so you want some way to show the capitals
    implicitly when you list all cities.  If you're really clever you
    might invent some scheme like this:

<programlisting>
CREATE TABLE capitals (
    name            text,
    population      real,
    altitude        int,    -- (in ft)
    state           char(2)
);

CREATE TABLE non_capitals (
    name            text,
    population      real,
    altitude        int     -- (in ft)
);

CREATE VIEW cities AS
    SELECT name, population, altitude FROM capitals
        UNION
    SELECT name, population, altitude FROM non_capitals;
</programlisting>

    This works OK as far as querying goes, but it gets ugly when you
    need to update several rows, to name one thing.
   </para>

   <para>
    A better solution is this:

<programlisting>
CREATE TABLE cities (
    name            text,
    population      real,
    altitude        int     -- (in ft)
);

CREATE TABLE capitals (
    state           char(2)
) INHERITS (cities);
</programlisting>

    In this case, a row of <classname>capitals</classname>
    <firstterm>inherits</firstterm> all columns (<structfield>name</>,
    <structfield>population</>, and <structfield>altitude</>) from its
    <firstterm>parent</firstterm>, <classname>cities</classname>.  The
    type of the column <structfield>name</structfield> is
    <type>text</type>, a native <productname>Postgres</productname>
    type for variable length character strings.  State capitals have
    an extra column, state, that shows their state.  In
    <productname>PostgreSQL</productname>, a table can inherit from
    zero or more other tables.
   </para>

   <para>
    For example, the  following  query finds the  names  of  all  cities,
    including  state capitals, that are located at an altitude 
    over 500 ft.:

<programlisting>
SELECT name, altitude
    FROM cities
    WHERE altitude &gt; 500;
</programlisting>

    which returns:

<screen>
   name    | altitude
-----------+----------
 Las Vegas |     2174
 Mariposa  |     1953
 Madison   |      845
(3 rows)
</screen>
   </para>

   <para>
    On the other hand, the  following  query  finds
    all  the cities that are not state capitals and
    are situated at an altitude of 500 ft. or higher:

<programlisting>
SELECT name, altitude
    FROM ONLY cities
    WHERE altitude &gt; 500;
</programlisting>

<screen>
   name    | altitude
-----------+----------
 Las Vegas |     2174
 Mariposa  |     1953
(2 rows)
</screen>         
   </para>

   <para>
    Here the <literal>ONLY</literal> before <literal>cities</literal>
    indicates that the query should be run over only the
    <classname>cities</classname> table, and not tables below
    <classname>cities</classname> in the inheritance hierarchy.  Many
    of the commands that we have already discussed --
    <command>SELECT</command>, <command>UPDATE</command> and
    <command>DELETE</command> -- support this <literal>ONLY</literal>
    notation.
   </para>
  </sect1>


  <sect1 id="tutorial-conclusion">
   <title>Conclusion</title>
 
   <para>
    <productname>PostgreSQL</productname> has many features not
    touched upon in this tutorial introduction, which has been
    oriented toward newer users of <acronym>SQL</acronym>.  These
    features are discussed in more detail in both the
    <citetitle>User's Guide</citetitle> and the
    <citetitle>Programmer's Guide</citetitle>.
   </para>

   <para>
    If you feel you need more introductory material, please visit the
    <ulink url="http://www.postgresql.org">PostgreSQL web
    site</ulink> for links to more resources.
   </para>
  </sect1>

 </chapter>

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