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 <chapter id="advanced">
  <title>Advanced <productname>Postgres</productname> <acronym>SQL</acronym> Features</title>

  <para>
   Having covered the basics  of  using
   <productname>Postgres</productname> <acronym>SQL</acronym>  to
   access your data, we will now discuss those features of
   <productname>Postgres</productname> that distinguish  it  from  conventional  data
   managers.   These  features  include  inheritance, time
   travel and non-atomic  data  values  (array-  and  
   set-valued attributes).
   Examples   in   this  section  can  also  be  found  in
   <filename>advance.sql</filename> in the tutorial directory.
   (Refer to <xref linkend="QUERY"> for how to use it.)
  </para>

  <sect1>
   <title>Inheritance</title>

   <para>
    Let's create two classes. The capitals  class  contains
    state  capitals  which  are also cities. Naturally, the
    capitals class should inherit from cities.

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

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

    In this case, an  instance  of  capitals  <firstterm>inherits</firstterm>  all
    attributes  (name,  population,  and altitude) from its
    parent, cities.  The type  of  the  attribute  name  is
    <type>text</type>,  a  native  <productname>Postgres</productname>
    type  for variable length
    ASCII strings.  The type of the attribute population is
    <type>float</type>,  a  native <productname>Postgres</productname>
    type for double precision
    floating point numbers.  State capitals have  an  extra
    attribute, state, that shows their state.
    In <productname>Postgres</productname>,
    a  class  can inherit from zero or more other classes,
    and a query can reference either  all  instances  of  a
    class  or  all  instances  of  a  class plus all of its
    descendants.

    <note>
     <para>
      The inheritance hierarchy is a  directed  acyclic graph.
     </para>
    </note>

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

   <programlisting>
    SELECT c.name, c.altitude
    FROM cities c
    WHERE c.altitude > 500;
   </programlisting>

   which returns:

   <programlisting>
+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
|Madison   | 845      |
+----------+----------+
   </programlisting>
  </para>

  <para>
   On the other hand, the  following  query  finds
   all  the cities, but not capital cities 
   that are situated at an attitude of 500ft or higher:

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

+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
   </programlisting>         
  </para>


   Here the <quote>ONLY</quote> before cities indicates that the query should
   be  run over only cities and not classes below cities 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 <quote>ONLY</quote> notation.
  </para>
  <para>
  Deprecated: In previous versions of postgres, the default was not to
  get access to child classes. By experience this was found to be error
  prone. Under the old syntax, to get the sub-classes you append "*"
  to the table name. For example
   <programlisting>
   SELECT * from cities*;
   </programlisting>     
   This old behaviour is still available by using a SET command...    
   <programlisting>
   SET EXAMINE_SUBCLASS TO on;
   </programlisting>     
  </para>
  </sect1>

  <sect1>
   <title>Non-Atomic Values</title>

   <para>
    One  of  the tenets of the relational model is that the
    attributes of a relation are atomic.
    <productname>Postgres</productname> does not
    have  this  restriction; attributes can themselves contain 
    sub-values that can be  accessed  from  the  query
    language.   For example, you can create attributes that
    are arrays of base types.
   </para>

   <sect2>
    <title>Arrays</title>

    <para>
     <productname>Postgres</productname> allows attributes of an
     instance to be defined
     as  fixed-length  or  variable-length multi-dimensional
     arrays. Arrays of any base type  or  user-defined  type
     can  be created. To illustrate their use, we first create a 
     class with arrays of base types.

     <programlisting>
CREATE TABLE SAL_EMP (
    name            text,
    pay_by_quarter  int4[],
    schedule        text[][]
);
     </programlisting>
    </para>

    <para>
     The above query will create a class named SAL_EMP  with
     a  <firstterm>text</firstterm>  string (name), a one-dimensional
     array of <firstterm>int4</firstterm>
     (pay_by_quarter),  which  represents   the   employee's
     salary by quarter and a two-dimensional array of
     <firstterm>text</firstterm>
     (schedule),  which  represents  the  employee's  weekly
     schedule.   Now  we  do  some  <firstterm>INSERTS</firstterm>s;
     note that when
     appending to an array, we  enclose  the  values  within
     braces  and  separate  them  by commas.  If you know
     <firstterm>C</firstterm>,
     this is not unlike the syntax for  initializing  structures.

     <programlisting>
INSERT INTO SAL_EMP
    VALUES ('Bill',
    '{10000, 10000, 10000, 10000}',
    '{{"meeting", "lunch"}, {}}');

INSERT INTO SAL_EMP
    VALUES ('Carol',
    '{20000, 25000, 25000, 25000}',
    '{{"talk", "consult"}, {"meeting"}}');
     </programlisting>

     By  default,  <productname>Postgres</productname>  uses  the
     "one-based" numbering
     convention for arrays -- that is, an array  of  n  elements
     starts with array[1] and ends with array[n].
     Now,  we  can  run  some queries on SAL_EMP.  First, we
     show how to access a single element of an  array  at  a
     time.   This query retrieves the names of the employees
     whose pay changed in the second quarter:

     <programlisting>
SELECT name
    FROM SAL_EMP
    WHERE SAL_EMP.pay_by_quarter[1] &lt;&gt;
    SAL_EMP.pay_by_quarter[2];

+------+
|name  |
+------+
|Carol |
+------+
     </programlisting>
    </para>

    <para>
     This query retrieves  the  third  quarter  pay  of  all
     employees:
     
     <programlisting>
SELECT SAL_EMP.pay_by_quarter[3] FROM SAL_EMP;


+---------------+
|pay_by_quarter |
+---------------+
|10000          |
+---------------+
|25000          |
+---------------+
     </programlisting>
    </para>

    <para>
     We  can  also  access  arbitrary slices of an array, or
     subarrays.  This query  retrieves  the  first  item  on
     Bill's schedule for the first two days of the week.

     <programlisting>
SELECT SAL_EMP.schedule[1:2][1:1]
    FROM SAL_EMP
    WHERE SAL_EMP.name = 'Bill';

+-------------------+
|schedule           |
+-------------------+
|{{"meeting"},{""}} |
+-------------------+
     </programlisting>
    </para>
   </sect2>
  </sect1>

  <sect1>
   <title>More Advanced Features</title>

   <para>
    <productname>Postgres</productname> has many features not touched
    upon in this
    tutorial introduction, which has been oriented toward newer users of
    <acronym>SQL</acronym>.
    These are discussed in more detail in both the User's and
    Programmer's Guides.
   </para>

  </sect1>

 </chapter>

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