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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)
) INHERITS (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>

    For example, the  following  query  finds
    all  the cities that are situated at an attitude of 500ft or higher:
     
    <programlisting>
SELECT name, altitude
    FROM cities
    WHERE altitude &gt; 500;

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

   <para>
    On the other hand, to find 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>

    Here the <quote>*</quote> after cities indicates that the query should
    be  run over cities and all 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>*</quote> notation, as do others, like
    <command>alter</command>.
   </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>Time Travel</title>

   <para>
    As of <productname>Postgres</productname> v6.2, <emphasis>time
     travel is no longer supported</emphasis>. There are
    several reasons for this: performance impact, storage size, and a
    pg_time file which grows
    toward infinite size in a short period of time.
   </para>

   <para>
    New features such as triggers allow one to mimic the behavior of time travel when desired, without
    incurring the overhead when it is not needed (for most users, this is most of the time).
    See examples in the <filename>contrib</filename> directory for more information.
   </para>

   <note>
    <title>Time travel is deprecated</title>
    <para>
     The remaining text in this section is retained only until it can be rewritten in the context
     of new techniques to accomplish the same purpose. Volunteers? - thomas 1998-01-12
    </para>
   </note>

   <para>
    <productname>Postgres</productname> supports the notion of time travel.  This feature 
    allows a user  to  run  historical  queries.   For
    example,  to  find  the  current population of Mariposa
    city, one would query:

    <programlisting>
SELECT * FROM cities WHERE name = 'Mariposa';

+---------+------------+----------+
|name     | population | altitude |
+---------+------------+----------+
|Mariposa | 1320       | 1953     |
+---------+------------+----------+
    </programlisting>

    <productname>Postgres</productname> will automatically find the version  of  Mariposa's 
    record valid at the current time.
    One can also give a time range.  For example to see the
    past and present populations  of  Mariposa,  one  would
    query:

    <programlisting>
SELECT name, population
    FROM cities['epoch', 'now']
    WHERE name = 'Mariposa';
</programlisting>

    where  "epoch"  indicates  the  beginning of the system
    clock.

    <note>
     <para>
      On Unix systems, this is always  midnight,  January  1, 1970 GMT.
     </para>
    </note>
   </para>

   <para>
    If  you  have  executed all of the examples so
    far, then the above query returns:

    <programlisting>
+---------+------------+
|name     | population |
+---------+------------+
|Mariposa | 1200       |
+---------+------------+
|Mariposa | 1320       |
+---------+------------+
    </programlisting>
   </para>

   <para>
    The default beginning of a time range is  the  earliest
    time representable by the system and the default end is
    the current time; thus, the above  time  range  can  be
    abbreviated as ``[,].''
   </para>
  </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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