s.sl_unit,
           s.sl_len * u.un_fact AS sl_len_cm
      FROM shoelace_data s, unit u
     WHERE s.sl_unit = u.un_name;

CREATE VIEW shoe_ready AS
    SELECT rsh.shoename,
           rsh.sh_avail,
           rsl.sl_name,
           rsl.sl_avail,
           least(rsh.sh_avail, rsl.sl_avail) AS total_avail
      FROM shoe rsh, shoelace rsl
     WHERE rsl.sl_color = rsh.slcolor
       AND rsl.sl_len_cm >= rsh.slminlen_cm
       AND rsl.sl_len_cm <= rsh.slmaxlen_cm;
</programlisting>

    The <command>CREATE VIEW</command> command for the
    <literal>shoelace</literal> view (which is the simplest one we
    have) will create a relation <literal>shoelace</literal> and an entry in
    <structname>pg_rewrite</structname> that tells that there is a
    rewrite rule that must be applied whenever the relation <literal>shoelace</literal>
    is referenced in a query's range table.  The rule has no rule
    qualification (discussed later, with the non-<command>SELECT</command> rules, since
    <command>SELECT</command> rules currently cannot have them) and it is <literal>INSTEAD</literal>. Note
    that rule qualifications are not the same as query qualifications.
    The action of our rule has a query qualification.
    The action of the rule is one query tree that is a copy of the
    <command>SELECT</command> statement in the view creation command.
</para>

    <note>
    <para>
    The two extra range
    table entries for <literal>NEW</literal> and <literal>OLD</literal> that you can see in
    the <structname>pg_rewrite</structname> entry aren't of interest
    for <command>SELECT</command> rules.
    </para>
    </note>

<para>
    Now we populate <literal>unit</literal>, <literal>shoe_data</literal>
    and <literal>shoelace_data</literal> and run a simple query on a view:

<programlisting>
INSERT INTO unit VALUES ('cm', 1.0);
INSERT INTO unit VALUES ('m', 100.0);
INSERT INTO unit VALUES ('inch', 2.54);

INSERT INTO shoe_data VALUES ('sh1', 2, 'black', 70.0, 90.0, 'cm');
INSERT INTO shoe_data VALUES ('sh2', 0, 'black', 30.0, 40.0, 'inch');
INSERT INTO shoe_data VALUES ('sh3', 4, 'brown', 50.0, 65.0, 'cm');
INSERT INTO shoe_data VALUES ('sh4', 3, 'brown', 40.0, 50.0, 'inch');

INSERT INTO shoelace_data VALUES ('sl1', 5, 'black', 80.0, 'cm');
INSERT INTO shoelace_data VALUES ('sl2', 6, 'black', 100.0, 'cm');
INSERT INTO shoelace_data VALUES ('sl3', 0, 'black', 35.0 , 'inch');
INSERT INTO shoelace_data VALUES ('sl4', 8, 'black', 40.0 , 'inch');
INSERT INTO shoelace_data VALUES ('sl5', 4, 'brown', 1.0 , 'm');
INSERT INTO shoelace_data VALUES ('sl6', 0, 'brown', 0.9 , 'm');
INSERT INTO shoelace_data VALUES ('sl7', 7, 'brown', 60 , 'cm');
INSERT INTO shoelace_data VALUES ('sl8', 1, 'brown', 40 , 'inch');

SELECT * FROM shoelace;

 sl_name   | sl_avail | sl_color | sl_len | sl_unit | sl_len_cm
-----------+----------+----------+--------+---------+-----------
 sl1       |        5 | black    |     80 | cm      |        80
 sl2       |        6 | black    |    100 | cm      |       100
 sl7       |        7 | brown    |     60 | cm      |        60
 sl3       |        0 | black    |     35 | inch    |      88.9
 sl4       |        8 | black    |     40 | inch    |     101.6
 sl8       |        1 | brown    |     40 | inch    |     101.6
 sl5       |        4 | brown    |      1 | m       |       100
 sl6       |        0 | brown    |    0.9 | m       |        90
(8 rows)
</programlisting>
   </para>

   <para>
    This is the simplest <command>SELECT</command> you can do on our
    views, so we take this opportunity to explain the basics of view
    rules.  The <literal>SELECT * FROM shoelace</literal> was
    interpreted by the parser and produced the query tree:

<programlisting>
SELECT shoelace.sl_name, shoelace.sl_avail,
       shoelace.sl_color, shoelace.sl_len,
       shoelace.sl_unit, shoelace.sl_len_cm
  FROM shoelace shoelace;
</programlisting>

    and this is given to the rule system. The rule system walks through