<literal>software</literal>
    that reference a deleted computer.  The trigger would use this command:

<programlisting>
DELETE FROM software WHERE hostname = $1;
</programlisting>

    Since the trigger is called for each individual row deleted from
    <literal>computer</literal>, it can prepare and save the plan for this
    command and pass the <structfield>hostname</structfield> value in the
    parameter.  The rule would be written as:

<programlisting>
CREATE RULE computer_del AS ON DELETE TO computer
    DO DELETE FROM software WHERE hostname = OLD.hostname;
</programlisting>
   </para>

   <para>
    Now we look at different types of deletes. In the case of a:

<programlisting>
DELETE FROM computer WHERE hostname = 'mypc.local.net';
</programlisting>

    the table <literal>computer</literal> is scanned by index (fast), and the
    command issued by the trigger would also use an index scan (also fast).
    The extra command from the rule would be:

<programlisting>
DELETE FROM software WHERE computer.hostname = 'mypc.local.net'
                       AND software.hostname = computer.hostname;
</programlisting>

    Since there are appropriate indexes set up, the planner
    will create a plan of

<literallayout class="monospaced">
Nestloop
  -&gt;  Index Scan using comp_hostidx on computer
  -&gt;  Index Scan using soft_hostidx on software
</literallayout>

    So there would be not that much difference in speed between
    the trigger and the rule implementation.
   </para>

   <para>
    With the next delete we want to get rid of all the 2000 computers
    where the <structfield>hostname</structfield> starts with
    <literal>old</literal>. There are two possible commands to do that. One
    is:

<programlisting>
DELETE FROM computer WHERE hostname &gt;= 'old'
                       AND hostname &lt;  'ole'
</programlisting>

    The command added by the rule will be:

<programlisting>
DELETE FROM software WHERE computer.hostname &gt;= 'old' AND computer.hostname &lt; 'ole'
                       AND software.hostname = computer.hostname;
</programlisting>

    with the plan

<literallayout class="monospaced">
Hash Join
  -&gt;  Seq Scan on software
  -&gt;  Hash
    -&gt;  Index Scan using comp_hostidx on computer
</literallayout>

    The other possible command is:

<programlisting>
DELETE FROM computer WHERE hostname ~ '^old';
</programlisting>

    which results