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<title>40.4.Visibility of Data Changes</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="spi-visibility"></a>40.4.Visibility of Data Changes</h2></div></div></div>
<p> The following rules govern the visibility of data changes in
functions that use SPI (or any other C function):
</p>
<div class="itemizedlist"><ul type="disc">
<li>
<p> During the execution of an SQL command, any data changes made by
the command are invisible to the command itself. For
example, in
</p>
<pre class="programlisting">INSERT INTO a SELECT * FROM a;</pre>
<p>
the inserted rows are invisible to the <code class="command">SELECT</code>
part.
</p>
</li>
<li><p> Changes made by a command C are visible to all commands that are
started after C, no matter whether they are started inside C
(during the execution of C) or after C is done.
</p></li>
<li><p> Commands executed via SPI inside a function called by an SQL command
(either an ordinary function or a trigger) follow one or the
other of the above rules depending on the read/write flag passed
to SPI. Commands executed in read-only mode follow the first
rule: they can't see changes of the calling command. Commands executed
in read-write mode follow the second rule: they can see all changes made
so far.
</p></li>
<li><p> All standard procedural languages set the SPI read-write mode
depending on the volatility attribute of the function. Commands of
<code class="literal">STABLE</code> and <code class="literal">IMMUTABLE</code> functions are done in
read-only mode, while commands of <code class="literal">VOLATILE</code> functions are
done in read-write mode. While authors of C functions are able to
violate this convention, it's unlikely to be a good idea to do so.
</p></li>
</ul></div>
<p>
</p>
<p> The next section contains an example that illustrates the
application of these rules.
</p>
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