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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>47.5. Visibility of Data Changes</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="spi-spi-start-transaction.html" title="SPI_start_transaction" /><link rel="next" href="spi-examples.html" title="47.6. Examples" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">47.5. Visibility of Data Changes</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="spi-spi-start-transaction.html" title="SPI_start_transaction">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="spi.html" title="Chapter 47. Server Programming Interface">Up</a></td><th width="60%" align="center">Chapter 47. Server Programming Interface</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="spi-examples.html" title="47.6. Examples">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="SPI-VISIBILITY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">47.5. 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 class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><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 class="listitem"><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 class="listitem"><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 cannot 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 class="listitem"><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.
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