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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>46.7. Explicit Subtransactions</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="plpython-database.html" title="46.6. Database Access" /><link rel="next" href="plpython-transactions.html" title="46.8. Transaction Management" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">46.7. Explicit Subtransactions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpython-database.html" title="46.6. Database Access">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpython.html" title="Chapter 46. PL/Python — Python Procedural Language">Up</a></td><th width="60%" align="center">Chapter 46. PL/Python — Python Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpython-transactions.html" title="46.8. Transaction Management">Next</a></td></tr></table><hr /></div><div class="sect1" id="PLPYTHON-SUBTRANSACTION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">46.7. Explicit Subtransactions</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="plpython-subtransaction.html#id-1.8.11.15.3">46.7.1. Subtransaction Context Managers</a></span></dt></dl></div><p>
Recovering from errors caused by database access as described in
<a class="xref" href="plpython-database.html#PLPYTHON-TRAPPING" title="46.6.2. Trapping Errors">Section 46.6.2</a> can lead to an undesirable
situation where some operations succeed before one of them fails,
and after recovering from that error the data is left in an
inconsistent state. PL/Python offers a solution to this problem in
the form of explicit subtransactions.
</p><div class="sect2" id="id-1.8.11.15.3"><div class="titlepage"><div><div><h3 class="title">46.7.1. Subtransaction Context Managers</h3></div></div></div><p>
Consider a function that implements a transfer between two
accounts:
</p><pre class="programlisting">
CREATE FUNCTION transfer_funds() RETURNS void AS $$
try:
plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
except plpy.SPIError as e:
result = "error transferring funds: %s" % e.args
else:
result = "funds transferred correctly"
plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
plpy.execute(plan, [result])
$$ LANGUAGE plpython3u;
</pre><p>
If the second <code class="literal">UPDATE</code> statement results in an
exception being raised, this function will report the error, but
the result of the first <code class="literal">UPDATE</code> will
nevertheless be committed. In other words, the funds will be
withdrawn from Joe's account, but will not be transferred to
Mary's account.
</p><p>
To avoid such issues, you can wrap your
<code class="literal">plpy.execute</code> calls in an explicit
subtransaction. The <code class="literal">plpy</code> module provides a
helper object to manage explicit subtransactions that gets created
with the <code class="literal">plpy.subtransaction()</code> function.
Objects created by this function implement the
<a class="ulink" href="https://docs.python.org/library/stdtypes.html#context-manager-types" target="_top">
context manager interface</a>. Using explicit subtransactions
we can rewrite our function as:
</p><pre class="programlisting">
CREATE FUNCTION transfer_funds2() RETURNS void AS $$
try:
with plpy.subtransaction():
plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
except plpy.SPIError as e:
result = "error transferring funds: %s" % e.args
else:
result = "funds transferred correctly"
plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
plpy.execute(plan, [result])
$$ LANGUAGE plpython3u;
</pre><p>
Note that the use of <code class="literal">try</code>/<code class="literal">except</code> is still
required. Otherwise the exception would propagate to the top of
the Python stack and would cause the whole function to abort with
a <span class="productname">PostgreSQL</span> error, so that the
<code class="literal">operations</code> table would not have any row
inserted into it. The subtransaction context manager does not
trap errors, it only assures that all database operations executed
inside its scope will be atomically committed or rolled back. A
rollback of the subtransaction block occurs on any kind of
exception exit, not only ones caused by errors originating from
database access. A regular Python exception raised inside an
explicit subtransaction block would also cause the subtransaction
to be rolled back.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="plpython-database.html" title="46.6. Database Access">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpython.html" title="Chapter 46. PL/Python — Python Procedural Language">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpython-transactions.html" title="46.8. Transaction Management">Next</a></td></tr><tr><td width="40%" align="left" valign="top">46.6. Database Access </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 46.8. Transaction Management</td></tr></table></div></body></html>
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