|
- <?xml version="1.0" encoding="UTF-8" standalone="no"?>
- <!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>45.8. 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 V1.79.1" /><link rel="prev" href="plpython-database.html" title="45.7. Database Access" /><link rel="next" href="plpython-transactions.html" title="45.9. Transaction Management" /></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">45.8. Explicit Subtransactions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpython-database.html" title="45.7. Database Access">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpython.html" title="Chapter 45. PL/Python - Python Procedural Language">Up</a></td><th width="60%" align="center">Chapter 45. PL/Python - Python Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpython-transactions.html" title="45.9. Transaction Management">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PLPYTHON-SUBTRANSACTION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">45.8. Explicit Subtransactions</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="plpython-subtransaction.html#id-1.8.11.16.3">45.8.1. Subtransaction Context Managers</a></span></dt><dt><span class="sect2"><a href="plpython-subtransaction.html#id-1.8.11.16.4">45.8.2. Older Python Versions</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="45.7.2. Trapping Errors">Section 45.7.2</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-1.8.11.16.3"><div class="titlepage"><div><div><h3 class="title">45.8.1. 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 $$
- try:
- plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
- plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
- except plpy.SPIError, e:
- result = "error transferring funds: %s" % e.args
- else:
- result = "funds transferred correctly"
- plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
- plpy.execute(plan, [result])
- $$ LANGUAGE plpythonu;
- </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 $$
- try:
- with plpy.subtransaction():
- plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
- plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
- except plpy.SPIError, e:
- result = "error transferring funds: %s" % e.args
- else:
- result = "funds transferred correctly"
- plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
- plpy.execute(plan, [result])
- $$ LANGUAGE plpythonu;
- </pre><p>
- Note that the use of <code class="literal">try/catch</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 class="sect2" id="id-1.8.11.16.4"><div class="titlepage"><div><div><h3 class="title">45.8.2. Older Python Versions</h3></div></div></div><p>
- Context managers syntax using the <code class="literal">with</code> keyword
- is available by default in Python 2.6. If using PL/Python with an
- older Python version, it is still possible to use explicit
- subtransactions, although not as transparently. You can call the
- subtransaction manager's <code class="literal">__enter__</code> and
- <code class="literal">__exit__</code> functions using the
- <code class="literal">enter</code> and <code class="literal">exit</code> convenience
- aliases. The example function that transfers funds could be
- written as:
- </p><pre class="programlisting">
- CREATE FUNCTION transfer_funds_old() RETURNS void AS $$
- try:
- subxact = plpy.subtransaction()
- subxact.enter()
- try:
- plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
- plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
- except:
- import sys
- subxact.exit(*sys.exc_info())
- raise
- else:
- subxact.exit(None, None, None)
- except plpy.SPIError, e:
- result = "error transferring funds: %s" % e.args
- else:
- result = "funds transferred correctly"
-
- plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
- plpy.execute(plan, [result])
- $$ LANGUAGE plpythonu;
- </pre><p>
- </p><div class="note"><h3 class="title">Note</h3><p>
- Although context managers were implemented in Python 2.5, to use
- the <code class="literal">with</code> syntax in that version you need to
- use a <a class="ulink" href="https://docs.python.org/release/2.5/ref/future.html" target="_top">future
- statement</a>. Because of implementation details, however,
- you cannot use future statements in PL/Python functions.
- </p></div></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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpython.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpython-transactions.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">45.7. Database Access </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 45.9. Transaction Management</td></tr></table></div></body></html>
|
