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diff --git a/doc/src/sgml/html/plpython-subtransaction.html b/doc/src/sgml/html/plpython-subtransaction.html new file mode 100644 index 0000000..1a1951e --- /dev/null +++ b/doc/src/sgml/html/plpython-subtransaction.html @@ -0,0 +1,100 @@ +<?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>46.8. 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 Vsnapshot" /><link rel="prev" href="plpython-database.html" title="46.7. Database Access" /><link rel="next" href="plpython-transactions.html" title="46.9. Transaction Management" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">46.8. Explicit Subtransactions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpython-database.html" title="46.7. 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 14.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpython-transactions.html" title="46.9. 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">46.8. 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">46.8.1. Subtransaction Context Managers</a></span></dt><dt><span class="sect2"><a href="plpython-subtransaction.html#id-1.8.11.16.4">46.8.2. 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="46.7.2. Trapping Errors">Section 46.7.2</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">46.8.1. 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 $$ +try: + plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'") + plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'") +except plpy.SPIError as e: + result = "error transferring funds: %s" % e.args +else: + result = "funds transferred correctly" +plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"]) +plpy.execute(plan, [result]) +$$ LANGUAGE 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 $$ +try: + with plpy.subtransaction(): + plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'") + plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'") +except plpy.SPIError as e: + result = "error transferring funds: %s" % e.args +else: + result = "funds transferred correctly" +plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"]) +plpy.execute(plan, [result]) +$$ LANGUAGE 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">46.8.2. 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. For compatibility with + older Python versions, 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 $$ +try: + subxact = plpy.subtransaction() + subxact.enter() + try: + plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'") + plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'") + except: + import sys + subxact.exit(*sys.exc_info()) + raise + else: + subxact.exit(None, None, None) +except plpy.SPIError as e: + result = "error transferring funds: %s" % e.args +else: + result = "funds transferred correctly" + +plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"]) +plpy.execute(plan, [result]) +$$ LANGUAGE plpythonu; +</pre><p> + </p></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="plpython-database.html" title="46.7. 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