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<?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>44.9. Explicit Subtransactions in PL/Tcl</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="pltcl-error-handling.html" title="44.8. Error Handling in PL/Tcl" /><link rel="next" href="pltcl-transactions.html" title="44.10. 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">44.9. Explicit Subtransactions in PL/Tcl</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pltcl-error-handling.html" title="44.8. Error Handling in PL/Tcl">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="pltcl.html" title="Chapter 44. PL/Tcl — Tcl Procedural Language">Up</a></td><th width="60%" align="center">Chapter 44. PL/Tcl — Tcl Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pltcl-transactions.html" title="44.10. Transaction Management">Next</a></td></tr></table><hr /></div><div class="sect1" id="PLTCL-SUBTRANSACTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">44.9. Explicit Subtransactions in PL/Tcl</h2></div></div></div><a id="id-1.8.9.13.2" class="indexterm"></a><p>
Recovering from errors caused by database access as described in
<a class="xref" href="pltcl-error-handling.html" title="44.8. Error Handling in PL/Tcl">Section 44.8</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/Tcl offers a solution to this problem in
the form of explicit subtransactions.
</p><p>
Consider a function that implements a transfer between two accounts:
</p><pre class="programlisting">
CREATE FUNCTION transfer_funds() RETURNS void AS $$
if [catch {
spi_exec "UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'"
spi_exec "UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'"
} errormsg] {
set result [format "error transferring funds: %s" $errormsg]
} else {
set result "funds transferred successfully"
}
spi_exec "INSERT INTO operations (result) VALUES ('[quote $result]')"
$$ LANGUAGE pltcl;
</pre><p>
If the second <code class="command">UPDATE</code> statement results in an
exception being raised, this function will log the failure, but
the result of the first <code class="command">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. This happens because each <code class="function">spi_exec</code>
is a separate subtransaction, and only one of those subtransactions
got rolled back.
</p><p>
To handle such cases, you can wrap multiple database operations in an
explicit subtransaction, which will succeed or roll back as a whole.
PL/Tcl provides a <code class="function">subtransaction</code> command to manage
this. We can rewrite our function as:
</p><pre class="programlisting">
CREATE FUNCTION transfer_funds2() RETURNS void AS $$
if [catch {
subtransaction {
spi_exec "UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'"
spi_exec "UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'"
}
} errormsg] {
set result [format "error transferring funds: %s" $errormsg]
} else {
set result "funds transferred successfully"
}
spi_exec "INSERT INTO operations (result) VALUES ('[quote $result]')"
$$ LANGUAGE pltcl;
</pre><p>
Note that use of <code class="function">catch</code> is still required for this
purpose. Otherwise the error would propagate to the top level of the
function, preventing the desired insertion into
the <code class="structname">operations</code> table.
The <code class="function">subtransaction</code> command does not trap errors, it
only assures that all database operations executed inside its scope will
be rolled back together when an error is reported.
</p><p>
A rollback of an explicit subtransaction occurs on any error reported
by the contained Tcl code, not only errors originating from database
access. Thus a regular Tcl exception raised inside
a <code class="function">subtransaction</code> command will also cause the
subtransaction to be rolled back. However, non-error exits out of the
contained Tcl code (for instance, due to <code class="function">return</code>) do
not cause a rollback.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pltcl-error-handling.html" title="44.8. Error Handling in PL/Tcl">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pltcl.html" title="Chapter 44. PL/Tcl — Tcl Procedural Language">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pltcl-transactions.html" title="44.10. Transaction Management">Next</a></td></tr><tr><td width="40%" align="left" valign="top">44.8. Error Handling in PL/Tcl </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 44.10. Transaction Management</td></tr></table></div></body></html>
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