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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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 16.3 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 <a href="#PLTCL-SUBTRANSACTIONS" class="id_link">#</a></h2></div></div></div><a id="id-1.8.9.13.2" class="indexterm"></a><p>3     Recovering from errors caused by database access as described in4     <a class="xref" href="pltcl-error-handling.html" title="44.8. Error Handling in PL/Tcl">Section 44.8</a> can lead to an undesirable5     situation where some operations succeed before one of them fails,6     and after recovering from that error the data is left in an7     inconsistent state.  PL/Tcl offers a solution to this problem in8     the form of explicit subtransactions.9    </p><p>10     Consider a function that implements a transfer between two accounts:11</p><pre class="programlisting">12CREATE FUNCTION transfer_funds() RETURNS void AS $$13    if [catch {14        spi_exec "UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'"15        spi_exec "UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'"16    } errormsg] {17        set result [format "error transferring funds: %s" $errormsg]18    } else {19        set result "funds transferred successfully"20    }21    spi_exec "INSERT INTO operations (result) VALUES ('[quote $result]')"22$$ LANGUAGE pltcl;23</pre><p>24     If the second <code class="command">UPDATE</code> statement results in an25     exception being raised, this function will log the failure, but26     the result of the first <code class="command">UPDATE</code> will27     nevertheless be committed.  In other words, the funds will be28     withdrawn from Joe's account, but will not be transferred to29     Mary's account.  This happens because each <code class="function">spi_exec</code>30     is a separate subtransaction, and only one of those subtransactions31     got rolled back.32    </p><p>33     To handle such cases, you can wrap multiple database operations in an34     explicit subtransaction, which will succeed or roll back as a whole.35     PL/Tcl provides a <code class="function">subtransaction</code> command to manage36     this.  We can rewrite our function as:37</p><pre class="programlisting">38CREATE FUNCTION transfer_funds2() RETURNS void AS $$39    if [catch {40        subtransaction {41            spi_exec "UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'"42            spi_exec "UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'"43        }44    } errormsg] {45        set result [format "error transferring funds: %s" $errormsg]46    } else {47        set result "funds transferred successfully"48    }49    spi_exec "INSERT INTO operations (result) VALUES ('[quote $result]')"50$$ LANGUAGE pltcl;51</pre><p>52     Note that use of <code class="function">catch</code> is still required for this53     purpose.  Otherwise the error would propagate to the top level of the54     function, preventing the desired insertion into55     the <code class="structname">operations</code> table.56     The <code class="function">subtransaction</code> command does not trap errors, it57     only assures that all database operations executed inside its scope will58     be rolled back together when an error is reported.59    </p><p>60     A rollback of an explicit subtransaction occurs on any error reported61     by the contained Tcl code, not only errors originating from database62     access.  Thus a regular Tcl exception raised inside63     a <code class="function">subtransaction</code> command will also cause the64     subtransaction to be rolled back.  However, non-error exits out of the65     contained Tcl code (for instance, due to <code class="function">return</code>) do66     not cause a rollback.67    </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 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 44.10. Transaction Management</td></tr></table></div></body></html>