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diff --git a/doc/src/sgml/html/pltcl-subtransactions.html b/doc/src/sgml/html/pltcl-subtransactions.html new file mode 100644 index 0000000..59bd6c3 --- /dev/null +++ b/doc/src/sgml/html/pltcl-subtransactions.html @@ -0,0 +1,67 @@ +<?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 xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 14.5 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></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 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="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 14.5 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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