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<!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>74.3. 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="xact-locking.html" title="74.2. Transactions and Locking" /><link rel="next" href="two-phase.html" title="74.4. Two-Phase Transactions" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">74.3. Subtransactions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xact-locking.html" title="74.2. Transactions and Locking">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="transactions.html" title="Chapter 74. Transaction Processing">Up</a></td><th width="60%" align="center">Chapter 74. Transaction Processing</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="two-phase.html" title="74.4. Two-Phase Transactions">Next</a></td></tr></table><hr /></div><div class="sect1" id="SUBXACTS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">74.3. Subtransactions <a href="#SUBXACTS" class="id_link">#</a></h2></div></div></div><p>
Subtransactions are started inside transactions, allowing large
transactions to be broken into smaller units. Subtransactions can
commit or abort without affecting their parent transactions, allowing
parent transactions to continue. This allows errors to be handled
more easily, which is a common application development pattern.
The word subtransaction is often abbreviated as
<em class="firstterm">subxact</em>.
</p><p>
Subtransactions can be started explicitly using the
<code class="command">SAVEPOINT</code> command, but can also be started in
other ways, such as PL/pgSQL's <code class="command">EXCEPTION</code> clause.
PL/Python and PL/Tcl also support explicit subtransactions.
Subtransactions can also be started from other subtransactions.
The top-level transaction and its child subtransactions form a
hierarchy or tree, which is why we refer to the main transaction as
the top-level transaction.
</p><p>
If a subtransaction is assigned a non-virtual transaction ID,
its transaction ID is referred to as a <span class="quote">“<span class="quote">subxid</span>”</span>.
Read-only subtransactions are not assigned subxids, but once they
attempt to write, they will be assigned one. This also causes all of
a subxid's parents, up to and including the top-level transaction,
to be assigned non-virtual transaction ids. We ensure that a parent
xid is always lower than any of its child subxids.
</p><p>
The immediate parent xid of each subxid is recorded in the
<code class="filename">pg_subtrans</code> directory. No entry is made for
top-level xids since they do not have a parent, nor is an entry made
for read-only subtransactions.
</p><p>
When a subtransaction commits, all of its committed child
subtransactions with subxids will also be considered subcommitted
in that transaction. When a subtransaction aborts, all of its child
subtransactions will also be considered aborted.
</p><p>
When a top-level transaction with an xid commits, all of its
subcommitted child subtransactions are also persistently recorded
as committed in the <code class="filename">pg_xact</code> subdirectory. If the
top-level transaction aborts, all its subtransactions are also aborted,
even if they were subcommitted.
</p><p>
The more subtransactions each transaction keeps open (not
rolled back or released), the greater the transaction management
overhead. Up to 64 open subxids are cached in shared memory for
each backend; after that point, the storage I/O overhead increases
significantly due to additional lookups of subxid entries in
<code class="filename">pg_subtrans</code>.
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