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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>74.1. Transactions and Identifiers</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="transactions.html" title="Chapter 74. Transaction Processing" /><link rel="next" href="xact-locking.html" title="74.2. Transactions and Locking" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">74.1. Transactions and Identifiers</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="transactions.html" title="Chapter 74. Transaction Processing">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 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="xact-locking.html" title="74.2. Transactions and Locking">Next</a></td></tr></table><hr /></div><div class="sect1" id="TRANSACTION-ID"><div class="titlepage"><div><div><h2 class="title" style="clear: both">74.1. Transactions and Identifiers <a href="#TRANSACTION-ID" class="id_link">#</a></h2></div></div></div><p>
Transactions can be created explicitly using <code class="command">BEGIN</code>
or <code class="command">START TRANSACTION</code> and ended using
<code class="command">COMMIT</code> or <code class="command">ROLLBACK</code>. SQL
statements outside of explicit transactions automatically use
single-statement transactions.
</p><p>
Every transaction is identified by a unique
<code class="literal">VirtualTransactionId</code> (also called
<code class="literal">virtualXID</code> or <code class="literal">vxid</code>), which
is comprised of a backend ID (or <code class="literal">backendID</code>)
and a sequentially-assigned number local to each backend, known as
<code class="literal">localXID</code>. For example, the virtual transaction
ID <code class="literal">4/12532</code> has a <code class="literal">backendID</code>
of <code class="literal">4</code> and a <code class="literal">localXID</code> of
<code class="literal">12532</code>.
</p><p>
Non-virtual <code class="literal">TransactionId</code>s (or <code class="type">xid</code>),
e.g., <code class="literal">278394</code>, are assigned sequentially to
transactions from a global counter used by all databases within
the <span class="productname">PostgreSQL</span> cluster. This assignment
happens when a transaction first writes to the database. This means
lower-numbered xids started writing before higher-numbered xids.
Note that the order in which transactions perform their first database
write might be different from the order in which the transactions
started, particularly if the transaction started with statements that
only performed database reads.
</p><p>
The internal transaction ID type <code class="type">xid</code> is 32 bits wide
and <a class="link" href="routine-vacuuming.html#VACUUM-FOR-WRAPAROUND" title="25.1.5. Preventing Transaction ID Wraparound Failures">wraps around</a> every
4 billion transactions. A 32-bit epoch is incremented during each
wraparound. There is also a 64-bit type <code class="type">xid8</code> which
includes this epoch and therefore does not wrap around during the
life of an installation; it can be converted to xid by casting.
The functions in <a class="xref" href="functions-info.html#FUNCTIONS-PG-SNAPSHOT" title="Table 9.80. Transaction ID and Snapshot Information Functions">Table 9.80</a>
return <code class="type">xid8</code> values. Xids are used as the
basis for <span class="productname">PostgreSQL</span>'s <a class="link" href="mvcc.html" title="Chapter 13. Concurrency Control">MVCC</a> concurrency mechanism and streaming
replication.
</p><p>
When a top-level transaction with a (non-virtual) xid commits,
it is marked as committed in the <code class="filename">pg_xact</code>
directory. Additional information is recorded in the
<code class="filename">pg_commit_ts</code> directory if <a class="xref" href="runtime-config-replication.html#GUC-TRACK-COMMIT-TIMESTAMP">track_commit_timestamp</a> is enabled.
</p><p>
In addition to <code class="literal">vxid</code> and <code class="type">xid</code>,
prepared transactions are also assigned Global Transaction
Identifiers (<acronym class="acronym">GID</acronym>). GIDs are string literals up
to 200 bytes long, which must be unique amongst other currently
prepared transactions. The mapping of GID to xid is shown in <a class="link" href="view-pg-prepared-xacts.html" title="54.16. pg_prepared_xacts"><code class="structname">pg_prepared_xacts</code></a>.
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