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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>8.20. pg_lsn Type</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="datatype-oid.html" title="8.19. Object Identifier Types" /><link rel="next" href="datatype-pseudo.html" title="8.21. Pseudo-Types" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">8.20. <code class="type">pg_lsn</code> Type</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="datatype-oid.html" title="8.19. Object Identifier Types">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="datatype.html" title="Chapter 8. Data Types">Up</a></td><th width="60%" align="center">Chapter 8. Data Types</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="datatype-pseudo.html" title="8.21. Pseudo-Types">Next</a></td></tr></table><hr /></div><div class="sect1" id="DATATYPE-PG-LSN"><div class="titlepage"><div><div><h2 class="title" style="clear: both">8.20. <code class="type">pg_lsn</code> Type <a href="#DATATYPE-PG-LSN" class="id_link">#</a></h2></div></div></div><a id="id-1.5.7.28.2" class="indexterm"></a><p>
The <code class="type">pg_lsn</code> data type can be used to store LSN (Log Sequence
Number) data which is a pointer to a location in the WAL. This type is a
representation of <code class="type">XLogRecPtr</code> and an internal system type of
<span class="productname">PostgreSQL</span>.
</p><p>
Internally, an LSN is a 64-bit integer, representing a byte position in
the write-ahead log stream. It is printed as two hexadecimal numbers of
up to 8 digits each, separated by a slash; for example,
<code class="literal">16/B374D848</code>. The <code class="type">pg_lsn</code> type supports the
standard comparison operators, like <code class="literal">=</code> and
<code class="literal">></code>. Two LSNs can be subtracted using the
<code class="literal">-</code> operator; the result is the number of bytes separating
those write-ahead log locations. Also the number of bytes can be
added into and subtracted from LSN using the
<code class="literal">+(pg_lsn,numeric)</code> and
<code class="literal">-(pg_lsn,numeric)</code> operators, respectively. Note that
the calculated LSN should be in the range of <code class="type">pg_lsn</code> type,
i.e., between <code class="literal">0/0</code> and
<code class="literal">FFFFFFFF/FFFFFFFF</code>.
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