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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>5.5. System Columns</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="ddl-constraints.html" title="5.4. Constraints" /><link rel="next" href="ddl-alter.html" title="5.6. Modifying Tables" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.5. System Columns</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-constraints.html" title="5.4. Constraints">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-alter.html" title="5.6. Modifying Tables">Next</a></td></tr></table><hr /></div><div class="sect1" id="DDL-SYSTEM-COLUMNS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.5. System Columns</h2></div></div></div><p>
Every table has several <em class="firstterm">system columns</em> that are
implicitly defined by the system. Therefore, these names cannot be
used as names of user-defined columns. (Note that these
restrictions are separate from whether the name is a key word or
not; quoting a name will not allow you to escape these
restrictions.) You do not really need to be concerned about these
columns; just know they exist.
</p><a id="id-1.5.4.7.3" class="indexterm"></a><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="structfield">tableoid</code></span></dt><dd><a id="id-1.5.4.7.4.1.2.1" class="indexterm"></a><p>
The OID of the table containing this row. This column is
particularly handy for queries that select from partitioned
tables (see <a class="xref" href="ddl-partitioning.html" title="5.11. Table Partitioning">Section 5.11</a>) or inheritance
hierarchies (see <a class="xref" href="ddl-inherit.html" title="5.10. Inheritance">Section 5.10</a>), since without it,
it's difficult to tell which individual table a row came from. The
<code class="structfield">tableoid</code> can be joined against the
<code class="structfield">oid</code> column of
<code class="structname">pg_class</code> to obtain the table name.
</p></dd><dt><span class="term"><code class="structfield">xmin</code></span></dt><dd><a id="id-1.5.4.7.4.2.2.1" class="indexterm"></a><p>
The identity (transaction ID) of the inserting transaction for
this row version. (A row version is an individual state of a
row; each update of a row creates a new row version for the same
logical row.)
</p></dd><dt><span class="term"><code class="structfield">cmin</code></span></dt><dd><a id="id-1.5.4.7.4.3.2.1" class="indexterm"></a><p>
The command identifier (starting at zero) within the inserting
transaction.
</p></dd><dt><span class="term"><code class="structfield">xmax</code></span></dt><dd><a id="id-1.5.4.7.4.4.2.1" class="indexterm"></a><p>
The identity (transaction ID) of the deleting transaction, or
zero for an undeleted row version. It is possible for this column to
be nonzero in a visible row version. That usually indicates that the
deleting transaction hasn't committed yet, or that an attempted
deletion was rolled back.
</p></dd><dt><span class="term"><code class="structfield">cmax</code></span></dt><dd><a id="id-1.5.4.7.4.5.2.1" class="indexterm"></a><p>
The command identifier within the deleting transaction, or zero.
</p></dd><dt><span class="term"><code class="structfield">ctid</code></span></dt><dd><a id="id-1.5.4.7.4.6.2.1" class="indexterm"></a><p>
The physical location of the row version within its table. Note that
although the <code class="structfield">ctid</code> can be used to
locate the row version very quickly, a row's
<code class="structfield">ctid</code> will change if it is
updated or moved by <code class="command">VACUUM FULL</code>. Therefore
<code class="structfield">ctid</code> is useless as a long-term row
identifier. A primary key should be used to identify logical rows.
</p></dd></dl></div><p>
Transaction identifiers are also 32-bit quantities. In a
long-lived database it is possible for transaction IDs to wrap
around. This is not a fatal problem given appropriate maintenance
procedures; see <a class="xref" href="maintenance.html" title="Chapter 25. Routine Database Maintenance Tasks">Chapter 25</a> for details. It is
unwise, however, to depend on the uniqueness of transaction IDs
over the long term (more than one billion transactions).
</p><p>
Command identifiers are also 32-bit quantities. This creates a hard limit
of 2<sup>32</sup> (4 billion) <acronym class="acronym">SQL</acronym> commands
within a single transaction. In practice this limit is not a
problem — note that the limit is on the number of
<acronym class="acronym">SQL</acronym> commands, not the number of rows processed.
Also, only commands that actually modify the database contents will
consume a command identifier.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-constraints.html" title="5.4. Constraints">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-alter.html" title="5.6. Modifying Tables">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.4. Constraints </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 5.6. Modifying Tables</td></tr></table></div></body></html>
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