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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" ""><html xmlns=""><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="" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><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><div xmlns="" 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 12.0 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></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-" class="indexterm"></a><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="structfield">tableoid</code></span></dt><dd><a id="id-" class="indexterm"></a><p>
+ The OID of the table containing this row. This column is
+ particularly handy for queries that select from 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-" 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-" 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-" 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-" 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-" 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 24. Routine Database Maintenance Tasks">Chapter 24</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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-alter.html">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">Home</a></td><td width="40%" align="right" valign="top"> 5.6. Modifying Tables</td></tr></table></div></body></html> \ No newline at end of file