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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>5.3. Generated 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-default.html" title="5.2. Default Values" /><link rel="next" href="ddl-constraints.html" title="5.4. Constraints" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.3. Generated Columns</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-default.html" title="5.2. Default Values">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-constraints.html" title="5.4. Constraints">Next</a></td></tr></table><hr /></div><div class="sect1" id="DDL-GENERATED-COLUMNS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.3. Generated Columns</h2></div></div></div><a id="id-1.5.4.5.2" class="indexterm"></a><p>
A generated column is a special column that is always computed from other
columns. Thus, it is for columns what a view is for tables. There are two
kinds of generated columns: stored and virtual. A stored generated column
is computed when it is written (inserted or updated) and occupies storage
as if it were a normal column. A virtual generated column occupies no
storage and is computed when it is read. Thus, a virtual generated column
is similar to a view and a stored generated column is similar to a
materialized view (except that it is always updated automatically).
PostgreSQL currently implements only stored generated columns.
</p><p>
To create a generated column, use the <code class="literal">GENERATED ALWAYS
AS</code> clause in <code class="command">CREATE TABLE</code>, for example:
</p><pre class="programlisting">
CREATE TABLE people (
...,
height_cm numeric,
height_in numeric <span class="emphasis"><strong>GENERATED ALWAYS AS (height_cm / 2.54) STORED</strong></span>
);
</pre><p>
The keyword <code class="literal">STORED</code> must be specified to choose the
stored kind of generated column. See <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> for
more details.
</p><p>
A generated column cannot be written to directly. In
<code class="command">INSERT</code> or <code class="command">UPDATE</code> commands, a value
cannot be specified for a generated column, but the keyword
<code class="literal">DEFAULT</code> may be specified.
</p><p>
Consider the differences between a column with a default and a generated
column. The column default is evaluated once when the row is first
inserted if no other value was provided; a generated column is updated
whenever the row changes and cannot be overridden. A column default may
not refer to other columns of the table; a generation expression would
normally do so. A column default can use volatile functions, for example
<code class="literal">random()</code> or functions referring to the current time;
this is not allowed for generated columns.
</p><p>
Several restrictions apply to the definition of generated columns and
tables involving generated columns:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
The generation expression can only use immutable functions and cannot
use subqueries or reference anything other than the current row in any
way.
</p></li><li class="listitem"><p>
A generation expression cannot reference another generated column.
</p></li><li class="listitem"><p>
A generation expression cannot reference a system column, except
<code class="varname">tableoid</code>.
</p></li><li class="listitem"><p>
A generated column cannot have a column default or an identity definition.
</p></li><li class="listitem"><p>
A generated column cannot be part of a partition key.
</p></li><li class="listitem"><p>
Foreign tables can have generated columns. See <a class="xref" href="sql-createforeigntable.html" title="CREATE FOREIGN TABLE"><span class="refentrytitle">CREATE FOREIGN TABLE</span></a> for details.
</p></li><li class="listitem"><p>For inheritance:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p>
If a parent column is a generated column, a child column must also be
a generated column using the same expression. In the definition of
the child column, leave off the <code class="literal">GENERATED</code> clause,
as it will be copied from the parent.
</p></li><li class="listitem"><p>
In case of multiple inheritance, if one parent column is a generated
column, then all parent columns must be generated columns and with the
same expression.
</p></li><li class="listitem"><p>
If a parent column is not a generated column, a child column may be
defined to be a generated column or not.
</p></li></ul></div></li></ul></div><p>
</p><p>
Additional considerations apply to the use of generated columns.
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
Generated columns maintain access privileges separately from their
underlying base columns. So, it is possible to arrange it so that a
particular role can read from a generated column but not from the
underlying base columns.
</p></li><li class="listitem"><p>
Generated columns are, conceptually, updated after
<code class="literal">BEFORE</code> triggers have run. Therefore, changes made to
base columns in a <code class="literal">BEFORE</code> trigger will be reflected in
generated columns. But conversely, it is not allowed to access
generated columns in <code class="literal">BEFORE</code> triggers.
</p></li></ul></div><p>
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-default.html" title="5.2. Default Values">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-constraints.html" title="5.4. Constraints">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.2. Default Values </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"> 5.4. Constraints</td></tr></table></div></body></html>
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