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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!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 16.3 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 <a href="#DDL-GENERATED-COLUMNS" class="id_link">#</a></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 and partitioning:</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p>
If a parent column is a generated column, its child column must also
be a generated column; however, the child column can have a
different generation expression. The generation expression that is
actually applied during insert or update of a row is the one
associated with the table that the row is physically in.
(This is unlike the behavior for column defaults: for those, the
default value associated with the table named in the query applies.)
</p></li><li class="listitem"><p>
If a parent column is not a generated column, its child column must
not be generated either.
</p></li><li class="listitem"><p>
For inherited tables, if you write a child column definition without
any <code class="literal">GENERATED</code> clause in <code class="command">CREATE TABLE
... INHERITS</code>, then its <code class="literal">GENERATED</code> clause
will automatically be copied from the parent. <code class="command">ALTER TABLE
... INHERIT</code> will insist that parent and child columns
already match as to generation status, but it will not require their
generation expressions to match.
</p></li><li class="listitem"><p>
Similarly for partitioned tables, if you write a child column
definition without any <code class="literal">GENERATED</code> clause
in <code class="command">CREATE TABLE ... PARTITION OF</code>, then
its <code class="literal">GENERATED</code> clause will automatically be copied
from the parent. <code class="command">ALTER TABLE ... ATTACH PARTITION</code>
will insist that parent and child columns already match as to
generation status, but it will not require their generation
expressions to match.
</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. If they
do not all have the same generation expression, then the desired
expression for the child must be specified explicitly.
</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 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 5.4. Constraints</td></tr></table></div></body></html>
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