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diff --git a/doc/src/sgml/html/ddl-generated-columns.html b/doc/src/sgml/html/ddl-generated-columns.html new file mode 100644 index 0000000..cbe2247 --- /dev/null +++ b/doc/src/sgml/html/ddl-generated-columns.html @@ -0,0 +1,85 @@ +<?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 V1.79.1" /><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 xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 13.4 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></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 xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></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 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 5.4. Constraints</td></tr></table></div></body></html>
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