From 5d06e99a3cfc23bbc217b4d78b8c070ad52f720e Mon Sep 17 00:00:00 2001
From: Peter Eisentraut
Date: Thu, 4 Jan 2024 15:45:35 +0100
Subject: ALTER TABLE command to change generation expression

This adds a new ALTER TABLE subcommand ALTER COLUMN ... SET EXPRESSION
that changes the generation expression of a generated column.

The syntax is not standard but was adapted from other SQL
implementations.

This command causes a table rewrite, using the usual ALTER TABLE
mechanisms.  The implementation is similar to and makes use of some of
the infrastructure of the SET DATA TYPE subcommand (for example,
rebuilding constraints and indexes afterwards).  The new command
requires a new pass in AlterTablePass, and the ADD COLUMN pass had to
be moved earlier so that combinations of ADD COLUMN and SET EXPRESSION
can work.

Author: Amul Sul <sulamul@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b94yyJeGA-5M951_Lr+KfZokOp-2kXicpmEhi5FXhBeTog@mail.gmail.com
---
 doc/src/sgml/ref/alter_table.sgml | 12 ++++++++++++
 1 file changed, 12 insertions(+)

(limited to 'doc/src')

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index e1d207bc60b..eaada230248 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -46,6 +46,7 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET DEFAULT <replaceable class="parameter">expression</replaceable>
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP DEFAULT
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET | DROP } NOT NULL
+    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET EXPRESSION AS ( <replaceable class="parameter">expression</replaceable> )
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP EXPRESSION [ IF EXISTS ]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ]
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET GENERATED { ALWAYS | BY DEFAULT } | SET <replaceable>sequence_option</replaceable> | RESTART [ [ WITH ] <replaceable class="parameter">restart</replaceable> ] } [...]
@@ -256,6 +257,17 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     </listitem>
    </varlistentry>
 
+   <varlistentry id="sql-altertable-desc-set-expression">
+    <term><literal>SET EXPRESSION AS</literal></term>
+    <listitem>
+     <para>
+      This form replaces the expression of a generated column.  Existing data
+      in the column is rewritten and all the future changes will apply the new
+      generation expression.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry id="sql-altertable-desc-drop-expression">
     <term><literal>DROP EXPRESSION [ IF EXISTS ]</literal></term>
     <listitem>
-- 
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