Virtual generated columns

This adds a new variant of generated columns that are computed on read
(like a view, unlike the existing stored generated columns, which are
computed on write, like a materialized view).

The syntax for the column definition is

    ... GENERATED ALWAYS AS (...) VIRTUAL

and VIRTUAL is also optional.  VIRTUAL is the default rather than
STORED to match various other SQL products.  (The SQL standard makes
no specification about this, but it also doesn't know about VIRTUAL or
STORED.)  (Also, virtual views are the default, rather than
materialized views.)

Virtual generated columns are stored in tuples as null values.  (A
very early version of this patch had the ambition to not store them at
all.  But so much stuff breaks or gets confused if you have tuples
where a column in the middle is completely missing.  This is a
compromise, and it still saves space over being forced to use stored
generated columns.  If we ever find a way to improve this, a bit of
pg_upgrade cleverness could allow for upgrades to a newer scheme.)

The capabilities and restrictions of virtual generated columns are
mostly the same as for stored generated columns.  In some cases, this
patch keeps virtual generated columns more restricted than they might
technically need to be, to keep the two kinds consistent.  Some of
that could maybe be relaxed later after separate careful
considerations.

Some functionality that is currently not supported, but could possibly
be added as incremental features, some easier than others:

- index on or using a virtual column
- hence also no unique constraints on virtual columns
- extended statistics on virtual columns
- foreign-key constraints on virtual columns
- not-null constraints on virtual columns (check constraints are supported)
- ALTER TABLE / DROP EXPRESSION
- virtual column cannot have domain type
- virtual columns are not supported in logical replication

The tests in generated_virtual.sql have been copied over from
generated_stored.sql with the keyword replaced.  This way we can make
sure the behavior is mostly aligned, and the differences can be
visible.  Some tests for currently not supported features are
currently commented out.

Reviewed-by: Jian He <jian.universality@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Tested-by: Shlok Kyal <shlok.kyal.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/a368248e-69e4-40be-9c07-6c3b5880b0a6@eisentraut.org

6 files changed, 74 insertions, 30 deletions

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 088fb175cce..ee59a7e15d0 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -1307,8 +1307,10 @@
       </para>
       <para>
        If a zero byte (<literal>''</literal>), then not a generated column.
-       Otherwise, <literal>s</literal> = stored.  (Other values might be added
-       in the future.)
+       Otherwise, <literal>s</literal> = stored, <literal>v</literal> =
+       virtual.  A stored generated column is physically stored like a normal
+       column.  A virtual generated column is physically stored as a null
+       value, with the actual value being computed at run time.
       </para></entry>
      </row>
 
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 7ff39ae8c67..ae156b6b1cd 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -361,7 +361,6 @@ INSERT INTO people (id, name, address) VALUES (<emphasis>DEFAULT</emphasis>, 'C'
    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).
-   <productname>PostgreSQL</productname> currently implements only stored generated columns.
   </para>
 
   <para>
@@ -371,12 +370,12 @@ INSERT INTO people (id, name, address) VALUES (<emphasis>DEFAULT</emphasis>, 'C'
 CREATE TABLE people (
     ...,
     height_cm numeric,
-    height_in numeric <emphasis>GENERATED ALWAYS AS (height_cm / 2.54) STORED</emphasis>
+    height_in numeric <emphasis>GENERATED ALWAYS AS (height_cm / 2.54)</emphasis>
 );
 </programlisting>
-   The keyword <literal>STORED</literal> must be specified to choose the
-   stored kind of generated column.  See <xref linkend="sql-createtable"/> for
-   more details.
+   A generated column is by default of the virtual kind.  Use the keywords
+   <literal>VIRTUAL</literal> or <literal>STORED</literal> to make the choice
+   explicit.  See <xref linkend="sql-createtable"/> for more details.
   </para>
 
   <para>
@@ -442,12 +441,18 @@ CREATE TABLE people (
       <listitem>
        <para>
         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
+        be a generated column of the same kind (stored or virtual); however,
+        the child column can have a different generation expression.
+       </para>
+
+       <para>
+        For stored generated columns, 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.)
+        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.)  For virtual
+        generated columns, the generation expression of the table named in the
+        query applies when a table is read.
        </para>
       </listitem>
       <listitem>
@@ -502,6 +507,26 @@ CREATE TABLE people (
       particular role can read from a generated column but not from the
       underlying base columns.
      </para>
+
+     <para>
+      For virtual generated columns, this is only fully secure if the
+      generation expression uses only leakproof functions (see <xref
+      linkend="sql-createfunction"/>), but this is not enforced by the system.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      Privileges of functions used in generation expressions are checked when
+      the expression is actually executed, on write or read respectively, as
+      if the generation expression had been called directly from the query
+      using the generated column.  The user of a generated column must have
+      permissions to call all functions used by the generation expression.
+      Functions in the generation expression are executed with the privileges
+      of the user executing the query or the function owner, depending on
+      whether the functions are defined as <literal>SECURITY INVOKER</literal>
+      or <literal>SECURITY DEFINER</literal>.
+      <!-- matches create_view.sgml -->
+     </para>
     </listitem>
     <listitem>
      <para>
@@ -519,6 +544,7 @@ CREATE TABLE people (
       <link linkend="sql-createpublication-params-with-publish-generated-columns">
       <literal>publish_generated_columns</literal></link> or by including them
       in the column list of the <command>CREATE PUBLICATION</command> command.
+      This is currently only supported for stored generated columns.
       See <xref linkend="logical-replication-gencols"/> for details.
      </para>
     </listitem>
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index f9576da435e..8e56b8e59b0 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -102,7 +102,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
   NULL |
   CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
   DEFAULT <replaceable>default_expr</replaceable> |
-  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED |
+  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ] |
   GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
   UNIQUE [ NULLS [ NOT ] DISTINCT ] <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
@@ -264,8 +264,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     <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.
+      in a stored generated column is rewritten and all the future changes
+      will apply the new generation expression.
      </para>
     </listitem>
    </varlistentry>
@@ -280,9 +280,14 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      </para>
 
      <para>
+      This form is currently only supported for stored generated columns (not
+      virtual ones).
+     </para>
+
+     <para>
       If <literal>DROP EXPRESSION IF EXISTS</literal> is specified and the
-      column is not a stored generated column, no error is thrown.  In this
-      case a notice is issued instead.
+      column is not a generated column, no error is thrown.  In this case a
+      notice is issued instead.
      </para>
     </listitem>
    </varlistentry>
diff --git a/doc/src/sgml/ref/create_foreign_table.sgml b/doc/src/sgml/ref/create_foreign_table.sgml
index 0dcd9ca6f87..e0b0e075c2c 100644
--- a/doc/src/sgml/ref/create_foreign_table.sgml
+++ b/doc/src/sgml/ref/create_foreign_table.sgml
@@ -47,7 +47,7 @@ CREATE FOREIGN TABLE [ IF NOT EXISTS ] <replaceable class="parameter">table_name
   NULL |
   CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
   DEFAULT <replaceable>default_expr</replaceable> |
-  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED }
+  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ] }
 [ ENFORCED | NOT ENFORCED ]
 
 <phrase>and <replaceable class="parameter">table_constraint</replaceable> is:</phrase>
@@ -283,7 +283,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    </varlistentry>
 
    <varlistentry>
-    <term><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED</literal><indexterm><primary>generated column</primary></indexterm></term>
+    <term><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ]</literal><indexterm><primary>generated column</primary></indexterm></term>
     <listitem>
      <para>
       This clause creates the column as a <firstterm>generated
@@ -292,10 +292,13 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      </para>
 
      <para>
-      The keyword <literal>STORED</literal> is required to signify that the
+      When <literal>VIRTUAL</literal> is specified, the column will be
+      computed when it is read.  (The foreign-data wrapper will see it as a
+      null value in new rows and may choose to store it as a null value or
+      ignore it altogether.)  When <literal>STORED</literal> is specified, the
       column will be computed on write.  (The computed value will be presented
       to the foreign-data wrapper for storage and must be returned on
-      reading.)
+      reading.)  <literal>VIRTUAL</literal> is the default.
      </para>
 
      <para>
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 417498f71db..9acbc4dd34d 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -65,7 +65,7 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
   NULL |
   CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
   DEFAULT <replaceable>default_expr</replaceable> |
-  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED |
+  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ] |
   GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
   UNIQUE [ NULLS [ NOT ] DISTINCT ] <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
@@ -725,8 +725,9 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
         <term><literal>INCLUDING GENERATED</literal></term>
         <listitem>
          <para>
-          Any generation expressions of copied column definitions will be
-          copied.  By default, new columns will be regular base columns.
+          Any generation expressions as well as the stored/virtual choice of
+          copied column definitions will be copied.  By default, new columns
+          will be regular base columns.
          </para>
         </listitem>
        </varlistentry>
@@ -907,7 +908,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
    </varlistentry>
 
    <varlistentry id="sql-createtable-parms-generated-stored">
-    <term><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED</literal><indexterm><primary>generated column</primary></indexterm></term>
+    <term><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ]</literal><indexterm><primary>generated column</primary></indexterm></term>
     <listitem>
      <para>
       This clause creates the column as a <firstterm>generated
@@ -916,8 +917,11 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      </para>
 
      <para>
-      The keyword <literal>STORED</literal> is required to signify that the
-      column will be computed on write and will be stored on disk.
+      When <literal>VIRTUAL</literal> is specified, the column will be
+      computed when it is read, and it will not occupy any storage.  When
+      <literal>STORED</literal> is specified, the column will be computed on
+      write and will be stored on disk.  <literal>VIRTUAL</literal> is the
+      default.
      </para>
 
      <para>
@@ -2504,9 +2508,9 @@ CREATE TABLE cities_partdef
    <title>Generated Columns</title>
 
    <para>
-    The option <literal>STORED</literal> is not standard but is also used by
-    other SQL implementations.  The SQL standard does not specify the storage
-    of generated columns.
+    The options <literal>STORED</literal> and <literal>VIRTUAL</literal> are
+    not standard but are also used by other SQL implementations.  The SQL
+    standard does not specify the storage of generated columns.
    </para>
   </refsect2>
 
diff --git a/doc/src/sgml/trigger.sgml b/doc/src/sgml/trigger.sgml
index c3c0faf7a1b..e9214dcf1b1 100644
--- a/doc/src/sgml/trigger.sgml
+++ b/doc/src/sgml/trigger.sgml
@@ -293,6 +293,10 @@
     <literal>BEFORE</literal> trigger.  Changes to the value of a generated
     column in a <literal>BEFORE</literal> trigger are ignored and will be
     overwritten.
+    Virtual generated columns are never computed when triggers fire.  In the C
+    language interface, their content is undefined in a trigger function.
+    Higher-level programming languages should prevent access to virtual
+    generated columns in triggers.
    </para>
 
    <para>