Support automatically-updatable views.

This patch makes "simple" views automatically updatable, without the need
to create either INSTEAD OF triggers or INSTEAD rules.  "Simple" views
are those classified as updatable according to SQL-92 rules.  The rewriter
transforms INSERT/UPDATE/DELETE commands on such views directly into an
equivalent command on the underlying table, which will generally have
noticeably better performance than is possible with either triggers or
user-written rules.  A view that has INSTEAD OF triggers or INSTEAD rules
continues to operate the same as before.

For the moment, security_barrier views are not considered simple.
Also, we do not support WITH CHECK OPTION.  These features may be
added in future.

Dean Rasheed, reviewed by Amit Kapila

doc/src/sgml/ref/create_view.sgml

@@ -127,17 +127,6 @@ CREATE [ OR REPLACE ] [ TEMP | TEMPORARY ] VIEW <replaceable class="PARAMETER">n
  <refsect1>
   <title>Notes</title>
 
-   <para>
-    Currently, views are read only: the system will not allow an insert,
-    update, or delete on a view.  You can get the effect of an updatable
-    view by creating <literal>INSTEAD</> triggers on the view, which
-    must convert attempted inserts, etc. on the view into
-    appropriate actions on other tables.  For more information see
-    <xref linkend="sql-createtrigger">.  Another possibility is to create
-    rules (see <xref linkend="sql-createrule">), but in practice triggers
-    are easier to understand and use correctly.
-   </para>
-
    <para>
     Use the <xref linkend="sql-dropview">
     statement to drop views.
@@ -175,6 +164,105 @@ CREATE VIEW vista AS SELECT text 'Hello World' AS hello;
     to replace it (this includes being a member of the owning role).
    </para>
 
+  <refsect2 id="SQL-CREATEVIEW-updatable-views">
+   <title id="SQL-CREATEVIEW-updatable-views-title">Updatable Views</title>
+
+   <indexterm zone="sql-createview-updatable-views">
+    <primary>updatable views</primary>
+   </indexterm>
+
+   <para>
+    Simple views are automatically updatable: the system will allow
+    <command>INSERT</>, <command>UPDATE</> and <command>DELETE</> statements
+    to be used on the view in the same way as on a regular table.  A view is
+    automatically updatable if it satisfies all of the following conditions:
+
+    <itemizedlist>
+     <listitem>
+      <para>
+       The view must have exactly one entry in its <literal>FROM</> list,
+       which must be a table or another updatable view.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       The view definition must not contain <literal>WITH</>,
+       <literal>DISTINCT</>, <literal>GROUP BY</>, <literal>HAVING</>,
+       <literal>LIMIT</>, or <literal>OFFSET</> clauses at the top level.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       The view definition must not contain set operations (<literal>UNION</>,
+       <literal>INTERSECT</> or <literal>EXCEPT</>) at the top level.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       All columns in the view's select list must be simple references to
+       columns of the underlying relation.  They cannot be expressions,
+       literals or functions.  System columns cannot be referenced, either.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       No column of the underlying relation can appear more than once in
+       the view's select list.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       The view must not have the <literal>security_barrier</> property.
+      </para>
+     </listitem>
+    </itemizedlist>
+   </para>
+
+   <para>
+    If the view is automatically updatable the system will convert any
+    <command>INSERT</>, <command>UPDATE</> or <command>DELETE</> statement
+    on the view into the corresponding statement on the underlying base
+    relation.
+   </para>
+
+   <para>
+    If an automatically updatable view contains a <literal>WHERE</>
+    condition, the condition restricts which rows of the base relation are
+    available to be modified by <command>UPDATE</> and <command>DELETE</>
+    statements on the view.  However, an <command>UPDATE</> is allowed to
+    change a row so that it no longer satisfies the <literal>WHERE</>
+    condition, and thus is no longer visible through the view.  Similarly,
+    an <command>INSERT</> command can potentially insert base-relation rows
+    that do not satisfy the <literal>WHERE</> condition and thus are not
+    visible through the view.
+   </para>
+
+   <para>
+    A more complex view that does not satisfy all these conditions is
+    read-only by default: the system will not allow an insert, update, or
+    delete on the view.  You can get the effect of an updatable view by
+    creating <literal>INSTEAD OF</> triggers on the view, which must
+    convert attempted inserts, etc. on the view into appropriate actions
+    on other tables.  For more information see <xref
+    linkend="sql-createtrigger">.  Another possibility is to create rules
+    (see <xref linkend="sql-createrule">), but in practice triggers are
+    easier to understand and use correctly.
+   </para>
+
+   <para>
+    Note that the user performing the insert, update or delete on the view
+    must have the corresponding insert, update or delete privilege on the
+    view.  In addition the view's owner must have the relevant privileges on
+    the underlying base relations, but the user performing the update does
+    not need any permissions on the underlying base relations (see
+    <xref linkend="rules-privileges">).
+   </para>
+  </refsect2>
  </refsect1>
 
  <refsect1>
@@ -217,11 +305,15 @@ CREATE VIEW <replaceable class="parameter">name</replaceable> [ ( <replaceable c
       <term><literal>CHECK OPTION</literal></term>
       <listitem>
        <para>
-        This option has to do with updatable views.  All
-        <command>INSERT</> and <command>UPDATE</> commands on the view
-        will be checked to ensure data satisfy the view-defining
-        condition (that is, the new data would be visible through the
-        view). If they do not, the update will be rejected.
+        This option controls the behavior of automatically updatable views.
+        When given, <command>INSERT</> and <command>UPDATE</> commands on
+        the view will be checked to ensure new rows satisfy the
+        view-defining condition (that is, the new rows would be visible
+        through the view). If they do not, the update will be rejected.
+        Without <literal>CHECK OPTION</literal>, <command>INSERT</> and
+        <command>UPDATE</> commands on the view are allowed to create rows
+        that are not visible through the view.  (The latter behavior is the
+        only one currently provided by <productname>PostgreSQL</>.)
        </para>
       </listitem>
      </varlistentry>
@@ -252,6 +344,7 @@ CREATE VIEW <replaceable class="parameter">name</replaceable> [ ( <replaceable c
    <command>CREATE OR REPLACE VIEW</command> is a
    <productname>PostgreSQL</productname> language extension.
    So is the concept of a temporary view.
+   The <literal>WITH</> clause is an extension as well.
   </para>
  </refsect1>