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

6 files changed, 175 insertions, 41 deletions

diff --git a/doc/src/sgml/intro.sgml b/doc/src/sgml/intro.sgml
index 4d3f93f3174..f0dba6f56fb 100644
--- a/doc/src/sgml/intro.sgml
+++ b/doc/src/sgml/intro.sgml
@@ -110,7 +110,7 @@
      <simpara>triggers</simpara>
     </listitem>
     <listitem>
-     <simpara>views</simpara>
+     <simpara>updatable views</simpara>
     </listitem>
     <listitem>
      <simpara>transactional integrity</simpara>
diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 356419e2d08..5437626c3fe 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -147,11 +147,9 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
     <listitem>
      <para>
       These forms set or remove the default value for a column.
-      The default values only apply to subsequent <command>INSERT</command>
-      commands; they do not cause rows already in the table to change.
-      Defaults can also be created for views, in which case they are
-      inserted into <command>INSERT</> statements on the view before
-      the view's <literal>ON INSERT</literal> rule is applied.
+      Default values only apply in subsequent <command>INSERT</command>
+      or <command>UPDATE</> commands; they do not cause rows already in the
+      table to change.
      </para>
     </listitem>
    </varlistentry>
diff --git a/doc/src/sgml/ref/alter_view.sgml b/doc/src/sgml/ref/alter_view.sgml
index 521f05b84a1..0e2b140241e 100644
--- a/doc/src/sgml/ref/alter_view.sgml
+++ b/doc/src/sgml/ref/alter_view.sgml
@@ -80,10 +80,11 @@ ALTER VIEW [ IF EXISTS ] <replaceable class="parameter">name</replaceable> RESET
     <listitem>
      <para>
       These forms set or remove the default value for a column.
-      A default value associated with a view column is
-      inserted into <command>INSERT</> statements on the view before
-      the view's <literal>ON INSERT</literal> rule is applied, if
-      the <command>INSERT</> does not specify a value for the column.
+      A view column's default value is substituted into any
+      <command>INSERT</> or <command>UPDATE</> command whose target is the
+      view, before applying any rules or triggers for the view.  The view's
+      default will therefore take precedence over any default values from
+      underlying relations.
      </para>
     </listitem>
    </varlistentry>
diff --git a/doc/src/sgml/ref/create_rule.sgml b/doc/src/sgml/ref/create_rule.sgml
index d4c33921290..381ea3ed6b4 100644
--- a/doc/src/sgml/ref/create_rule.sgml
+++ b/doc/src/sgml/ref/create_rule.sgml
@@ -45,10 +45,10 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
    additional commands to be executed when a given command on a given
    table is executed.  Alternatively, an <literal>INSTEAD</literal>
    rule can replace a given command by another, or cause a command
-   not to be executed at all.  Rules are used to implement table
+   not to be executed at all.  Rules are used to implement SQL
    views as well.  It is important to realize that a rule is really
    a command transformation mechanism, or command macro.  The
-   transformation happens before the execution of the commands starts.
+   transformation happens before the execution of the command starts.
    If you actually want an operation that fires independently for each
    physical row, you probably want to use a trigger, not a rule.
    More information about the rules system is in <xref linkend="rules">.
@@ -73,13 +73,11 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
    sufficient for your purposes) to replace update actions on the view
    with appropriate updates on other tables.  If you want to support
    <command>INSERT RETURNING</> and so on, then be sure to put a suitable
-   <literal>RETURNING</> clause into each of these rules.  Alternatively,
-   an updatable view can be implemented using <literal>INSTEAD OF</>
-   triggers (see <xref linkend="sql-createtrigger">).
+   <literal>RETURNING</> clause into each of these rules.
   </para>
 
   <para>
-   There is a catch if you try to use conditional rules for view
+   There is a catch if you try to use conditional rules for complex view
    updates: there <emphasis>must</> be an unconditional
    <literal>INSTEAD</literal> rule for each action you wish to allow
    on the view.  If the rule is conditional, or is not
@@ -95,6 +93,21 @@ CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS
    <literal>INSTEAD NOTHING</literal> action.  (This method does not
    currently work to support <literal>RETURNING</> queries, however.)
   </para>
+
+  <note>
+   <para>
+    A view that is simple enough to be automatically updatable (see <xref
+    linkend="sql-createview">) does not require a user-created rule in
+    order to be updatable.  While you can create an explicit rule anyway,
+    the automatic update transformation will generally outperform an
+    explicit rule.
+   </para>
+
+   <para>
+    Another alternative worth considering is to use <literal>INSTEAD OF</>
+    triggers (see <xref linkend="sql-createtrigger">) in place of rules.
+   </para>
+  </note>
  </refsect1>
 
  <refsect1>
diff --git a/doc/src/sgml/ref/create_view.sgml b/doc/src/sgml/ref/create_view.sgml
index 9e3bc2954f2..abbde94772c 100644
--- a/doc/src/sgml/ref/create_view.sgml
+++ b/doc/src/sgml/ref/create_view.sgml
@@ -128,17 +128,6 @@ CREATE [ OR REPLACE ] [ TEMP | TEMPORARY ] VIEW <replaceable class="PARAMETER">n
   <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.
    </para>
@@ -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>
 
diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index cc02ada7c71..5811de7942f 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -808,13 +808,28 @@ SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
 <para>
     What happens if a view is named as the target relation for an
     <command>INSERT</command>, <command>UPDATE</command>, or
-    <command>DELETE</command>?  Simply doing the substitutions
+    <command>DELETE</command>?  Doing the substitutions
     described above would give a query tree in which the result
     relation points at a subquery range-table entry, which will not
-    work.  Instead, the rewriter assumes that the operation will be
-    handled by an <literal>INSTEAD OF</> trigger on the view.
-    (If there is no such trigger, the executor will throw an error
-    when execution starts.)  Rewriting works slightly differently
+    work.  There are several ways in which <productname>PostgreSQL</>
+    can support the appearance of updating a view, however.
+</para>
+
+<para>
+    If the subquery selects from a single base relation and is simple
+    enough, the rewriter can automatically replace the subquery with the
+    underlying base relation so that the <command>INSERT</command>,
+    <command>UPDATE</command>, or <command>DELETE</command> is applied to
+    the base relation in the appropriate way.  Views that are
+    <quote>simple enough</> for this are called <firstterm>automatically
+    updatable</>.  For detailed information on the kinds of view that can
+    be automatically updated, see <xref linkend="sql-createview">.
+</para>
+
+<para>
+    Alternatively, the operation may be handled by a user-provided
+    <literal>INSTEAD OF</> trigger on the view.
+    Rewriting works slightly differently
     in this case.  For <command>INSERT</command>, the rewriter does
     nothing at all with the view, leaving it as the result relation
     for the query.  For <command>UPDATE</command> and
@@ -842,10 +857,8 @@ SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
 </para>
 
 <para>
-    If there are no <literal>INSTEAD OF</> triggers to update the view,
-    the executor will throw an error, because it cannot automatically
-    update a view by itself. To change this, we can define rules that
-    modify the behavior of <command>INSERT</command>,
+    Another possibility is for the user to define <literal>INSTEAD</>
+    rules that specify substitute actions for <command>INSERT</command>,
     <command>UPDATE</command>, and <command>DELETE</command> commands on
     a view. These rules will rewrite the command, typically into a command
     that updates one or more tables, rather than views. That is the topic
@@ -860,6 +873,22 @@ SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
     evaluated first, and depending on the result, the triggers may not be
     used at all.
 </para>
+
+<para>
+    Automatic rewriting of an <command>INSERT</command>,
+    <command>UPDATE</command>, or <command>DELETE</command> query on a
+    simple view is always tried last. Therefore, if a view has rules or
+    triggers, they will override the default behavior of automatically
+    updatable views.
+</para>
+
+<para>
+    If there are no <literal>INSTEAD</> rules or <literal>INSTEAD OF</>
+    triggers for the view, and the rewriter cannot automatically rewrite
+    the query as an update on the underlying base relation, an error will
+    be thrown because the executor cannot update a view as such.
+</para>
+
 </sect2>
 
 </sect1>