Implement syntax for transition tables in AFTER triggers.

This is infrastructure for the complete SQL standard feature.  No
support is included at this point for execution nodes or PLs.  The
intent is to add that soon.

As this patch leaves things, standard syntax can create tuplestores
to contain old and/or new versions of rows affected by a statement.
References to these tuplestores are in the TriggerData structure.
C triggers can access the tuplestores directly, so they are usable,
but they cannot yet be referenced within a SQL statement.

doc/src/sgml/catalogs.sgml

@@ -6231,6 +6231,22 @@
        representation) for the trigger's <literal>WHEN</> condition, or null
        if none</entry>
      </row>
+
+     <row>
+      <entry><structfield>tgoldtable</structfield></entry>
+      <entry><type>name</type></entry>
+      <entry></entry>
+      <entry><literal>REFERENCING</> clause name for <literal>OLD TABLE</>,
+       or null if none</entry>
+     </row>
+
+     <row>
+      <entry><structfield>tgnewtable</structfield></entry>
+      <entry><type>name</type></entry>
+      <entry></entry>
+      <entry><literal>REFERENCING</> clause name for <literal>NEW TABLE</>,
+       or null if none</entry>
+     </row>
     </tbody>
    </tgroup>
   </table>

doc/src/sgml/ref/create_trigger.sgml

@@ -25,6 +25,7 @@ CREATE [ CONSTRAINT ] TRIGGER <replaceable class="PARAMETER">name</replaceable>
     ON <replaceable class="PARAMETER">table_name</replaceable>
     [ FROM <replaceable class="parameter">referenced_table_name</replaceable> ]
     [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
+    [ REFERENCING { { OLD | NEW } TABLE [ AS ] <replaceable class="PARAMETER">transition_relation_name</replaceable> } [ ... ] ]
     [ FOR [ EACH ] { ROW | STATEMENT } ]
     [ WHEN ( <replaceable class="parameter">condition</replaceable> ) ]
     EXECUTE PROCEDURE <replaceable class="PARAMETER">function_name</replaceable> ( <replaceable class="PARAMETER">arguments</replaceable> )
@@ -177,6 +178,15 @@ CREATE [ CONSTRAINT ] TRIGGER <replaceable class="PARAMETER">name</replaceable>
    when the constraints they implement are violated.
   </para>
 
+  <para>
+   The <literal>REFERENCING</> option is only allowed for an <literal>AFTER</>
+   trigger which is not a constraint trigger.  <literal>OLD TABLE</> may only
+   be specified once, and only on a trigger which can fire on
+   <literal>UPDATE</> or <literal>DELETE</>.  <literal>NEW TABLE</> may only
+   be specified once, and only on a trigger which can fire on
+   <literal>UPDATE</> or <literal>INSERT</>.
+  </para>
+
   <para>
    <command>SELECT</command> does not modify any rows so you cannot
    create <command>SELECT</command> triggers. Rules and views are more
@@ -281,6 +291,40 @@ UPDATE OF <replaceable>column_name1</replaceable> [, <replaceable>column_name2</
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>REFERENCING</literal></term>
+    <listitem>
+     <para>
+      This immediately preceeds the declaration of one or two relations which
+      can be used to read the before and/or after images of all rows directly
+      affected by the triggering statement.  An <literal>AFTER EACH ROW</>
+      trigger is allowed to use both these transition relation names and the
+      row names (<literal>OLD</> and <literal>NEW</>) which reference each
+      individual row for which the trigger fires.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>OLD TABLE</literal></term>
+    <term><literal>NEW TABLE</literal></term>
+    <listitem>
+     <para>
+      This specifies whether the named relation contains the before or after
+      images for rows affected by the statement which fired the trigger.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><replaceable class="PARAMETER">transition_relation_name</replaceable></term>
+    <listitem>
+     <para>
+      The (unqualified) name to be used within the trigger for this relation.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>FOR EACH ROW</literal></term>
     <term><literal>FOR EACH STATEMENT</literal></term>
@@ -474,6 +518,30 @@ CREATE TRIGGER view_insert
     FOR EACH ROW
     EXECUTE PROCEDURE view_insert_row();
 </programlisting>
+
+   Execute the function <function>check_transfer_balances_to_zero</> for each
+   statement to confirm that the <literal>transfer</> rows offset to a net of
+   zero:
+
+<programlisting>
+CREATE TRIGGER transfer_insert
+    AFTER INSERT ON transfer
+    FOR EACH STATEMENT
+    REFERENCING NEW TABLE AS inserted
+    EXECUTE PROCEDURE check_transfer_balances_to_zero();
+</programlisting>
+
+   Execute the function <function>check_matching_pairs</> for each row to
+   confirm that changes are made to matching pairs at the same time (by the
+   same statement):
+
+<programlisting>
+CREATE TRIGGER paired_items_update
+    AFTER UPDATE ON paired_items
+    FOR EACH ROW
+    REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
+    EXECUTE PROCEDURE check_matching_pairs();
+</programlisting>
   </para>
 
   <para>
@@ -502,24 +570,14 @@ CREATE TRIGGER view_insert
    <itemizedlist>
     <listitem>
      <para>
-      SQL allows you to define aliases for the <quote>old</quote>
-      and <quote>new</quote> rows or tables for use in the definition
-      of the triggered action (e.g., <literal>CREATE TRIGGER ... ON
-      tablename REFERENCING OLD ROW AS somename NEW ROW AS othername
-      ...</literal>).  Since <productname>PostgreSQL</productname>
-      allows trigger procedures to be written in any number of
-      user-defined languages, access to the data is handled in a
-      language-specific way.
-     </para>
-    </listitem>
-
-    <listitem>
-     <para>
-      <productname>PostgreSQL</productname> does not allow the old and new
-      tables to be referenced in statement-level triggers, i.e., the tables
-      that contain all the old and/or new rows, which are referred to by the
-      <literal>OLD TABLE</literal> and <literal>NEW TABLE</literal> clauses in
-      the <acronym>SQL</> standard.
+      While transition tables for <literal>AFTER</> triggers are specified
+      using the <literal>REFERENCING</> clause in the standard way, the row
+      variables used in <literal>FOR EACH ROW</> triggers may not be
+      specified in <literal>REFERENCING</> clause.  They are available in a
+      manner which is dependent on the language in which the trigger function
+      is written.  Some languages effectively behave as though there is a
+      <literal>REFERENCING</> clause containing <literal>OLD ROW AS OLD NEW
+      ROW AS NEW</>.
      </para>
     </listitem>