doc: Add better description for rewrite functions in event triggers

michaelpq · michaelpq · commit 709ce29b1656 · 2024-10-29T15:35:14.000+09:00
There are two functions that can be used in event triggers to get more details about a rewrite happening on a relation. Both had a limited documentation: - pg_event_trigger_table_rewrite_reason() and pg_event_trigger_table_rewrite_oid() were not mentioned in the main event trigger section in the paragraph dedicated to the event table_rewrite. - pg_event_trigger_table_rewrite_reason() returns an integer which is a bitmap of the reasons why a rewrite happens. There was no explanation about the meaning of these values, forcing the reader to look at the code to find out that these are defined in event_trigger.h. While on it, let's add a comment in event_trigger.h where the AT_REWRITE_* are defined, telling to update the documentation when these values are changed. Backpatch down to 13 as a consequence of 1ad2333, where this area of the documentation has been heavily reworked. Author: Greg Sabino Mullane Discussion: https://postgr.es/m/CAKAnmmL+Z6j-C8dAx1tVrnBmZJu+BSoc68WSg3sR+CVNjBCqbw@mail.gmail.com Backpatch-through: 13
diff --git a/doc/src/sgml/event-trigger.sgml b/doc/src/sgml/event-trigger.sgml
@@ -99,6 +99,11 @@
     control statements are available to rewrite a table,
     like <literal>CLUSTER</literal> and <literal>VACUUM</literal>,
     the <literal>table_rewrite</literal> event is not triggered by them.
+    To find the OID of the table that was rewritten, use the function
+    <literal>pg_event_trigger_table_rewrite_oid()</literal> (see
+    <xref linkend="functions-event-triggers"/>). To discover the reason(s)
+    for the rewrite, use the function
+    <literal>pg_event_trigger_table_rewrite_reason()</literal>.
    </para>
 
    <para>
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
@@ -31110,8 +31110,12 @@ CREATE EVENT TRIGGER test_event_trigger_for_drops
         <returnvalue>integer</returnvalue>
        </para>
        <para>
-        Returns a code explaining the reason(s) for rewriting.  The exact
-        meaning of the codes is release dependent.
+        Returns a code explaining the reason(s) for rewriting. The value is
+        a bitmap built from the following values: <literal>1</literal>
+        (the table has changed its persistence), <literal>2</literal>
+        (default value of a column has changed), <literal>4</literal>
+        (a column has a new data type) and <literal>8</literal>
+        (the table access method has changed).
        </para></entry>
       </row>
      </tbody>
diff --git a/src/include/commands/event_trigger.h b/src/include/commands/event_trigger.h
@@ -31,6 +31,12 @@ typedef struct EventTriggerData
 
 extern PGDLLIMPORT bool event_triggers;
 
+/*
+ * Reasons for relation rewrites.
+ *
+ * pg_event_trigger_table_rewrite_reason() uses these values, so make sure to
+ * update the documentation when changing this list.
+ */
 #define AT_REWRITE_ALTER_PERSISTENCE	0x01
 #define AT_REWRITE_DEFAULT_VAL			0x02
 #define AT_REWRITE_COLUMN_REWRITE		0x04
