1 files changed, 57 insertions, 0 deletions
diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index bf76490cbc0..6b029a5a35f 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3864,6 +3864,63 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats">
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>