v38-0005-Doc-changes-describing-details-about-logical-dec.patch

From 8d996fba629ec796d84c85d947e3ecb1f615a7d7 Mon Sep 17 00:00:00 2001
From: bdrouvotAWS <bdrouvot@amazon.com>
Date: Wed, 11 Jan 2023 17:21:12 +0000
Subject: [PATCH v38 5/6] Doc changes describing details about logical
 decoding.

Author: Andres Freund (in an older version), Amit Khandekar, Bertrand Drouvot
Reviewed-By: Bertrand Drouvot, Andres Freund, Robert Haas, Fabrizio de Royes Mello
---
 doc/src/sgml/logicaldecoding.sgml | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)
 100.0% doc/src/sgml/

diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml
index 4cf863a76f..0387558d75 100644
--- a/doc/src/sgml/logicaldecoding.sgml
+++ b/doc/src/sgml/logicaldecoding.sgml
@@ -316,6 +316,27 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU
      may consume changes from a slot at any given time.
     </para>
 
+    <para>
+     A logical replication slot can also be created on a hot standby. To prevent
+     <command>VACUUM</command> from removing required rows from the system
+     catalogs, <varname>hot_standby_feedback</varname> should be set on the
+     standby. In spite of that, if any required rows get removed, the slot gets
+     invalidated. It's highly recommended to use a physical slot between the primary
+     and the standby. Otherwise, hot_standby_feedback will work, but only while the
+     connection is alive (for example a node restart would break it). Existing
+     logical slots on standby also get invalidated if wal_level on primary is reduced to
+     less than 'logical'.
+    </para>
+
+    <para>
+     For a logical slot to be created, it builds a historic snapshot, for which
+     information of all the currently running transactions is essential. On
+     primary, this information is available, but on standby, this information
+     has to be obtained from primary. So, slot creation may wait for some
+     activity to happen on the primary. If the primary is idle, creating a
+     logical slot on standby may take a noticeable time.
+    </para>
+
     <caution>
      <para>
       Replication slots persist across crashes and know nothing about the state
-- 
2.34.1