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From: Amit Khandekar <[email protected]>
Subject: [PATCH v6 5/5] Doc changes describing details about logical decoding on standby.
Date: Thu, 16 Jan 2020 10:05:16 +0530

---
 doc/src/sgml/logicaldecoding.sgml | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml
index bce6d379bf..745c65ff02 100644
--- a/doc/src/sgml/logicaldecoding.sgml
+++ b/doc/src/sgml/logicaldecoding.sgml
@@ -248,6 +248,24 @@ $ pg_recvlogical -d postgres --slot=test --drop-slot
      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
+     dropped.  Existing logical slots on standby also get dropped 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
-- 
2.20.1


--/9DWx/yDrRhgMJTb--

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