From: Noah Misch <noah@leadboat.com>

Doc: cover index CONCURRENTLY causing errors in INSERT ... ON CONFLICT.

Author: Mikhail Nikalayeu <mihailnikalayeu@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CANtu0ojXmqjmEzp-=aJSxjsdE76iAsRgHBoK0QtYHimb_mEfsg@mail.gmail.com
Backpatch-through: 13

diff --git a/doc/src/sgml/ref/insert.sgml b/doc/src/sgml/ref/insert.sgml
index 3f13991..f88ea78 100644
--- a/doc/src/sgml/ref/insert.sgml
+++ b/doc/src/sgml/ref/insert.sgml
@@ -594,6 +594,17 @@ INSERT INTO <replaceable class="parameter">table_name</replaceable> [ AS <replac
     </para>
    </tip>
 
+   <warning>
+    <para>
+      While <command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX
+      CONCURRENTLY</command> is running on a unique index, <command>INSERT
+      ... ON CONFLICT</command> statements on the same table may unexpectedly
+      fail with a unique violation.  Using <literal>ON CONFLICT ON
+      CONSTRAINT</literal> <replaceable class="parameter">
+      constraint_name</replaceable> avoids this, by disabling inference.
+    </para>
+   </warning>
+
   </refsect2>
  </refsect1>