Re: COPY ON_CONFLICT TABLE; save duplicated record to another table.

I also noticed the early relation close mentioned by Jim, which can
crash the patch.

+      This uses the same mechanism as <link
linkend="sql-on-conflict"><command>INSERT ... ON
CONFLICT</command></link>.
+      However, exclusion constraints are not supported; only
<literal>NOT DEFERRABLE</literal>
+      unique constraints are checked for violations.

EXCLUDE USING gist (... WITH =, ... WITH &&) seems to work fine?
Except that the message mentions unique constraint violation.

I also checked the same trigger behaviors as in the other thread[1],
especially before triggers on the conflict table, and this patch
behaves similarly, it silently drops rows.
I think this could also use some more visibility/documentation about that.

1: https://www.postgresql.org/message-id/CAN4CZFPoohFvQTSE0wC%2BwcrfYiZOxFmUdOq0%2B9TCVR6Hk8n6iw%40mail.gmail.com