From 401a6956fa69c9202cbc14c09ba8a9c430b90cac Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Thu, 6 Feb 2025 15:23:40 -0600
Subject: [PATCH] Disallow COPY FREEZE on foreign tables.

This didn't actually work: the COPY succeeds, but the FREEZE
optimization isn't applied.  There doesn't seem to be an easy way
to support FREEZE on foreign tables, so let's follow the precedent
established by commit 5c9a5513a3 by raising an error early.  This
is arguably a bug fix, but due to the lack of reports, the minimal
discussion on the mailing list, and the potential to break existing
scripts, I am not back-patching it for now.

Author: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0ujeNgKpE3OrLtR%3DeJGa5LkGMekFzQTwjgw%3DrzaLufQLQ%40mail.gmail.com
---
 doc/src/sgml/ref/copy.sgml         |  2 +-
 src/backend/commands/copyfrom.c    |  6 ++++++
 src/test/regress/expected/copy.out |  8 ++++++++
 src/test/regress/sql/copy.sql      | 10 ++++++++++
 4 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/doc/src/sgml/ref/copy.sgml b/doc/src/sgml/ref/copy.sgml
index 8394402f096..df093da97c5 100644
--- a/doc/src/sgml/ref/copy.sgml
+++ b/doc/src/sgml/ref/copy.sgml
@@ -237,7 +237,7 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
       or truncated in the current subtransaction, there are no cursors
       open and there are no older snapshots held by this transaction.  It is
       currently not possible to perform a <command>COPY FREEZE</command> on
-      a partitioned table.
+      a partitioned table or foreign table.
       This option is only allowed in <command>COPY FROM</command>.
      </para>
      <para>
diff --git a/src/backend/commands/copyfrom.c b/src/backend/commands/copyfrom.c
index 0cbd05f5602..b70f4691b72 100644
--- a/src/backend/commands/copyfrom.c
+++ b/src/backend/commands/copyfrom.c
@@ -740,6 +740,12 @@ CopyFrom(CopyFromState cstate)
 					 errmsg("cannot perform COPY FREEZE on a partitioned table")));
 		}
 
+		/* There's currently no support for COPY FREEZE on foreign tables. */
+		if (cstate->rel->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("cannot perform COPY FREEZE on a foreign table")));
+
 		/*
 		 * Tolerate one registration for the benefit of FirstXactSnapshot.
 		 * Scan-bearing queries generally create at least two registrations,
diff --git a/src/test/regress/expected/copy.out b/src/test/regress/expected/copy.out
index f554d42c84c..e69e34c69b8 100644
--- a/src/test/regress/expected/copy.out
+++ b/src/test/regress/expected/copy.out
@@ -325,3 +325,11 @@ SELECT tableoid::regclass, id % 2 = 0 is_even, count(*) from parted_si GROUP BY
 (2 rows)
 
 DROP TABLE parted_si;
+-- ensure COPY FREEZE errors for foreign tables
+begin;
+create foreign data wrapper copytest_wrapper;
+create server copytest_server foreign data wrapper copytest_wrapper;
+create foreign table copytest_foreign_table (a int) server copytest_server;
+copy copytest_foreign_table from stdin (freeze);
+ERROR:  cannot perform COPY FREEZE on a foreign table
+rollback;
diff --git a/src/test/regress/sql/copy.sql b/src/test/regress/sql/copy.sql
index f1699b66b04..895479d2d0f 100644
--- a/src/test/regress/sql/copy.sql
+++ b/src/test/regress/sql/copy.sql
@@ -348,3 +348,13 @@ COPY parted_si(id, data) FROM :'filename';
 SELECT tableoid::regclass, id % 2 = 0 is_even, count(*) from parted_si GROUP BY 1, 2 ORDER BY 1;
 
 DROP TABLE parted_si;
+
+-- ensure COPY FREEZE errors for foreign tables
+begin;
+create foreign data wrapper copytest_wrapper;
+create server copytest_server foreign data wrapper copytest_wrapper;
+create foreign table copytest_foreign_table (a int) server copytest_server;
+copy copytest_foreign_table from stdin (freeze);
+1
+\.
+rollback;