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 5c9a551 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

Author: nathan-bossart

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>

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,

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;

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;