Allow TRUNCATE command to truncate foreign tables.

This commit introduces new foreign data wrapper API for TRUNCATE.
It extends TRUNCATE command so that it accepts foreign tables as
the targets to truncate and invokes that API. Also it extends postgres_fdw
so that it can issue TRUNCATE command to foreign servers, by adding
new routine for that TRUNCATE API.

The information about options specified in TRUNCATE command, e.g.,
ONLY, CACADE, etc is passed to FDW via API. The list of foreign tables to
truncate is also passed to FDW. FDW truncates the foreign data sources
that the passed foreign tables specify, based on those information.
For example, postgres_fdw constructs TRUNCATE command using them
and issues it to the foreign server.

For performance, TRUNCATE command invokes the FDW routine for
TRUNCATE once per foreign server that foreign tables to truncate belong to.

Author: Kazutaka Onishi, Kohei KaiGai, slightly modified by Fujii Masao
Reviewed-by: Bharath Rupireddy, Michael Paquier, Zhihong Yu, Alvaro Herrera, Stephen Frost, Ashutosh Bapat, Amit Langote, Daniel Gustafsson, Ibrar Ahmed, Fujii Masao
Discussion: https://postgr.es/m/CAOP8fzb_gkReLput7OvOK+8NHgw-RKqNv59vem7=524krQTcWA@mail.gmail.com
Discussion: https://postgr.es/m/CAJuF6cMWDDqU-vn_knZgma+2GMaout68YUgn1uyDnexRhqqM5Q@mail.gmail.com

contrib/postgres_fdw/connection.c

@@ -92,7 +92,6 @@ static PGconn *connect_pg_server(ForeignServer *server, UserMapping *user);
 static void disconnect_pg_server(ConnCacheEntry *entry);
 static void check_conn_params(const char **keywords, const char **values, UserMapping *user);
 static void configure_remote_session(PGconn *conn);
-static void do_sql_command(PGconn *conn, const char *sql);
 static void begin_remote_xact(ConnCacheEntry *entry);
 static void pgfdw_xact_callback(XactEvent event, void *arg);
 static void pgfdw_subxact_callback(SubXactEvent event,
@@ -568,7 +567,7 @@ configure_remote_session(PGconn *conn)
 /*
  * Convenience subroutine to issue a non-data-returning SQL command to remote
  */
-static void
+void
 do_sql_command(PGconn *conn, const char *sql)
 {
 	PGresult   *res;

contrib/postgres_fdw/deparse.c

@@ -56,6 +56,7 @@
 #include "utils/rel.h"
 #include "utils/syscache.h"
 #include "utils/typcache.h"
+#include "commands/tablecmds.h"
 
 /*
  * Global context for foreign_expr_walker's search of an expression tree.
@@ -2172,6 +2173,43 @@ deparseAnalyzeSql(StringInfo buf, Relation rel, List **retrieved_attrs)
 	deparseRelation(buf, rel);
 }
 
+/*
+ * Construct a simple "TRUNCATE rel" statement
+ */
+void
+deparseTruncateSql(StringInfo buf,
+				   List *rels,
+				   List *rels_extra,
+				   DropBehavior behavior,
+				   bool restart_seqs)
+{
+	ListCell   *lc1,
+			   *lc2;
+
+	appendStringInfoString(buf, "TRUNCATE ");
+
+	forboth(lc1, rels, lc2, rels_extra)
+	{
+		Relation	rel = lfirst(lc1);
+		int			extra = lfirst_int(lc2);
+
+		if (lc1 != list_head(rels))
+			appendStringInfoString(buf, ", ");
+		if (extra & TRUNCATE_REL_CONTEXT_ONLY)
+			appendStringInfoString(buf, "ONLY ");
+
+		deparseRelation(buf, rel);
+	}
+
+	appendStringInfo(buf, " %s IDENTITY",
+					 restart_seqs ? "RESTART" : "CONTINUE");
+
+	if (behavior == DROP_RESTRICT)
+		appendStringInfoString(buf, " RESTRICT");
+	else if (behavior == DROP_CASCADE)
+		appendStringInfoString(buf, " CASCADE");
+}
+
 /*
  * Construct name to use for given column, and emit it into buf.
  * If it has a column_name FDW option, use that instead of attribute name.

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -8215,6 +8215,205 @@ select * from rem3;
 drop foreign table rem3;
 drop table loc3;
 -- ===================================================================
+-- test for TRUNCATE
+-- ===================================================================
+CREATE TABLE tru_rtable0 (id int primary key);
+CREATE TABLE tru_rtable1 (id int primary key);
+CREATE FOREIGN TABLE tru_ftable (id int)
+       SERVER loopback OPTIONS (table_name 'tru_rtable0');
+INSERT INTO tru_rtable0 (SELECT x FROM generate_series(1,10) x);
+CREATE TABLE tru_ptable (id int) PARTITION BY HASH(id);
+CREATE TABLE tru_ptable__p0 PARTITION OF tru_ptable
+                            FOR VALUES WITH (MODULUS 2, REMAINDER 0);
+CREATE FOREIGN TABLE tru_ftable__p1 PARTITION OF tru_ptable
+                                    FOR VALUES WITH (MODULUS 2, REMAINDER 1)
+       SERVER loopback OPTIONS (table_name 'tru_rtable1');
+INSERT INTO tru_ptable (SELECT x FROM generate_series(11,20) x);
+CREATE TABLE tru_pk_table(id int primary key);
+CREATE TABLE tru_fk_table(fkey int references tru_pk_table(id));
+INSERT INTO tru_pk_table (SELECT x FROM generate_series(1,10) x);
+INSERT INTO tru_fk_table (SELECT x % 10 + 1 FROM generate_series(5,25) x);
+CREATE FOREIGN TABLE tru_pk_ftable (id int)
+       SERVER loopback OPTIONS (table_name 'tru_pk_table');
+CREATE TABLE tru_rtable_parent (id int);
+CREATE TABLE tru_rtable_child (id int);
+CREATE FOREIGN TABLE tru_ftable_parent (id int)
+       SERVER loopback OPTIONS (table_name 'tru_rtable_parent');
+CREATE FOREIGN TABLE tru_ftable_child () INHERITS (tru_ftable_parent)
+       SERVER loopback OPTIONS (table_name 'tru_rtable_child');
+INSERT INTO tru_rtable_parent (SELECT x FROM generate_series(1,8) x);
+INSERT INTO tru_rtable_child  (SELECT x FROM generate_series(10, 18) x);
+-- normal truncate
+SELECT sum(id) FROM tru_ftable;        -- 55
+ sum 
+-----
+  55
+(1 row)
+
+TRUNCATE tru_ftable;
+SELECT count(*) FROM tru_rtable0;		-- 0
+ count 
+-------
+     0
+(1 row)
+
+SELECT count(*) FROM tru_ftable;		-- 0
+ count 
+-------
+     0
+(1 row)
+
+-- 'truncatable' option
+ALTER SERVER loopback OPTIONS (ADD truncatable 'false');
+TRUNCATE tru_ftable;			-- error
+ERROR:  foreign table "tru_ftable" does not allow truncates
+ALTER FOREIGN TABLE tru_ftable OPTIONS (ADD truncatable 'true');
+TRUNCATE tru_ftable;			-- accepted
+ALTER FOREIGN TABLE tru_ftable OPTIONS (SET truncatable 'false');
+TRUNCATE tru_ftable;			-- error
+ERROR:  foreign table "tru_ftable" does not allow truncates
+ALTER SERVER loopback OPTIONS (DROP truncatable);
+ALTER FOREIGN TABLE tru_ftable OPTIONS (SET truncatable 'false');
+TRUNCATE tru_ftable;			-- error
+ERROR:  foreign table "tru_ftable" does not allow truncates
+ALTER FOREIGN TABLE tru_ftable OPTIONS (SET truncatable 'true');
+TRUNCATE tru_ftable;			-- accepted
+-- partitioned table with both local and foreign tables as partitions
+SELECT sum(id) FROM tru_ptable;        -- 155
+ sum 
+-----
+ 155
+(1 row)
+
+TRUNCATE tru_ptable;
+SELECT count(*) FROM tru_ptable;		-- 0
+ count 
+-------
+     0
+(1 row)
+
+SELECT count(*) FROM tru_ptable__p0;	-- 0
+ count 
+-------
+     0
+(1 row)
+
+SELECT count(*) FROM tru_ftable__p1;	-- 0
+ count 
+-------
+     0
+(1 row)
+
+SELECT count(*) FROM tru_rtable1;		-- 0
+ count 
+-------
+     0
+(1 row)
+
+-- 'CASCADE' option
+SELECT sum(id) FROM tru_pk_ftable;      -- 55
+ sum 
+-----
+  55
+(1 row)
+
+TRUNCATE tru_pk_ftable;	-- failed by FK reference
+ERROR:  cannot truncate a table referenced in a foreign key constraint
+DETAIL:  Table "tru_fk_table" references "tru_pk_table".
+HINT:  Truncate table "tru_fk_table" at the same time, or use TRUNCATE ... CASCADE.
+CONTEXT:  remote SQL command: TRUNCATE public.tru_pk_table CONTINUE IDENTITY RESTRICT
+TRUNCATE tru_pk_ftable CASCADE;
+SELECT count(*) FROM tru_pk_ftable;    -- 0
+ count 
+-------
+     0
+(1 row)
+
+SELECT count(*) FROM tru_fk_table;		-- also truncated,0
+ count 
+-------
+     0
+(1 row)
+
+-- truncate two tables at a command
+INSERT INTO tru_ftable (SELECT x FROM generate_series(1,8) x);
+INSERT INTO tru_pk_ftable (SELECT x FROM generate_series(3,10) x);
+SELECT count(*) from tru_ftable; -- 8
+ count 
+-------
+     8
+(1 row)
+
+SELECT count(*) from tru_pk_ftable; -- 8
+ count 
+-------
+     8
+(1 row)
+
+TRUNCATE tru_ftable, tru_pk_ftable CASCADE;
+SELECT count(*) from tru_ftable; -- 0
+ count 
+-------
+     0
+(1 row)
+
+SELECT count(*) from tru_pk_ftable; -- 0
+ count 
+-------
+     0
+(1 row)
+
+-- truncate with ONLY clause
+TRUNCATE ONLY tru_ftable_parent;
+SELECT sum(id) FROM tru_ftable_parent;  -- 126
+ sum 
+-----
+ 126
+(1 row)
+
+TRUNCATE tru_ftable_parent;
+SELECT count(*) FROM tru_ftable_parent; -- 0
+ count 
+-------
+     0
+(1 row)
+
+-- in case when remote table has inherited children
+CREATE TABLE tru_rtable0_child () INHERITS (tru_rtable0);
+INSERT INTO tru_rtable0 (SELECT x FROM generate_series(5,9) x);
+INSERT INTO tru_rtable0_child (SELECT x FROM generate_series(10,14) x);
+SELECT sum(id) FROM tru_ftable;   -- 95
+ sum 
+-----
+  95
+(1 row)
+
+TRUNCATE ONLY tru_ftable;		-- truncate only parent portion
+SELECT sum(id) FROM tru_ftable;   -- 60
+ sum 
+-----
+  60
+(1 row)
+
+INSERT INTO tru_rtable0 (SELECT x FROM generate_series(21,25) x);
+SELECT sum(id) FROM tru_ftable;		-- 175
+ sum 
+-----
+ 175
+(1 row)
+
+TRUNCATE tru_ftable;			-- truncate both of parent and child
+SELECT count(*) FROM tru_ftable;    -- empty
+ count 
+-------
+     0
+(1 row)
+
+-- cleanup
+DROP FOREIGN TABLE tru_ftable_parent, tru_ftable_child, tru_pk_ftable,tru_ftable__p1,tru_ftable;
+DROP TABLE tru_rtable0, tru_rtable1, tru_ptable, tru_ptable__p0, tru_pk_table, tru_fk_table,
+tru_rtable_parent,tru_rtable_child, tru_rtable0_child;
+-- ===================================================================
 -- test IMPORT FOREIGN SCHEMA
 -- ===================================================================
 CREATE SCHEMA import_source;
@@ -8917,7 +9116,7 @@ DO $d$
     END;
 $d$;
 ERROR:  invalid option "password"
-HINT:  Valid options in this context are: service, passfile, channel_binding, connect_timeout, dbname, host, hostaddr, port, options, application_name, keepalives, keepalives_idle, keepalives_interval, keepalives_count, tcp_user_timeout, sslmode, sslcompression, sslcert, sslkey, sslrootcert, sslcrl, sslcrldir, sslsni, requirepeer, ssl_min_protocol_version, ssl_max_protocol_version, gssencmode, krbsrvname, gsslib, target_session_attrs, use_remote_estimate, fdw_startup_cost, fdw_tuple_cost, extensions, updatable, fetch_size, batch_size, async_capable, keep_connections
+HINT:  Valid options in this context are: service, passfile, channel_binding, connect_timeout, dbname, host, hostaddr, port, options, application_name, keepalives, keepalives_idle, keepalives_interval, keepalives_count, tcp_user_timeout, sslmode, sslcompression, sslcert, sslkey, sslrootcert, sslcrl, sslcrldir, sslsni, requirepeer, ssl_min_protocol_version, ssl_max_protocol_version, gssencmode, krbsrvname, gsslib, target_session_attrs, use_remote_estimate, fdw_startup_cost, fdw_tuple_cost, extensions, updatable, truncatable, fetch_size, batch_size, async_capable, keep_connections
 CONTEXT:  SQL statement "ALTER SERVER loopback_nopw OPTIONS (ADD password 'dummypw')"
 PL/pgSQL function inline_code_block line 3 at EXECUTE
 -- If we add a password for our user mapping instead, we should get a different

contrib/postgres_fdw/option.c

@@ -108,6 +108,7 @@ postgres_fdw_validator(PG_FUNCTION_ARGS)
 		 */
 		if (strcmp(def->defname, "use_remote_estimate") == 0 ||
 			strcmp(def->defname, "updatable") == 0 ||
+			strcmp(def->defname, "truncatable") == 0 ||
 			strcmp(def->defname, "async_capable") == 0 ||
 			strcmp(def->defname, "keep_connections") == 0)
 		{
@@ -213,6 +214,9 @@ InitPgFdwOptions(void)
 		/* updatable is available on both server and table */
 		{"updatable", ForeignServerRelationId, false},
 		{"updatable", ForeignTableRelationId, false},
+		/* truncatable is available on both server and table */
+		{"truncatable", ForeignServerRelationId, false},
+		{"truncatable", ForeignTableRelationId, false},
 		/* fetch_size is available on both server and table */
 		{"fetch_size", ForeignServerRelationId, false},
 		{"fetch_size", ForeignTableRelationId, false},

contrib/postgres_fdw/postgres_fdw.c

@@ -400,6 +400,10 @@ static void postgresExplainForeignModify(ModifyTableState *mtstate,
 										 ExplainState *es);
 static void postgresExplainDirectModify(ForeignScanState *node,
 										ExplainState *es);
+static void postgresExecForeignTruncate(List *rels,
+										List *rels_extra,
+										DropBehavior behavior,
+										bool restart_seqs);
 static bool postgresAnalyzeForeignTable(Relation relation,
 										AcquireSampleRowsFunc *func,
 										BlockNumber *totalpages);
@@ -588,6 +592,9 @@ postgres_fdw_handler(PG_FUNCTION_ARGS)
 	routine->ExplainForeignModify = postgresExplainForeignModify;
 	routine->ExplainDirectModify = postgresExplainDirectModify;
 
+	/* Support function for TRUNCATE */
+	routine->ExecForeignTruncate = postgresExecForeignTruncate;
+
 	/* Support functions for ANALYZE */
 	routine->AnalyzeForeignTable = postgresAnalyzeForeignTable;
 
@@ -2868,6 +2875,102 @@ postgresExplainDirectModify(ForeignScanState *node, ExplainState *es)
 	}
 }
 
+/*
+ * postgresExecForeignTruncate
+ *		Truncate one or more foreign tables
+ */
+static void
+postgresExecForeignTruncate(List *rels,
+							List *rels_extra,
+							DropBehavior behavior,
+							bool restart_seqs)
+{
+	Oid			serverid = InvalidOid;
+	UserMapping *user = NULL;
+	PGconn	   *conn = NULL;
+	StringInfoData sql;
+	ListCell   *lc;
+	bool		server_truncatable = true;
+
+	/*
+	 * By default, all postgres_fdw foreign tables are assumed truncatable.
+	 * This can be overridden by a per-server setting, which in turn can be
+	 * overridden by a per-table setting.
+	 */
+	foreach(lc, rels)
+	{
+		ForeignServer *server = NULL;
+		Relation	rel = lfirst(lc);
+		ForeignTable *table = GetForeignTable(RelationGetRelid(rel));
+		ListCell   *cell;
+		bool		truncatable;
+
+		/*
+		 * First time through, determine whether the foreign server allows
+		 * truncates. Since all specified foreign tables are assumed to belong
+		 * to the same foreign server, this result can be used for other
+		 * foreign tables.
+		 */
+		if (!OidIsValid(serverid))
+		{
+			serverid = table->serverid;
+			server = GetForeignServer(serverid);
+
+			foreach(cell, server->options)
+			{
+				DefElem    *defel = (DefElem *) lfirst(cell);
+
+				if (strcmp(defel->defname, "truncatable") == 0)
+				{
+					server_truncatable = defGetBoolean(defel);
+					break;
+				}
+			}
+		}
+
+		/*
+		 * Confirm that all specified foreign tables belong to the same
+		 * foreign server.
+		 */
+		Assert(table->serverid == serverid);
+
+		/* Determine whether this foreign table allows truncations */
+		truncatable = server_truncatable;
+		foreach(cell, table->options)
+		{
+			DefElem    *defel = (DefElem *) lfirst(cell);
+
+			if (strcmp(defel->defname, "truncatable") == 0)
+			{
+				truncatable = defGetBoolean(defel);
+				break;
+			}
+		}
+
+		if (!truncatable)
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("foreign table \"%s\" does not allow truncates",
+							RelationGetRelationName(rel))));
+	}
+	Assert(OidIsValid(serverid));
+
+	/*
+	 * Get connection to the foreign server.  Connection manager will
+	 * establish new connection if necessary.
+	 */
+	user = GetUserMapping(GetUserId(), serverid);
+	conn = GetConnection(user, false, NULL);
+
+	/* Construct the TRUNCATE command string */
+	initStringInfo(&sql);
+	deparseTruncateSql(&sql, rels, rels_extra, behavior, restart_seqs);
+
+	/* Issue the TRUNCATE command to remote server */
+	do_sql_command(conn, sql.data);
+
+	pfree(sql.data);
+}
 
 /*
  * estimate_path_cost_size

contrib/postgres_fdw/postgres_fdw.h

@@ -145,6 +145,7 @@ extern PGconn *GetConnection(UserMapping *user, bool will_prep_stmt,
 extern void ReleaseConnection(PGconn *conn);
 extern unsigned int GetCursorNumber(PGconn *conn);
 extern unsigned int GetPrepStmtNumber(PGconn *conn);
+extern void do_sql_command(PGconn *conn, const char *sql);
 extern PGresult *pgfdw_get_result(PGconn *conn, const char *query);
 extern PGresult *pgfdw_exec_query(PGconn *conn, const char *query,
 								  PgFdwConnState *state);
@@ -206,6 +207,11 @@ extern void deparseDirectDeleteSql(StringInfo buf, PlannerInfo *root,
 extern void deparseAnalyzeSizeSql(StringInfo buf, Relation rel);
 extern void deparseAnalyzeSql(StringInfo buf, Relation rel,
 							  List **retrieved_attrs);
+extern void deparseTruncateSql(StringInfo buf,
+							   List *rels,
+							   List *rels_extra,
+							   DropBehavior behavior,
+							   bool restart_seqs);
 extern void deparseStringLiteral(StringInfo buf, const char *val);
 extern Expr *find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel);
 extern Expr *find_em_expr_for_input_target(PlannerInfo *root,

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -2351,6 +2351,107 @@ select * from rem3;
 drop foreign table rem3;
 drop table loc3;
 
+-- ===================================================================
+-- test for TRUNCATE
+-- ===================================================================
+CREATE TABLE tru_rtable0 (id int primary key);
+CREATE TABLE tru_rtable1 (id int primary key);
+CREATE FOREIGN TABLE tru_ftable (id int)
+       SERVER loopback OPTIONS (table_name 'tru_rtable0');
+INSERT INTO tru_rtable0 (SELECT x FROM generate_series(1,10) x);
+
+CREATE TABLE tru_ptable (id int) PARTITION BY HASH(id);
+CREATE TABLE tru_ptable__p0 PARTITION OF tru_ptable
+                            FOR VALUES WITH (MODULUS 2, REMAINDER 0);
+CREATE FOREIGN TABLE tru_ftable__p1 PARTITION OF tru_ptable
+                                    FOR VALUES WITH (MODULUS 2, REMAINDER 1)
+       SERVER loopback OPTIONS (table_name 'tru_rtable1');
+INSERT INTO tru_ptable (SELECT x FROM generate_series(11,20) x);
+
+CREATE TABLE tru_pk_table(id int primary key);
+CREATE TABLE tru_fk_table(fkey int references tru_pk_table(id));
+INSERT INTO tru_pk_table (SELECT x FROM generate_series(1,10) x);
+INSERT INTO tru_fk_table (SELECT x % 10 + 1 FROM generate_series(5,25) x);
+CREATE FOREIGN TABLE tru_pk_ftable (id int)
+       SERVER loopback OPTIONS (table_name 'tru_pk_table');
+
+CREATE TABLE tru_rtable_parent (id int);
+CREATE TABLE tru_rtable_child (id int);
+CREATE FOREIGN TABLE tru_ftable_parent (id int)
+       SERVER loopback OPTIONS (table_name 'tru_rtable_parent');
+CREATE FOREIGN TABLE tru_ftable_child () INHERITS (tru_ftable_parent)
+       SERVER loopback OPTIONS (table_name 'tru_rtable_child');
+INSERT INTO tru_rtable_parent (SELECT x FROM generate_series(1,8) x);
+INSERT INTO tru_rtable_child  (SELECT x FROM generate_series(10, 18) x);
+
+-- normal truncate
+SELECT sum(id) FROM tru_ftable;        -- 55
+TRUNCATE tru_ftable;
+SELECT count(*) FROM tru_rtable0;		-- 0
+SELECT count(*) FROM tru_ftable;		-- 0
+
+-- 'truncatable' option
+ALTER SERVER loopback OPTIONS (ADD truncatable 'false');
+TRUNCATE tru_ftable;			-- error
+ALTER FOREIGN TABLE tru_ftable OPTIONS (ADD truncatable 'true');
+TRUNCATE tru_ftable;			-- accepted
+ALTER FOREIGN TABLE tru_ftable OPTIONS (SET truncatable 'false');
+TRUNCATE tru_ftable;			-- error
+ALTER SERVER loopback OPTIONS (DROP truncatable);
+ALTER FOREIGN TABLE tru_ftable OPTIONS (SET truncatable 'false');
+TRUNCATE tru_ftable;			-- error
+ALTER FOREIGN TABLE tru_ftable OPTIONS (SET truncatable 'true');
+TRUNCATE tru_ftable;			-- accepted
+
+-- partitioned table with both local and foreign tables as partitions
+SELECT sum(id) FROM tru_ptable;        -- 155
+TRUNCATE tru_ptable;
+SELECT count(*) FROM tru_ptable;		-- 0
+SELECT count(*) FROM tru_ptable__p0;	-- 0
+SELECT count(*) FROM tru_ftable__p1;	-- 0
+SELECT count(*) FROM tru_rtable1;		-- 0
+
+-- 'CASCADE' option
+SELECT sum(id) FROM tru_pk_ftable;      -- 55
+TRUNCATE tru_pk_ftable;	-- failed by FK reference
+TRUNCATE tru_pk_ftable CASCADE;
+SELECT count(*) FROM tru_pk_ftable;    -- 0
+SELECT count(*) FROM tru_fk_table;		-- also truncated,0
+
+-- truncate two tables at a command
+INSERT INTO tru_ftable (SELECT x FROM generate_series(1,8) x);
+INSERT INTO tru_pk_ftable (SELECT x FROM generate_series(3,10) x);
+SELECT count(*) from tru_ftable; -- 8
+SELECT count(*) from tru_pk_ftable; -- 8
+TRUNCATE tru_ftable, tru_pk_ftable CASCADE;
+SELECT count(*) from tru_ftable; -- 0
+SELECT count(*) from tru_pk_ftable; -- 0
+
+-- truncate with ONLY clause
+TRUNCATE ONLY tru_ftable_parent;
+SELECT sum(id) FROM tru_ftable_parent;  -- 126
+TRUNCATE tru_ftable_parent;
+SELECT count(*) FROM tru_ftable_parent; -- 0
+
+-- in case when remote table has inherited children
+CREATE TABLE tru_rtable0_child () INHERITS (tru_rtable0);
+INSERT INTO tru_rtable0 (SELECT x FROM generate_series(5,9) x);
+INSERT INTO tru_rtable0_child (SELECT x FROM generate_series(10,14) x);
+SELECT sum(id) FROM tru_ftable;   -- 95
+
+TRUNCATE ONLY tru_ftable;		-- truncate only parent portion
+SELECT sum(id) FROM tru_ftable;   -- 60
+
+INSERT INTO tru_rtable0 (SELECT x FROM generate_series(21,25) x);
+SELECT sum(id) FROM tru_ftable;		-- 175
+TRUNCATE tru_ftable;			-- truncate both of parent and child
+SELECT count(*) FROM tru_ftable;    -- empty
+
+-- cleanup
+DROP FOREIGN TABLE tru_ftable_parent, tru_ftable_child, tru_pk_ftable,tru_ftable__p1,tru_ftable;
+DROP TABLE tru_rtable0, tru_rtable1, tru_ptable, tru_ptable__p0, tru_pk_table, tru_fk_table,
+tru_rtable_parent,tru_rtable_child, tru_rtable0_child;
+
 -- ===================================================================
 -- test IMPORT FOREIGN SCHEMA
 -- ===================================================================

doc/src/sgml/fdwhandler.sgml

@@ -1065,6 +1065,67 @@ EndDirectModify(ForeignScanState *node);
 
    </sect2>
 
+   <sect2 id="fdw-callbacks-truncate">
+    <title>FDW Routines for <command>TRUNCATE</command></title>
+
+    <para>
+<programlisting>
+void
+ExecForeignTruncate(List *rels, List *rels_extra,
+                    DropBehavior behavior, bool restart_seqs);
+</programlisting>
+
+     Truncate a set of foreign tables specified in <literal>rels</literal>.
+     This function is called when <xref linkend="sql-truncate"/> is executed
+     on foreign tables.  <literal>rels</literal> is the list of
+     <structname>Relation</structname> data structure that indicates
+     a foreign table to truncate.  <literal>rels_extra</literal> the list of
+     <literal>int</literal> value, which delivers extra information about
+     a foreign table to truncate.  Possible values are
+     <literal>TRUNCATE_REL_CONTEXT_NORMAL</literal>, which means that
+     the foreign table is specified WITHOUT <literal>ONLY</literal> clause
+     in <command>TRUNCATE</command>,
+     <literal>TRUNCATE_REL_CONTEXT_ONLY</literal>, which means that
+     the foreign table is specified WITH <literal>ONLY</literal> clause,
+     and <literal>TRUNCATE_REL_CONTEXT_CASCADING</literal>,
+     which means that the foreign table is not specified explicitly,
+     but will be truncated due to dependency (for example, partition table).
+     There is one-to-one mapping between <literal>rels</literal> and
+     <literal>rels_extra</literal>.  The number of entries is the same
+     between the two lists.
+    </para>
+
+    <para>
+     <literal>behavior</literal> defines how foreign tables should
+     be truncated, using as possible values <literal>DROP_RESTRICT</literal>,
+     which means that <literal>RESTRICT</literal> option is specified,
+     and <literal>DROP_CASCADE</literal>, which means that
+     <literal>CASCADE</literal> option is specified, in
+     <command>TRUNCATE</command> command.
+    </para>
+    
+    <para>
+     <literal>restart_seqs</literal> is set to <literal>true</literal>
+     if <literal>RESTART IDENTITY</literal> option is specified in
+     <command>TRUNCATE</command> command.  It is <literal>false</literal>
+     if <literal>CONTINUE IDENTITY</literal> option is specified.
+    </para>
+
+    <para>
+     <command>TRUNCATE</command> invokes
+     <function>ExecForeignTruncate</function> once per foreign server
+     that foreign tables to truncate belong to.  This means that all foreign
+     tables included in <literal>rels</literal> must belong to the same
+     server.
+    </para>
+
+    <para>
+     If the <function>ExecForeignTruncate</function> pointer is set to
+     <literal>NULL</literal>, attempts to truncate foreign tables will
+     fail with an error message.
+    </para>
+   </sect2>
+
    <sect2 id="fdw-callbacks-row-locking">
     <title>FDW Routines for Row Locking</title>
 

doc/src/sgml/postgres-fdw.sgml

@@ -63,9 +63,10 @@
  <para>
   Now you need only <command>SELECT</command> from a foreign table to access
   the data stored in its underlying remote table.  You can also modify
-  the remote table using <command>INSERT</command>, <command>UPDATE</command>, or
-  <command>DELETE</command>.  (Of course, the remote user you have specified
-  in your user mapping must have privileges to do these things.)
+  the remote table using <command>INSERT</command>, <command>UPDATE</command>,
+  <command>DELETE</command>, or <command>TRUNCATE</command>.
+  (Of course, the remote user you have specified in your user mapping must
+  have privileges to do these things.)
  </para>
 
  <para>
@@ -436,6 +437,31 @@ OPTIONS (ADD password_required 'false');
    </variablelist>
   </sect3>
 
+  <sect3>
+   <title>Truncatability Options</title>
+
+   <para>
+    By default all foreign tables using <filename>postgres_fdw</filename> are assumed
+    to be truncatable.  This may be overridden using the following option:
+   </para>
+
+   <variablelist>
+
+    <varlistentry>
+     <term><literal>truncatable</literal></term>
+     <listitem>
+      <para>
+       This option controls whether <filename>postgres_fdw</filename> allows
+       foreign tables to be truncated using <command>TRUNCATE</command>
+       command. It can be specified for a foreign table or a foreign server.
+       A table-level option overrides a server-level option.
+       The default is <literal>true</literal>.
+      </para>
+     </listitem>
+    </varlistentry>
+   </variablelist>
+  </sect3>
+
   <sect3>
    <title>Importing Options</title>
 

doc/src/sgml/ref/truncate.sgml

@@ -172,9 +172,9 @@ TRUNCATE [ TABLE ] [ ONLY ] <replaceable class="parameter">name</replaceable> [
   </para>
 
   <para>
-   <command>TRUNCATE</command> is not currently supported for foreign tables.
-   This implies that if a specified table has any descendant tables that are
-   foreign, the command will fail.
+   <command>TRUNCATE</command> can be used for foreign tables if
+   the foreign data wrapper supports, for instance,
+   see <xref linkend="postgres-fdw"/>.
   </para>
  </refsect1>
 

src/backend/commands/tablecmds.c

@@ -59,6 +59,7 @@
 #include "commands/typecmds.h"
 #include "commands/user.h"
 #include "executor/executor.h"
+#include "foreign/fdwapi.h"
 #include "foreign/foreign.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
@@ -310,6 +311,21 @@ struct DropRelationCallbackState
 #define		ATT_FOREIGN_TABLE		0x0020
 #define		ATT_PARTITIONED_INDEX	0x0040
 
+/*
+ * ForeignTruncateInfo
+ *
+ * Information related to truncation of foreign tables.  This is used for
+ * the elements in a hash table. It uses the server OID as lookup key,
+ * and includes a per-server list of all foreign tables involved in the
+ * truncation.
+ */
+typedef struct ForeignTruncateInfo
+{
+	Oid			serverid;
+	List	   *rels;
+	List	   *rels_extra;
+} ForeignTruncateInfo;
+
 /*
  * Partition tables are expected to be dropped when the parent partitioned
  * table gets dropped. Hence for partitioning we use AUTO dependency.
@@ -1589,7 +1605,10 @@ RangeVarCallbackForDropRelation(const RangeVar *rel, Oid relOid, Oid oldRelOid,
  *
  * This is a multi-relation truncate.  We first open and grab exclusive
  * lock on all relations involved, checking permissions and otherwise
- * verifying that the relation is OK for truncation.  In CASCADE mode,
+ * verifying that the relation is OK for truncation.  Note that if relations
+ * are foreign tables, at this stage, we have not yet checked that their
+ * foreign data in external data sources are OK for truncation.  These are
+ * checked when foreign data are actually truncated later.  In CASCADE mode,
  * relations having FK references to the targeted relations are automatically
  * added to the group; in RESTRICT mode, we check that all FK references are
  * internal to the group that's being truncated.  Finally all the relations
@@ -1600,6 +1619,7 @@ ExecuteTruncate(TruncateStmt *stmt)
 {
 	List	   *rels = NIL;
 	List	   *relids = NIL;
+	List	   *relids_extra = NIL;
 	List	   *relids_logged = NIL;
 	ListCell   *cell;
 
@@ -1636,6 +1656,9 @@ ExecuteTruncate(TruncateStmt *stmt)
 
 		rels = lappend(rels, rel);
 		relids = lappend_oid(relids, myrelid);
+		relids_extra = lappend_int(relids_extra, (recurse ?
+												  TRUNCATE_REL_CONTEXT_NORMAL :
+												  TRUNCATE_REL_CONTEXT_ONLY));
 		/* Log this relation only if needed for logical decoding */
 		if (RelationIsLogicallyLogged(rel))
 			relids_logged = lappend_oid(relids_logged, myrelid);
@@ -1683,6 +1706,8 @@ ExecuteTruncate(TruncateStmt *stmt)
 
 				rels = lappend(rels, rel);
 				relids = lappend_oid(relids, childrelid);
+				relids_extra = lappend_int(relids_extra,
+										   TRUNCATE_REL_CONTEXT_CASCADING);
 				/* Log this relation only if needed for logical decoding */
 				if (RelationIsLogicallyLogged(rel))
 					relids_logged = lappend_oid(relids_logged, childrelid);
@@ -1695,7 +1720,7 @@ ExecuteTruncate(TruncateStmt *stmt)
 					 errhint("Do not specify the ONLY keyword, or use TRUNCATE ONLY on the partitions directly.")));
 	}
 
-	ExecuteTruncateGuts(rels, relids, relids_logged,
+	ExecuteTruncateGuts(rels, relids, relids_extra, relids_logged,
 						stmt->behavior, stmt->restart_seqs);
 
 	/* And close the rels */
@@ -1716,21 +1741,28 @@ ExecuteTruncate(TruncateStmt *stmt)
  *
  * explicit_rels is the list of Relations to truncate that the command
  * specified.  relids is the list of Oids corresponding to explicit_rels.
- * relids_logged is the list of Oids (a subset of relids) that require
- * WAL-logging.  This is all a bit redundant, but the existing callers have
- * this information handy in this form.
+ * relids_extra is the list of integer values that deliver extra information
+ * about relations in explicit_rels.  relids_logged is the list of Oids
+ * (a subset of relids) that require WAL-logging.  This is all a bit redundant,
+ * but the existing callers have this information handy in this form.
  */
 void
-ExecuteTruncateGuts(List *explicit_rels, List *relids, List *relids_logged,
+ExecuteTruncateGuts(List *explicit_rels,
+					List *relids,
+					List *relids_extra,
+					List *relids_logged,
 					DropBehavior behavior, bool restart_seqs)
 {
 	List	   *rels;
 	List	   *seq_relids = NIL;
+	HTAB	   *ft_htab = NULL;
 	EState	   *estate;
 	ResultRelInfo *resultRelInfos;
 	ResultRelInfo *resultRelInfo;
 	SubTransactionId mySubid;
 	ListCell   *cell;
+	ListCell   *lc1,
+			*lc2;
 	Oid		   *logrelids;
 
 	/*
@@ -1768,6 +1800,8 @@ ExecuteTruncateGuts(List *explicit_rels, List *relids, List *relids_logged,
 				truncate_check_activity(rel);
 				rels = lappend(rels, rel);
 				relids = lappend_oid(relids, relid);
+				relids_extra = lappend_int(relids_extra,
+										   TRUNCATE_REL_CONTEXT_CASCADING);
 				/* Log this relation only if needed for logical decoding */
 				if (RelationIsLogicallyLogged(rel))
 					relids_logged = lappend_oid(relids_logged, relid);
@@ -1868,14 +1902,63 @@ ExecuteTruncateGuts(List *explicit_rels, List *relids, List *relids_logged,
 	 */
 	mySubid = GetCurrentSubTransactionId();
 
-	foreach(cell, rels)
+	Assert(list_length(rels) == list_length(relids_extra));
+	forboth(lc1, rels, lc2, relids_extra)
 	{
-		Relation	rel = (Relation) lfirst(cell);
+		Relation	rel = (Relation) lfirst(lc1);
+		int			extra = lfirst_int(lc2);
 
 		/* Skip partitioned tables as there is nothing to do */
 		if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
 			continue;
 
+		/*
+		 * Build the lists of foreign tables belonging to each foreign server
+		 * and pass each list to the foreign data wrapper's callback function,
+		 * so that each server can truncate its all foreign tables in bulk.
+		 * Each list is saved as a single entry in a hash table that uses the
+		 * server OID as lookup key.
+		 */
+		if (rel->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
+		{
+			Oid			serverid = GetForeignServerIdByRelId(RelationGetRelid(rel));
+			bool		found;
+			ForeignTruncateInfo *ft_info;
+
+			/* First time through, initialize hashtable for foreign tables */
+			if (!ft_htab)
+			{
+				HASHCTL		hctl;
+
+				memset(&hctl, 0, sizeof(HASHCTL));
+				hctl.keysize = sizeof(Oid);
+				hctl.entrysize = sizeof(ForeignTruncateInfo);
+				hctl.hcxt = CurrentMemoryContext;
+
+				ft_htab = hash_create("TRUNCATE for Foreign Tables",
+									  32,	/* start small and extend */
+									  &hctl,
+									  HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+			}
+
+			/* Find or create cached entry for the foreign table */
+			ft_info = hash_search(ft_htab, &serverid, HASH_ENTER, &found);
+			if (!found)
+			{
+				ft_info->serverid = serverid;
+				ft_info->rels = NIL;
+				ft_info->rels_extra = NIL;
+			}
+
+			/*
+			 * Save the foreign table in the entry of the server that the
+			 * foreign table belongs to.
+			 */
+			ft_info->rels = lappend(ft_info->rels, rel);
+			ft_info->rels_extra = lappend_int(ft_info->rels_extra, extra);
+			continue;
+		}
+
 		/*
 		 * Normally, we need a transaction-safe truncation here.  However, if
 		 * the table was either created in the current (sub)transaction or has
@@ -1938,6 +2021,36 @@ ExecuteTruncateGuts(List *explicit_rels, List *relids, List *relids_logged,
 		pgstat_count_truncate(rel);
 	}
 
+	/* Now go through the hash table, and truncate foreign tables */
+	if (ft_htab)
+	{
+		ForeignTruncateInfo *ft_info;
+		HASH_SEQ_STATUS seq;
+
+		hash_seq_init(&seq, ft_htab);
+
+		PG_TRY();
+		{
+			while ((ft_info = hash_seq_search(&seq)) != NULL)
+			{
+				FdwRoutine *routine = GetFdwRoutineByServerId(ft_info->serverid);
+
+				/* truncate_check_rel() has checked that already */
+				Assert(routine->ExecForeignTruncate != NULL);
+
+				routine->ExecForeignTruncate(ft_info->rels,
+											 ft_info->rels_extra,
+											 behavior,
+											 restart_seqs);
+			}
+		}
+		PG_FINALLY();
+		{
+			hash_destroy(ft_htab);
+		}
+		PG_END_TRY();
+	}
+
 	/*
 	 * Restart owned sequences if we were asked to.
 	 */
@@ -2023,12 +2136,24 @@ truncate_check_rel(Oid relid, Form_pg_class reltuple)
 	char	   *relname = NameStr(reltuple->relname);
 
 	/*
-	 * Only allow truncate on regular tables and partitioned tables (although,
-	 * the latter are only being included here for the following checks; no
-	 * physical truncation will occur in their case.)
+	 * Only allow truncate on regular tables, foreign tables using foreign
+	 * data wrappers supporting TRUNCATE and partitioned tables (although, the
+	 * latter are only being included here for the following checks; no
+	 * physical truncation will occur in their case.).
 	 */
-	if (reltuple->relkind != RELKIND_RELATION &&
-		reltuple->relkind != RELKIND_PARTITIONED_TABLE)
+	if (reltuple->relkind == RELKIND_FOREIGN_TABLE)
+	{
+		Oid			serverid = GetForeignServerIdByRelId(relid);
+		FdwRoutine *fdwroutine = GetFdwRoutineByServerId(serverid);
+
+		if (!fdwroutine->ExecForeignTruncate)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("cannot truncate foreign table \"%s\"",
+							relname)));
+	}
+	else if (reltuple->relkind != RELKIND_RELATION &&
+			 reltuple->relkind != RELKIND_PARTITIONED_TABLE)
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
 				 errmsg("\"%s\" is not a table", relname)));

src/backend/replication/logical/worker.c

@@ -1795,6 +1795,7 @@ apply_handle_truncate(StringInfo s)
 	List	   *rels = NIL;
 	List	   *part_rels = NIL;
 	List	   *relids = NIL;
+	List	   *relids_extra = NIL;
 	List	   *relids_logged = NIL;
 	ListCell   *lc;
 
@@ -1824,6 +1825,7 @@ apply_handle_truncate(StringInfo s)
 		remote_rels = lappend(remote_rels, rel);
 		rels = lappend(rels, rel->localrel);
 		relids = lappend_oid(relids, rel->localreloid);
+		relids_extra = lappend_int(relids_extra, TRUNCATE_REL_CONTEXT_NORMAL);
 		if (RelationIsLogicallyLogged(rel->localrel))
 			relids_logged = lappend_oid(relids_logged, rel->localreloid);
 
@@ -1862,6 +1864,7 @@ apply_handle_truncate(StringInfo s)
 				rels = lappend(rels, childrel);
 				part_rels = lappend(part_rels, childrel);
 				relids = lappend_oid(relids, childrelid);
+				relids_extra = lappend_int(relids_extra, TRUNCATE_REL_CONTEXT_CASCADING);
 				/* Log this relation only if needed for logical decoding */
 				if (RelationIsLogicallyLogged(childrel))
 					relids_logged = lappend_oid(relids_logged, childrelid);
@@ -1874,8 +1877,12 @@ apply_handle_truncate(StringInfo s)
 	 * to replaying changes without further cascading. This might be later
 	 * changeable with a user specified option.
 	 */
-	ExecuteTruncateGuts(rels, relids, relids_logged, DROP_RESTRICT, restart_seqs);
-
+	ExecuteTruncateGuts(rels,
+						relids,
+						relids_extra,
+						relids_logged,
+						DROP_RESTRICT,
+						restart_seqs);
 	foreach(lc, remote_rels)
 	{
 		LogicalRepRelMapEntry *rel = lfirst(lc);

src/include/commands/tablecmds.h

@@ -21,6 +21,16 @@
 #include "storage/lock.h"
 #include "utils/relcache.h"
 
+/*
+ * These values indicate how a relation was specified as the target to
+ * truncate in TRUNCATE command.
+ */
+#define TRUNCATE_REL_CONTEXT_NORMAL       1 /* specified without ONLY clause */
+#define TRUNCATE_REL_CONTEXT_ONLY         2 /* specified with ONLY clause */
+#define TRUNCATE_REL_CONTEXT_CASCADING     3	/* not specified but truncated
+												 * due to dependency (e.g.,
+												 * partition table) */
+
 struct AlterTableUtilityContext;	/* avoid including tcop/utility.h here */
 
 
@@ -56,8 +66,12 @@ extern void AlterRelationNamespaceInternal(Relation classRel, Oid relOid,
 extern void CheckTableNotInUse(Relation rel, const char *stmt);
 
 extern void ExecuteTruncate(TruncateStmt *stmt);
-extern void ExecuteTruncateGuts(List *explicit_rels, List *relids, List *relids_logged,
-								DropBehavior behavior, bool restart_seqs);
+extern void ExecuteTruncateGuts(List *explicit_rels,
+								List *relids,
+								List *relids_extra,
+								List *relids_logged,
+								DropBehavior behavior,
+								bool restart_seqs);
 
 extern void SetRelationHasSubclass(Oid relationId, bool relhassubclass);
 

src/include/foreign/fdwapi.h

@@ -160,6 +160,11 @@ typedef bool (*AnalyzeForeignTable_function) (Relation relation,
 typedef List *(*ImportForeignSchema_function) (ImportForeignSchemaStmt *stmt,
 											   Oid serverOid);
 
+typedef void (*ExecForeignTruncate_function) (List *rels,
+											  List *rels_extra,
+											  DropBehavior behavior,
+											  bool restart_seqs);
+
 typedef Size (*EstimateDSMForeignScan_function) (ForeignScanState *node,
 												 ParallelContext *pcxt);
 typedef void (*InitializeDSMForeignScan_function) (ForeignScanState *node,
@@ -255,6 +260,9 @@ typedef struct FdwRoutine
 	/* Support functions for IMPORT FOREIGN SCHEMA */
 	ImportForeignSchema_function ImportForeignSchema;
 
+	/* Support functions for TRUNCATE */
+	ExecForeignTruncate_function ExecForeignTruncate;
+
 	/* Support functions for parallelism under Gather node */
 	IsForeignScanParallelSafe_function IsForeignScanParallelSafe;
 	EstimateDSMForeignScan_function EstimateDSMForeignScan;

src/include/utils/rel.h

@@ -634,7 +634,8 @@ typedef struct ViewOptions
  *		WAL stream.
  *
  * We don't log information for unlogged tables (since they don't WAL log
- * anyway) and for system tables (their content is hard to make sense of, and
+ * anyway), for foreign tables (since they don't WAL log, either),
+ * and for system tables (their content is hard to make sense of, and
  * it would complicate decoding slightly for little gain). Note that we *do*
  * log information for user defined catalog tables since they presumably are
  * interesting to the user...
@@ -642,6 +643,7 @@ typedef struct ViewOptions
 #define RelationIsLogicallyLogged(relation) \
 	(XLogLogicalInfoActive() && \
 	 RelationNeedsWAL(relation) && \
+	 (relation)->rd_rel->relkind != RELKIND_FOREIGN_TABLE &&	\
 	 !IsCatalogRelation(relation))
 
 /* routines in utils/cache/relcache.c */

src/test/regress/expected/foreign_data.out

@@ -1807,9 +1807,9 @@ Inherits: fd_pt1
 
 -- TRUNCATE doesn't work on foreign tables, either directly or recursively
 TRUNCATE ft2;  -- ERROR
-ERROR:  "ft2" is not a table
+ERROR:  foreign-data wrapper "dummy" has no handler
 TRUNCATE fd_pt1;  -- ERROR
-ERROR:  "ft2" is not a table
+ERROR:  foreign-data wrapper "dummy" has no handler
 DROP TABLE fd_pt1 CASCADE;
 NOTICE:  drop cascades to foreign table ft2
 -- IMPORT FOREIGN SCHEMA
@@ -2032,9 +2032,9 @@ ALTER FOREIGN TABLE fd_pt2_1 ADD CONSTRAINT fd_pt2chk1 CHECK (c1 > 0);
 ALTER TABLE fd_pt2 ATTACH PARTITION fd_pt2_1 FOR VALUES IN (1);
 -- TRUNCATE doesn't work on foreign tables, either directly or recursively
 TRUNCATE fd_pt2_1;  -- ERROR
-ERROR:  "fd_pt2_1" is not a table
+ERROR:  foreign-data wrapper "dummy" has no handler
 TRUNCATE fd_pt2;  -- ERROR
-ERROR:  "fd_pt2_1" is not a table
+ERROR:  foreign-data wrapper "dummy" has no handler
 DROP FOREIGN TABLE fd_pt2_1;
 DROP TABLE fd_pt2;
 -- foreign table cannot be part of partition tree made of temporary

src/tools/pgindent/typedefs.list

@@ -704,6 +704,7 @@ ForeignScanState
 ForeignServer
 ForeignServerInfo
 ForeignTable
+ForeignTruncateInfo
 ForkNumber
 FormData_pg_aggregate
 FormData_pg_am