Fix tuple routing to initialize batching only for inserts

A cross-partition update on a partitioned table is implemented as a
delete followed by an insert. With foreign partitions, this was however
causing issues, because the FDW and core may disagree on when to enable
batching.  postgres_fdw was only allowing batching for plain inserts
(CMD_INSERT) while core was trying to batch the insert component of the
cross-partition update.  Fix by restricting core to apply batching only
to plain CMD_INSERT queries.

It's possible to allow batching for cross-partition updates, but that
will require more extensive changes, so better to leave that for a
separate patch.

Author: Amit Langote
Reviewed-by: Tomas Vondra, Takayuki Tsunakawa
Discussion: https://postgr.es/m/20200628151002.7x5laxwpgvkyiu3q@development

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -9414,5 +9414,26 @@ SELECT COUNT(*) FROM batch_table;
     66
 (1 row)
 
+-- Check that enabling batched inserts doesn't interfere with cross-partition
+-- updates
+CREATE TABLE batch_cp_upd_test (a int) PARTITION BY LIST (a);
+CREATE TABLE batch_cp_upd_test1 (LIKE batch_cp_upd_test);
+CREATE FOREIGN TABLE batch_cp_upd_test1_f
+	PARTITION OF batch_cp_upd_test
+	FOR VALUES IN (1)
+	SERVER loopback
+	OPTIONS (table_name 'batch_cp_upd_test1', batch_size '10');
+CREATE TABLE batch_cp_up_test1 PARTITION OF batch_cp_upd_test
+	FOR VALUES IN (2);
+INSERT INTO batch_cp_upd_test VALUES (1), (2);
+-- The following moves a row from the local partition to the foreign one
+UPDATE batch_cp_upd_test t SET a = 1 FROM (VALUES (1), (2)) s(a) WHERE t.a = s.a;
+SELECT tableoid::regclass, * FROM batch_cp_upd_test;
+       tableoid       | a 
+----------------------+---
+ batch_cp_upd_test1_f | 1
+ batch_cp_upd_test1_f | 1
+(2 rows)
+
 -- Clean up
-DROP TABLE batch_table CASCADE;
+DROP TABLE batch_table, batch_cp_upd_test CASCADE;

contrib/postgres_fdw/postgres_fdw.c

@@ -1934,17 +1934,26 @@ static int
 postgresGetForeignModifyBatchSize(ResultRelInfo *resultRelInfo)
 {
 	int	batch_size;
+	PgFdwModifyState *fmstate = resultRelInfo->ri_FdwState ?
+							(PgFdwModifyState *) resultRelInfo->ri_FdwState :
+							NULL;
 
 	/* should be called only once */
 	Assert(resultRelInfo->ri_BatchSize == 0);
 
+	/*
+	 * Should never get called when the insert is being performed as part of
+	 * a row movement operation.
+	 */
+	Assert(fmstate == NULL || fmstate->aux_fmstate == NULL);
+
 	/*
 	 * In EXPLAIN without ANALYZE, ri_fdwstate is NULL, so we have to lookup
 	 * the option directly in server/table options. Otherwise just use the
 	 * value we determined earlier.
 	 */
-	if (resultRelInfo->ri_FdwState)
-		batch_size = ((PgFdwModifyState *) resultRelInfo->ri_FdwState)->batch_size;
+	if (fmstate)
+		batch_size = fmstate->batch_size;
 	else
 		batch_size = get_batch_size_option(resultRelInfo->ri_RelationDesc);
 

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -2909,5 +2909,22 @@ CREATE TABLE batch_table_p2
 INSERT INTO batch_table SELECT * FROM generate_series(1, 66) i;
 SELECT COUNT(*) FROM batch_table;
 
+-- Check that enabling batched inserts doesn't interfere with cross-partition
+-- updates
+CREATE TABLE batch_cp_upd_test (a int) PARTITION BY LIST (a);
+CREATE TABLE batch_cp_upd_test1 (LIKE batch_cp_upd_test);
+CREATE FOREIGN TABLE batch_cp_upd_test1_f
+	PARTITION OF batch_cp_upd_test
+	FOR VALUES IN (1)
+	SERVER loopback
+	OPTIONS (table_name 'batch_cp_upd_test1', batch_size '10');
+CREATE TABLE batch_cp_up_test1 PARTITION OF batch_cp_upd_test
+	FOR VALUES IN (2);
+INSERT INTO batch_cp_upd_test VALUES (1), (2);
+
+-- The following moves a row from the local partition to the foreign one
+UPDATE batch_cp_upd_test t SET a = 1 FROM (VALUES (1), (2)) s(a) WHERE t.a = s.a;
+SELECT tableoid::regclass, * FROM batch_cp_upd_test;
+
 -- Clean up
-DROP TABLE batch_table CASCADE;
+DROP TABLE batch_table, batch_cp_upd_test CASCADE;

src/backend/executor/execPartition.c

@@ -1000,7 +1000,8 @@ ExecInitRoutingInfo(ModifyTableState *mtstate,
 	 *
 	 * If the FDW does not support batching, we set the batch size to 1.
 	 */
-	if (partRelInfo->ri_FdwRoutine != NULL &&
+	if (mtstate->operation == CMD_INSERT &&
+		partRelInfo->ri_FdwRoutine != NULL &&
 		partRelInfo->ri_FdwRoutine->GetForeignModifyBatchSize &&
 		partRelInfo->ri_FdwRoutine->ExecForeignBatchInsert)
 		partRelInfo->ri_BatchSize =