postgres_fdw: Account for triggers in non-direct remote UPDATE planning.

Previously, in postgresPlanForeignModify, we planned an UPDATE operation
on a foreign table so that we transmit only columns that were explicitly
targets of the UPDATE, so as to avoid unnecessary data transmission, but
if there were BEFORE ROW UPDATE triggers on the foreign table, those
triggers might change values for non-target columns, in which case we
would miss sending changed values for those columns.  Prevent optimizing
away transmitting all columns if there are BEFORE ROW UPDATE triggers on
the foreign table.

This is an oversight in commit 7cbe57c34 which added triggers on foreign
tables, so apply the patch all the way back to 9.4 where that came in.

Author: Shohei Mochizuki
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/201905270152.x4R1q3qi014550@toshiba.co.jp

1 files changed, 13 insertions, 6 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 11451c2055d..3b874467f0f 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -1686,12 +1686,19 @@ postgresPlanForeignModify(PlannerInfo *root,
 
 	/*
 	 * In an INSERT, we transmit all columns that are defined in the foreign
-	 * table.  In an UPDATE, we transmit only columns that were explicitly
-	 * targets of the UPDATE, so as to avoid unnecessary data transmission.
-	 * (We can't do that for INSERT since we would miss sending default values
-	 * for columns not listed in the source statement.)
-	 */
-	if (operation == CMD_INSERT)
+	 * table.  In an UPDATE, if there are BEFORE ROW UPDATE triggers on the
+	 * foreign table, we transmit all columns like INSERT; else we transmit
+	 * only columns that were explicitly targets of the UPDATE, so as to avoid
+	 * unnecessary data transmission.  (We can't do that for INSERT since we
+	 * would miss sending default values for columns not listed in the source
+	 * statement, and for UPDATE if there are BEFORE ROW UPDATE triggers since
+	 * those triggers might change values for non-target columns, in which
+	 * case we would miss sending changed values for those columns.)
+	 */
+	if (operation == CMD_INSERT ||
+		(operation == CMD_UPDATE &&
+		 rel->trigdesc &&
+		 rel->trigdesc->trig_update_before_row))
 	{
 		TupleDesc	tupdesc = RelationGetDescr(rel);
 		int			attnum;