Document trigger support.

In passing, simplify the check for AFTER triggers with FOR EACH ROW.

README.oracle_fdw

@@ -351,6 +351,11 @@ The RETURNING clause on INSERT, UPDATE and DELETE is supported except
 for columns with Oracle data types LONG and LONG RAW (Oracle doesn't support
 these data types in the RETURNING clause).
 
+Triggers on foreign tables are supported from PostgreSQL 9.4.
+Triggers defined with AFTER and FOR EACH ROW require that the foreign table
+has no columns with Oracle data type LONG or LONG RAW.  This is because
+such triggers make use of the RETURNING clause mentioned above.
+
 While modifying foreign data works, the performance is not particularly
 good, specifically when many rows are affected, because (owing to the way
 foreign data wrappers work) each row has to be treated individually.

oracle_fdw.c

@@ -661,9 +661,9 @@ oracleGetForeignRelSize(PlannerInfo *root, RelOptInfo *baserel, Oid foreigntable
 	rel = heap_open(foreigntableid, NoLock);
 
 	/* is there an AFTER trigger FOR EACH ROW? */
-	has_trigger = (baserel->relid == root->parse->resultRelation)
-					&& ((root->parse->commandType == CMD_UPDATE && rel->trigdesc && rel->trigdesc->trig_update_after_row)
-						|| (root->parse->commandType == CMD_DELETE && rel->trigdesc && rel->trigdesc->trig_delete_after_row));
+	has_trigger = (baserel->relid == root->parse->resultRelation) && rel->trigdesc
+					&& ((root->parse->commandType == CMD_UPDATE && rel->trigdesc->trig_update_after_row)
+						|| (root->parse->commandType == CMD_DELETE && rel->trigdesc->trig_delete_after_row));
 
 	heap_close(rel, NoLock);