Allow table AM tuple_insert() method to return the different slot

This allows table AM to return a native tuple slot even if
VirtualTupleTableSlot is given as an input.  Native tuple slots have knowledge
about system attributes, which could be accessed in the future.
table_multi_insert() method already can modify the input 'slots' array.

Discussion: https://postgr.es/m/CAPpHfdurb9ycV8udYqM%3Do0sPS66PJ4RCBM1g-bBpvzUfogY0EA%40mail.gmail.com
Reviewed-by: Matthias van de Meent, Mark Dilger, Pavel Borisov
Reviewed-by: Nikita Malakhov, Japin Li

Author: akorotkov

src/backend/access/heap/heapam_handler.c

@@ -237,7 +237,7 @@ heapam_tuple_satisfies_snapshot(Relation rel, TupleTableSlot *slot,
  * ----------------------------------------------------------------------------
  */
 
-static void
+static TupleTableSlot *
 heapam_tuple_insert(Relation relation, TupleTableSlot *slot, CommandId cid,
 					int options, BulkInsertState bistate)
 {
@@ -254,6 +254,8 @@ heapam_tuple_insert(Relation relation, TupleTableSlot *slot, CommandId cid,
 
 	if (shouldFree)
 		pfree(tuple);
+
+	return slot;
 }
 
 static void

src/backend/executor/nodeModifyTable.c

@@ -1125,9 +1125,9 @@ ExecInsert(ModifyTableContext *context,
 		else
 		{
 			/* insert the tuple normally */
-			table_tuple_insert(resultRelationDesc, slot,
-							   estate->es_output_cid,
-							   0, NULL);
+			slot = table_tuple_insert(resultRelationDesc, slot,
+									  estate->es_output_cid,
+									  0, NULL);
 
 			/* insert index entries for tuple */
 			if (resultRelInfo->ri_NumIndices > 0)

src/include/access/tableam.h

@@ -500,9 +500,9 @@ typedef struct TableAmRoutine
 	 */
 
 	/* see table_tuple_insert() for reference about parameters */
-	void		(*tuple_insert) (Relation rel, TupleTableSlot *slot,
-								 CommandId cid, int options,
-								 struct BulkInsertStateData *bistate);
+	TupleTableSlot *(*tuple_insert) (Relation rel, TupleTableSlot *slot,
+									 CommandId cid, int options,
+									 struct BulkInsertStateData *bistate);
 
 	/* see table_tuple_insert_speculative() for reference about parameters */
 	void		(*tuple_insert_speculative) (Relation rel,
@@ -1392,16 +1392,19 @@ table_index_delete_tuples(Relation rel, TM_IndexDeleteOp *delstate)
  * behavior) is also just passed through to RelationGetBufferForTuple. If
  * `bistate` is provided, table_finish_bulk_insert() needs to be called.
  *
- * On return the slot's tts_tid and tts_tableOid are updated to reflect the
- * insertion. But note that any toasting of fields within the slot is NOT
- * reflected in the slots contents.
+ * Returns the slot containing the inserted tuple, which may differ from the
+ * given slot. For instance, the source slot may be VirtualTupleTableSlot, but
+ * the result slot may correspond to the table AM. On return the slot's
+ * tts_tid and tts_tableOid are updated to reflect the insertion. But note
+ * that any toasting of fields within the slot is NOT reflected in the slots
+ * contents.
  */
-static inline void
+static inline TupleTableSlot *
 table_tuple_insert(Relation rel, TupleTableSlot *slot, CommandId cid,
 				   int options, struct BulkInsertStateData *bistate)
 {
-	rel->rd_tableam->tuple_insert(rel, slot, cid, options,
-								  bistate);
+	return rel->rd_tableam->tuple_insert(rel, slot, cid, options,
+										 bistate);
 }
 
 /*