Add TupleTableSlotOps.is_current_xact_tuple() method

This allows us to abstract how/whether table AM uses transaction identifiers.
A custom table AM can use a custom slot, which may not store xmin directly,
but determine the tuple belonging to the current transaction in the other way.

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

src/backend/executor/execTuples.c

@@ -60,6 +60,7 @@
 #include "access/heaptoast.h"
 #include "access/htup_details.h"
 #include "access/tupdesc_details.h"
+#include "access/xact.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"
 #include "nodes/nodeFuncs.h"
@@ -148,6 +149,22 @@ tts_virtual_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 	return 0;					/* silence compiler warnings */
 }
 
+/*
+ * VirtualTupleTableSlots never have storage tuples.  We generally
+ * shouldn't get here, but provide a user-friendly message if we do.
+ */
+static bool
+tts_virtual_is_current_xact_tuple(TupleTableSlot *slot)
+{
+	Assert(!TTS_EMPTY(slot));
+
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("don't have a storage tuple in this context")));
+
+	return false;				/* silence compiler warnings */
+}
+
 /*
  * To materialize a virtual slot all the datums that aren't passed by value
  * have to be copied into the slot's memory context.  To do so, compute the
@@ -354,6 +371,29 @@ tts_heap_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 						   slot->tts_tupleDescriptor, isnull);
 }
 
+static bool
+tts_heap_is_current_xact_tuple(TupleTableSlot *slot)
+{
+	HeapTupleTableSlot *hslot = (HeapTupleTableSlot *) slot;
+	TransactionId xmin;
+
+	Assert(!TTS_EMPTY(slot));
+
+	/*
+	 * In some code paths it's possible to get here with a non-materialized
+	 * slot, in which case we can't check if tuple is created by the current
+	 * transaction.
+	 */
+	if (!hslot->tuple)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("don't have a storage tuple in this context")));
+
+	xmin = HeapTupleHeaderGetRawXmin(hslot->tuple->t_data);
+
+	return TransactionIdIsCurrentTransactionId(xmin);
+}
+
 static void
 tts_heap_materialize(TupleTableSlot *slot)
 {
@@ -521,6 +561,18 @@ tts_minimal_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 	return 0;					/* silence compiler warnings */
 }
 
+static bool
+tts_minimal_is_current_xact_tuple(TupleTableSlot *slot)
+{
+	Assert(!TTS_EMPTY(slot));
+
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("don't have a storage tuple in this context")));
+
+	return false;				/* silence compiler warnings */
+}
+
 static void
 tts_minimal_materialize(TupleTableSlot *slot)
 {
@@ -714,6 +766,29 @@ tts_buffer_heap_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 						   slot->tts_tupleDescriptor, isnull);
 }
 
+static bool
+tts_buffer_is_current_xact_tuple(TupleTableSlot *slot)
+{
+	BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
+	TransactionId xmin;
+
+	Assert(!TTS_EMPTY(slot));
+
+	/*
+	 * In some code paths it's possible to get here with a non-materialized
+	 * slot, in which case we can't check if tuple is created by the current
+	 * transaction.
+	 */
+	if (!bslot->base.tuple)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("don't have a storage tuple in this context")));
+
+	xmin = HeapTupleHeaderGetRawXmin(bslot->base.tuple->t_data);
+
+	return TransactionIdIsCurrentTransactionId(xmin);
+}
+
 static void
 tts_buffer_heap_materialize(TupleTableSlot *slot)
 {
@@ -1029,6 +1104,7 @@ const TupleTableSlotOps TTSOpsVirtual = {
 	.getsomeattrs = tts_virtual_getsomeattrs,
 	.getsysattr = tts_virtual_getsysattr,
 	.materialize = tts_virtual_materialize,
+	.is_current_xact_tuple = tts_virtual_is_current_xact_tuple,
 	.copyslot = tts_virtual_copyslot,
 
 	/*
@@ -1048,6 +1124,7 @@ const TupleTableSlotOps TTSOpsHeapTuple = {
 	.clear = tts_heap_clear,
 	.getsomeattrs = tts_heap_getsomeattrs,
 	.getsysattr = tts_heap_getsysattr,
+	.is_current_xact_tuple = tts_heap_is_current_xact_tuple,
 	.materialize = tts_heap_materialize,
 	.copyslot = tts_heap_copyslot,
 	.get_heap_tuple = tts_heap_get_heap_tuple,
@@ -1065,6 +1142,7 @@ const TupleTableSlotOps TTSOpsMinimalTuple = {
 	.clear = tts_minimal_clear,
 	.getsomeattrs = tts_minimal_getsomeattrs,
 	.getsysattr = tts_minimal_getsysattr,
+	.is_current_xact_tuple = tts_minimal_is_current_xact_tuple,
 	.materialize = tts_minimal_materialize,
 	.copyslot = tts_minimal_copyslot,
 
@@ -1082,6 +1160,7 @@ const TupleTableSlotOps TTSOpsBufferHeapTuple = {
 	.clear = tts_buffer_heap_clear,
 	.getsomeattrs = tts_buffer_heap_getsomeattrs,
 	.getsysattr = tts_buffer_heap_getsysattr,
+	.is_current_xact_tuple = tts_buffer_is_current_xact_tuple,
 	.materialize = tts_buffer_heap_materialize,
 	.copyslot = tts_buffer_heap_copyslot,
 	.get_heap_tuple = tts_buffer_heap_get_heap_tuple,

src/backend/utils/adt/ri_triggers.c

@@ -1260,9 +1260,6 @@ RI_FKey_fk_upd_check_required(Trigger *trigger, Relation fk_rel,
 {
 	const RI_ConstraintInfo *riinfo;
 	int			ri_nullcheck;
-	Datum		xminDatum;
-	TransactionId xmin;
-	bool		isnull;
 
 	/*
 	 * AfterTriggerSaveEvent() handles things such that this function is never
@@ -1330,10 +1327,7 @@ RI_FKey_fk_upd_check_required(Trigger *trigger, Relation fk_rel,
 	 * this if we knew the INSERT trigger already fired, but there is no easy
 	 * way to know that.)
 	 */
-	xminDatum = slot_getsysattr(oldslot, MinTransactionIdAttributeNumber, &isnull);
+	if (slot_is_current_xact_tuple(oldslot))
-	Assert(!isnull);
-	xmin = DatumGetTransactionId(xminDatum);
-	if (TransactionIdIsCurrentTransactionId(xmin))
 		return true;
 
 	/* If all old and new key values are equal, no check is needed */

src/include/executor/tuptable.h

@@ -166,6 +166,12 @@ struct TupleTableSlotOps
 	 */
 	Datum		(*getsysattr) (TupleTableSlot *slot, int attnum, bool *isnull);
 
+	/*
+	 * Check if the tuple is created by the current transaction. Throws an
+	 * error if the slot doesn't contain the storage tuple.
+	 */
+	bool		(*is_current_xact_tuple) (TupleTableSlot *slot);
+
 	/*
 	 * Make the contents of the slot solely depend on the slot, and not on
 	 * underlying resources (like another memory context, buffers, etc).
@@ -426,6 +432,21 @@ slot_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 	return slot->tts_ops->getsysattr(slot, attnum, isnull);
 }
 
+/*
+ * slot_is_current_xact_tuple - check if the slot's current tuple is created
+ *								by the current transaction.
+ *
+ *  If the slot does not contain a storage tuple, this will throw an error.
+ *  Hence before calling this function, callers should make sure that the
+ *  slot type supports storage tuples and that there is currently one inside
+ *  the slot.
+ */
+static inline bool
+slot_is_current_xact_tuple(TupleTableSlot *slot)
+{
+	return slot->tts_ops->is_current_xact_tuple(slot);
+}
+
 /*
  * ExecClearTuple - clear the slot's contents
  */