tableam: Add and use table_fetch_row_version().

This is essentially the tableam version of heapam_fetch(),
i.e. fetching a tuple identified by a tid, performing visibility
checks.

Note that this different from table_index_fetch_tuple(), which is for
index lookups. It therefore has to handle a tid pointing to an earlier
version of a tuple if the AM uses an optimization like heap's HOT. Add
comments to that end.

This commit removes the stats_relation argument from heap_fetch, as
it's been unused for a long time.

Author: Andres Freund
Reviewed-By: Haribabu Kommi
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

src/backend/access/heap/heapam.c

@@ -1388,8 +1388,7 @@ bool
 heap_fetch(Relation relation,
 		   Snapshot snapshot,
 		   HeapTuple tuple,
-		   Buffer *userbuf,
-		   Relation stats_relation)
+		   Buffer *userbuf)
 {
 	ItemPointer tid = &(tuple->t_self);
 	ItemId		lp;
@@ -1468,10 +1467,6 @@ heap_fetch(Relation relation,
 		 */
 		*userbuf = buffer;
 
-		/* Count the successful fetch against appropriate rel, if any */
-		if (stats_relation != NULL)
-			pgstat_count_heap_fetch(stats_relation);
-
 		return true;
 	}
 
@@ -5097,7 +5092,7 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
 		block = ItemPointerGetBlockNumber(&tupid);
 		ItemPointerCopy(&tupid, &(mytup.t_self));
 
-		if (!heap_fetch(rel, SnapshotAny, &mytup, &buf, NULL))
+		if (!heap_fetch(rel, SnapshotAny, &mytup, &buf))
 		{
 			/*
 			 * if we fail to find the updated version of the tuple, it's

src/backend/access/heap/heapam_handler.c

@@ -148,6 +148,30 @@ heapam_index_fetch_tuple(struct IndexFetchTableData *scan,
  * ------------------------------------------------------------------------
  */
 
+static bool
+heapam_fetch_row_version(Relation relation,
+						 ItemPointer tid,
+						 Snapshot snapshot,
+						 TupleTableSlot *slot)
+{
+	BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
+	Buffer		buffer;
+
+	Assert(TTS_IS_BUFFERTUPLE(slot));
+
+	bslot->base.tupdata.t_self = *tid;
+	if (heap_fetch(relation, snapshot, &bslot->base.tupdata, &buffer))
+	{
+		/* store in slot, transferring existing pin */
+		ExecStorePinnedBufferHeapTuple(&bslot->base.tupdata, slot, buffer);
+		slot->tts_tableOid = RelationGetRelid(relation);
+
+		return true;
+	}
+
+	return false;
+}
+
 static bool
 heapam_tuple_satisfies_snapshot(Relation rel, TupleTableSlot *slot,
 								Snapshot snapshot)
@@ -338,7 +362,7 @@ tuple_lock_retry:
 							 errmsg("tuple to be locked was already moved to another partition due to concurrent update")));
 
 				tuple->t_self = *tid;
-				if (heap_fetch(relation, &SnapshotDirty, tuple, &buffer, NULL))
+				if (heap_fetch(relation, &SnapshotDirty, tuple, &buffer))
 				{
 					/*
 					 * If xmin isn't what we're expecting, the slot must have
@@ -517,6 +541,7 @@ static const TableAmRoutine heapam_methods = {
 	.tuple_update = heapam_tuple_update,
 	.tuple_lock = heapam_tuple_lock,
 
+	.tuple_fetch_row_version = heapam_fetch_row_version,
 	.tuple_satisfies_snapshot = heapam_tuple_satisfies_snapshot,
 };
 

src/backend/access/table/tableamapi.c

@@ -62,6 +62,7 @@ GetTableAmRoutine(Oid amhandler)
 	Assert(routine->index_fetch_end != NULL);
 	Assert(routine->index_fetch_tuple != NULL);
 
+	Assert(routine->tuple_fetch_row_version != NULL);
 	Assert(routine->tuple_satisfies_snapshot != NULL);
 
 	Assert(routine->tuple_insert != NULL);

src/backend/commands/trigger.c

@@ -14,10 +14,11 @@
 #include "postgres.h"
 
 #include "access/genam.h"
-#include "access/heapam.h"
-#include "access/tableam.h"
-#include "access/sysattr.h"
 #include "access/htup_details.h"
+#include "access/relation.h"
+#include "access/sysattr.h"
+#include "access/table.h"
+#include "access/tableam.h"
 #include "access/xact.h"
 #include "catalog/catalog.h"
 #include "catalog/dependency.h"
@@ -3379,42 +3380,12 @@ GetTupleForTrigger(EState *estate,
 	}
 	else
 	{
-		Page		page;
-		ItemId		lp;
-		Buffer		buffer;
-		BufferHeapTupleTableSlot *boldslot;
-		HeapTuple	tuple;
-
-		Assert(TTS_IS_BUFFERTUPLE(oldslot));
-		ExecClearTuple(oldslot);
-		boldslot = (BufferHeapTupleTableSlot *) oldslot;
-		tuple = &boldslot->base.tupdata;
-
-		buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
-
 		/*
-		 * Although we already know this tuple is valid, we must lock the
-		 * buffer to ensure that no one has a buffer cleanup lock; otherwise
-		 * they might move the tuple while we try to copy it.  But we can
-		 * release the lock before actually doing the heap_copytuple call,
-		 * since holding pin is sufficient to prevent anyone from getting a
-		 * cleanup lock they don't already hold.
+		 * We expect the tuple to be present, thus very simple error handling
+		 * suffices.
 		 */
-		LockBuffer(buffer, BUFFER_LOCK_SHARE);
-
-		page = BufferGetPage(buffer);
-		lp = PageGetItemId(page, ItemPointerGetOffsetNumber(tid));
-
-		Assert(ItemIdIsNormal(lp));
-
-		tuple->t_data = (HeapTupleHeader) PageGetItem(page, lp);
-		tuple->t_len = ItemIdGetLength(lp);
-		tuple->t_self = *tid;
-		tuple->t_tableOid = RelationGetRelid(relation);
-
-		LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
-
-		ExecStorePinnedBufferHeapTuple(tuple, oldslot, buffer);
+		if (!table_fetch_row_version(relation, tid, SnapshotAny, oldslot))
+			elog(ERROR, "failed to fetch tuple for trigger");
 	}
 
 	return true;
@@ -4193,8 +4164,6 @@ AfterTriggerExecute(EState *estate,
 	AfterTriggerShared evtshared = GetTriggerSharedData(event);
 	Oid			tgoid = evtshared->ats_tgoid;
 	TriggerData LocTriggerData;
-	HeapTupleData tuple1;
-	HeapTupleData tuple2;
 	HeapTuple	rettuple;
 	int			tgindx;
 	bool		should_free_trig = false;
@@ -4271,19 +4240,12 @@ AfterTriggerExecute(EState *estate,
 		default:
 			if (ItemPointerIsValid(&(event->ate_ctid1)))
 			{
-				Buffer buffer;
-
 				LocTriggerData.tg_trigslot = ExecGetTriggerOldSlot(estate, relInfo);
 
-				ItemPointerCopy(&(event->ate_ctid1), &(tuple1.t_self));
-				if (!heap_fetch(rel, SnapshotAny, &tuple1, &buffer, NULL))
+				if (!table_fetch_row_version(rel, &(event->ate_ctid1), SnapshotAny, LocTriggerData.tg_trigslot))
 					elog(ERROR, "failed to fetch tuple1 for AFTER trigger");
-				ExecStorePinnedBufferHeapTuple(&tuple1,
-											   LocTriggerData.tg_trigslot,
-											   buffer);
 				LocTriggerData.tg_trigtuple =
-					ExecFetchSlotHeapTuple(LocTriggerData.tg_trigslot, false,
-										   &should_free_trig);
+					ExecFetchSlotHeapTuple(LocTriggerData.tg_trigslot, false, &should_free_trig);
 			}
 			else
 			{
@@ -4295,19 +4257,12 @@ AfterTriggerExecute(EState *estate,
 				AFTER_TRIGGER_2CTID &&
 				ItemPointerIsValid(&(event->ate_ctid2)))
 			{
-				Buffer buffer;
-
 				LocTriggerData.tg_newslot = ExecGetTriggerNewSlot(estate, relInfo);
 
-				ItemPointerCopy(&(event->ate_ctid2), &(tuple2.t_self));
-				if (!heap_fetch(rel, SnapshotAny, &tuple2, &buffer, NULL))
+				if (!table_fetch_row_version(rel, &(event->ate_ctid2), SnapshotAny, LocTriggerData.tg_newslot))
 					elog(ERROR, "failed to fetch tuple2 for AFTER trigger");
-				ExecStorePinnedBufferHeapTuple(&tuple2,
-											   LocTriggerData.tg_newslot,
-											   buffer);
 				LocTriggerData.tg_newtuple =
-					ExecFetchSlotHeapTuple(LocTriggerData.tg_newslot, false,
-										   &should_free_new);
+					ExecFetchSlotHeapTuple(LocTriggerData.tg_newslot, false, &should_free_new);
 			}
 			else
 			{

src/backend/executor/execMain.c

@@ -2649,17 +2649,10 @@ EvalPlanQualFetchRowMarks(EPQState *epqstate)
 			else
 			{
 				/* ordinary table, fetch the tuple */
-				HeapTupleData tuple;
-				Buffer		buffer;
-
-				tuple.t_self = *((ItemPointer) DatumGetPointer(datum));
-				if (!heap_fetch(erm->relation, SnapshotAny, &tuple, &buffer,
-								NULL))
+				if (!table_fetch_row_version(erm->relation,
+											 (ItemPointer) DatumGetPointer(datum),
+											 SnapshotAny, slot))
 					elog(ERROR, "failed to fetch tuple for EvalPlanQual recheck");
-
-				/* successful, store tuple */
-				ExecStorePinnedBufferHeapTuple(&tuple, slot, buffer);
-				ExecMaterializeSlot(slot);
 			}
 		}
 		else

src/backend/executor/nodeModifyTable.c

@@ -229,17 +229,13 @@ ExecCheckTIDVisible(EState *estate,
 					TupleTableSlot *tempSlot)
 {
 	Relation	rel = relinfo->ri_RelationDesc;
-	Buffer		buffer;
-	HeapTupleData tuple;
 
 	/* Redundantly check isolation level */
 	if (!IsolationUsesXactSnapshot())
 		return;
 
-	tuple.t_self = *tid;
-	if (!heap_fetch(rel, SnapshotAny, &tuple, &buffer, NULL))
+	if (!table_fetch_row_version(rel, tid, SnapshotAny, tempSlot))
 		elog(ERROR, "failed to fetch conflicting tuple for ON CONFLICT");
-	ExecStorePinnedBufferHeapTuple(&tuple, tempSlot, buffer);
 	ExecCheckTupleVisible(estate, rel, tempSlot);
 	ExecClearTuple(tempSlot);
 }
@@ -874,21 +870,9 @@ ldelete:;
 			}
 			else
 			{
-				BufferHeapTupleTableSlot *bslot;
-				HeapTuple	deltuple;
-				Buffer		buffer;
-
-				Assert(TTS_IS_BUFFERTUPLE(slot));
-				ExecClearTuple(slot);
-				bslot = (BufferHeapTupleTableSlot *) slot;
-				deltuple = &bslot->base.tupdata;
-
-				deltuple->t_self = *tupleid;
-				if (!heap_fetch(resultRelationDesc, SnapshotAny,
-								deltuple, &buffer, NULL))
+				if (!table_fetch_row_version(resultRelationDesc, tupleid,
+											 SnapshotAny, slot))
 					elog(ERROR, "failed to fetch deleted tuple for DELETE RETURNING");
-
-				ExecStorePinnedBufferHeapTuple(deltuple, slot, buffer);
 			}
 		}
 

src/backend/executor/nodeTidscan.c

@@ -310,7 +310,6 @@ TidNext(TidScanState *node)
 	Relation	heapRelation;
 	HeapTuple	tuple;
 	TupleTableSlot *slot;
-	Buffer		buffer = InvalidBuffer;
 	ItemPointerData *tidList;
 	int			numTids;
 	bool		bBackward;
@@ -376,19 +375,10 @@ TidNext(TidScanState *node)
 		if (node->tss_isCurrentOf)
 			heap_get_latest_tid(heapRelation, snapshot, &tuple->t_self);
 
-		if (heap_fetch(heapRelation, snapshot, tuple, &buffer, NULL))
-		{
-			/*
-			 * Store the scanned tuple in the scan tuple slot of the scan
-			 * state, transferring the pin to the slot.
-			 */
-			ExecStorePinnedBufferHeapTuple(tuple, /* tuple to store */
-										   slot,	/* slot to store in */
-										   buffer);	/* buffer associated with
-													 * tuple */
-
+		if (table_fetch_row_version(heapRelation, &tuple->t_self, snapshot,
+									slot))
 			return slot;
-		}
+
 		/* Bad TID or failed snapshot qual; try next */
 		if (bBackward)
 			node->tss_TidPtr--;

src/include/access/heapam.h

@@ -128,7 +128,7 @@ extern bool heap_getnextslot(TableScanDesc sscan,
 				 ScanDirection direction, struct TupleTableSlot *slot);
 
 extern bool heap_fetch(Relation relation, Snapshot snapshot,
-		   HeapTuple tuple, Buffer *userbuf, Relation stats_relation);
+		   HeapTuple tuple, Buffer *userbuf);
 extern bool heap_hot_search_buffer(ItemPointer tid, Relation relation,
 					   Buffer buffer, Snapshot snapshot, HeapTuple heapTuple,
 					   bool *all_dead, bool first_call);

src/include/access/tableam.h

@@ -271,6 +271,17 @@ typedef struct TableAmRoutine
 	 * ------------------------------------------------------------------------
 	 */
 
+
+	/*
+	 * Fetch tuple at `tid` into `slot, after doing a visibility test
+	 * according to `snapshot`. If a tuple was found and passed the visibility
+	 * test, returns true, false otherwise.
+	 */
+	bool		(*tuple_fetch_row_version) (Relation rel,
+											ItemPointer tid,
+											Snapshot snapshot,
+											TupleTableSlot *slot);
+
 	/*
 	 * Does the tuple in `slot` satisfy `snapshot`?  The slot needs to be of
 	 * the appropriate type for the AM.
@@ -574,9 +585,9 @@ table_index_fetch_end(struct IndexFetchTableData *scan)
 }
 
 /*
- * Fetches tuple at `tid` into `slot`, after doing a visibility test according
- * to `snapshot`. If a tuple was found and passed the visibility test, returns
- * true, false otherwise.
+ * Fetches, as part of an index scan, tuple at `tid` into `slot`, after doing
+ * a visibility test according to `snapshot`. If a tuple was found and passed
+ * the visibility test, returns true, false otherwise.
  *
  * *call_again needs to be false on the first call to table_index_fetch_tuple() for
  * a tid. If there potentially is another tuple matching the tid, *call_again
@@ -586,6 +597,13 @@ table_index_fetch_end(struct IndexFetchTableData *scan)
  * *all_dead will be set to true by table_index_fetch_tuple() iff it is guaranteed
  * that no backend needs to see that tuple. Index AMs can use that do avoid
  * returning that tid in future searches.
+ *
+ * The difference between this function and table_fetch_row_version is that
+ * this function returns the currently visible version of a row if the AM
+ * supports storing multiple row versions reachable via a single index entry
+ * (like heap's HOT). Whereas table_fetch_row_version only evaluates the the
+ * tuple exactly at `tid`. Outside of index entry ->table tuple lookups,
+ * table_fetch_row_version is what's usually needed.
  */
 static inline bool
 table_index_fetch_tuple(struct IndexFetchTableData *scan,
@@ -606,6 +624,25 @@ table_index_fetch_tuple(struct IndexFetchTableData *scan,
  * ------------------------------------------------------------------------
  */
 
+
+/*
+ * Fetch tuple at `tid` into `slot, after doing a visibility test according to
+ * `snapshot`. If a tuple was found and passed the visibility test, returns
+ * true, false otherwise.
+ *
+ * See table_index_fetch_tuple's comment about what the difference between
+ * these functions is. This function is the correct to use outside of
+ * index entry->table tuple lookups.
+ */
+static inline bool
+table_fetch_row_version(Relation rel,
+						ItemPointer tid,
+						Snapshot snapshot,
+						TupleTableSlot *slot)
+{
+	return rel->rd_tableam->tuple_fetch_row_version(rel, tid, snapshot, slot);
+}
+
 /*
  * Return true iff tuple in slot satisfies the snapshot.
  *