Make nodeSort.c use Datum sorts for single column sorts

Datum sorts can be significantly faster than tuple sorts, especially when
the data type being sorted is a pass-by-value type.  Something in the
region of 50-70% performance improvements appear to be possible.

Just in case there's any confusion; the Datum sort is only used when the
targetlist of the Sort node contains a single column, not when there's a
single column in the sort key and multiple items in the target list.

Author: Ronan Dunklau
Reviewed-by: James Coleman, David Rowley, Ranier Vilela, Hou Zhijie
Tested-by: John Naylor
Discussion: https://postgr.es/m/3177670.itZtoPt7T5@aivenronan

src/backend/executor/nodeSort.c

@@ -29,6 +29,16 @@
  *		which saves the results in a temporary file or memory. After the
  *		initial call, returns a tuple from the file with each call.
  *
+ *		There are two distinct ways that this sort can be performed:
+ *
+ *		1) When the result is a single column we perform a Datum sort.
+ *
+ *		2) When the result contains multiple columns we perform a tuple sort.
+ *
+ *		We could do this by always performing a tuple sort, however sorting
+ *		Datums only can be significantly faster than sorting tuples,
+ *		especially when the Datums are of a pass-by-value type.
+ *
  *		Conditions:
  *		  -- none.
  *
@@ -86,6 +96,15 @@ ExecSort(PlanState *pstate)
 		outerNode = outerPlanState(node);
 		tupDesc = ExecGetResultType(outerNode);
 
+		if (node->datumSort)
+			tuplesortstate = tuplesort_begin_datum(TupleDescAttr(tupDesc, 0)->atttypid,
+												   plannode->sortOperators[0],
+												   plannode->collations[0],
+												   plannode->nullsFirst[0],
+												   work_mem,
+												   NULL,
+												   node->randomAccess);
+		else
 			tuplesortstate = tuplesort_begin_heap(tupDesc,
 												  plannode->numCols,
 												  plannode->sortColIdx,
@@ -100,18 +119,34 @@ ExecSort(PlanState *pstate)
 		node->tuplesortstate = (void *) tuplesortstate;
 
 		/*
-		 * Scan the subplan and feed all the tuples to tuplesort.
+		 * Scan the subplan and feed all the tuples to tuplesort using the
+		 * appropriate method based on the type of sort we're doing.
 		 */
-
+		if (node->datumSort)
+		{
 			for (;;)
 			{
 				slot = ExecProcNode(outerNode);
 
 				if (TupIsNull(slot))
 					break;
+				slot_getsomeattrs(slot, 1);
+				tuplesort_putdatum(tuplesortstate,
+								   slot->tts_values[0],
+								   slot->tts_isnull[0]);
+			}
+		}
+		else
+		{
+			for (;;)
+			{
+				slot = ExecProcNode(outerNode);
 
+				if (TupIsNull(slot))
+					break;
 				tuplesort_puttupleslot(tuplesortstate, slot);
 			}
+		}
 
 		/*
 		 * Complete the sort.
@@ -144,15 +179,27 @@ ExecSort(PlanState *pstate)
 	SO1_printf("ExecSort: %s\n",
 			   "retrieving tuple from tuplesort");
 
-	/*
-	 * Get the first or next tuple from tuplesort. Returns NULL if no more
-	 * tuples.  Note that we only rely on slot tuple remaining valid until the
-	 * next fetch from the tuplesort.
-	 */
 	slot = node->ss.ps.ps_ResultTupleSlot;
+
+	/*
+	 * Fetch the next sorted item from the appropriate tuplesort function. For
+	 * datum sorts we must manage the slot ourselves and leave it clear when
+	 * tuplesort_getdatum returns false to indicate there are no more datums.
+	 * For tuple sorts, tuplesort_gettupleslot manages the slot for us and
+	 * empties the slot when it runs out of tuples.
+	 */
+	if (node->datumSort)
+	{
+		ExecClearTuple(slot);
+		if (tuplesort_getdatum(tuplesortstate, ScanDirectionIsForward(dir),
+							   &(slot->tts_values[0]), &(slot->tts_isnull[0]), NULL))
+			ExecStoreVirtualTuple(slot);
+	}
+	else
 		(void) tuplesort_gettupleslot(tuplesortstate,
 									  ScanDirectionIsForward(dir),
 									  false, slot, NULL);
+
 	return slot;
 }
 
@@ -221,6 +268,15 @@ ExecInitSort(Sort *node, EState *estate, int eflags)
 	ExecInitResultTupleSlotTL(&sortstate->ss.ps, &TTSOpsMinimalTuple);
 	sortstate->ss.ps.ps_ProjInfo = NULL;
 
+	/*
+	 * We perform a Datum sort when we're sorting just a single column,
+	 * otherwise we perform a tuple sort.
+	 */
+	if (ExecGetResultType(outerPlanState(sortstate))->natts == 1)
+		sortstate->datumSort = true;
+	else
+		sortstate->datumSort = false;
+
 	SO1_printf("ExecInitSort: %s\n",
 			   "sort node initialized");
 

src/include/nodes/execnodes.h

@@ -2151,6 +2151,7 @@ typedef struct SortState
 	int64		bound_Done;		/* value of bound we did the sort with */
 	void	   *tuplesortstate; /* private state of tuplesort.c */
 	bool		am_worker;		/* are we a worker? */
+	bool		datumSort;		/* Datum sort instead of tuple sort? */
 	SharedSortInfo *shared_info;	/* one entry per worker */
 } SortState;