From: Ronan Dunklau <ronan.dunklau@aiven.io>
To: James Coleman <jtc331@gmail.com>
Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>,
 Ranier Vilela <ranier.vf@gmail.com>, David Rowley <dgrowleyml@gmail.com>,
 Dilip Kumar <dilipbalaut@gmail.com>
Subject: Re: [PATCH] Use optimized single-datum tuplesort in ExecSort
Date: Wed, 07 Jul 2021 11:32:03 +0200
Message-ID: <3060002.hb0XKQ11pn@aivenronan>
Organization: aiven
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Le mardi 6 juillet 2021, 17:37:53 CEST James Coleman a =E9crit :
> Yes and no. When incremental sort has to do a full sort there will
> always be at least 2 attributes. But in prefix sort mode (see
> prefixsort_state) only non-presorted columns are sorted (i.e., if
> given a,b already sorted by a, then only b is sorted). So the
> prefixsort_state could use this optimization.

The optimization is not when we actually sort on a single key, but when we =
get=20
a single attribute in / out of the tuplesort.  Since sorting always add=20
resjunk entries for the keys being sorted on, I don't think we can ever end=
 up=20
in a situation where the optimization would kick in, since the entries for =
the=20
already-performed-sort keys will need to be present in the output.

Maybe if instead of adding resjunk entries to the whole query's targetlist,=
=20
sort and incrementalsort nodes were able to do a projection from the input=
=20
(needed tle + resjunk sorting tle) to a tuple containing only the needed tl=
e=20
on output before actually sorting it, it would be possible, but that would =
be=20
quite a big design change.

In the meantime I fixed some formatting issues, please find attached a new=
=20
patch.


=2D-=20
Ronan Dunklau
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diff --git a/src/backend/executor/nodeSort.c b/src/backend/executor/nodeSort.c
index b99027e0d7..9d8b0a77da 100644
--- a/src/backend/executor/nodeSort.c
+++ b/src/backend/executor/nodeSort.c
@@ -29,6 +29,10 @@
  *		which saves the results in a temporary file or memory. After the
  *		initial call, returns a tuple from the file with each call.
  *
+ *		The tuplesort can either occur on the whole tuple (this is the nominal
+ *		case) or, when the input / output tuple consists of only one attribute,
+ *		we switch to the tuplesort_*_datum API, optimized for that specific case.
+ *
  *		Conditions:
  *		  -- none.
  *
@@ -86,32 +90,61 @@ ExecSort(PlanState *pstate)
 		outerNode = outerPlanState(node);
 		tupDesc = ExecGetResultType(outerNode);
 
-		tuplesortstate = tuplesort_begin_heap(tupDesc,
-											  plannode->numCols,
-											  plannode->sortColIdx,
-											  plannode->sortOperators,
-											  plannode->collations,
-											  plannode->nullsFirst,
-											  work_mem,
-											  NULL,
-											  node->randomAccess);
+		/*
+		 * Switch to the tuplesort_*_datum interface when we have only one
+		 * key, as it is much more efficient especially when the type is
+		 * pass-by-value.
+		 */
+		if (tupDesc->natts == 1)
+		{
+			node->is_single_val = true;
+			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,
+												  plannode->sortOperators,
+												  plannode->collations,
+												  plannode->nullsFirst,
+												  work_mem,
+												  NULL,
+												  node->randomAccess);
 		if (node->bounded)
 			tuplesort_set_bound(tuplesortstate, node->bound);
 		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 either
+		 * the _putdatum or _puttupleslot API as appropriate.
 		 */
-
-		for (;;)
-		{
-			slot = ExecProcNode(outerNode);
-
-			if (TupIsNull(slot))
-				break;
-
-			tuplesort_puttupleslot(tuplesortstate, slot);
-		}
+		if (node->is_single_val)
+			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.
@@ -150,9 +183,18 @@ ExecSort(PlanState *pstate)
 	 * next fetch from the tuplesort.
 	 */
 	slot = node->ss.ps.ps_ResultTupleSlot;
-	(void) tuplesort_gettupleslot(tuplesortstate,
-								  ScanDirectionIsForward(dir),
-								  false, slot, NULL);
+	if (node->is_single_val)
+	{
+		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;
 }
 
@@ -191,6 +233,7 @@ ExecInitSort(Sort *node, EState *estate, int eflags)
 	sortstate->bounded = false;
 	sortstate->sort_Done = false;
 	sortstate->tuplesortstate = NULL;
+	sortstate->is_single_val = false;
 
 	/*
 	 * Miscellaneous initialization
diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
index 22972071ff..f785259da7 100644
--- a/src/backend/utils/sort/tuplesort.c
+++ b/src/backend/utils/sort/tuplesort.c
@@ -4773,6 +4773,14 @@ leader_takeover_tapes(Tuplesortstate *state)
 static void
 free_sort_tuple(Tuplesortstate *state, SortTuple *stup)
 {
-	FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
-	pfree(stup->tuple);
+	/*
+	 * If the SortTuple is actually only a single Datum, which was not copied
+	 * as it is a byval type, do not try to free it nor account for it in
+	 * memory used.
+	 */
+	if (stup->tuple)
+	{
+		FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
+		pfree(stup->tuple);
+	}
 }
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 0ec5509e7e..643f416c54 100644
--- a/src/include/nodes/execnodes.h
+++ b/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		is_single_val;  /* are we using the single value optimization ? */
 	SharedSortInfo *shared_info;	/* one entry per worker */
 } SortState;
 
-- 
2.32.0


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