Optimize WindowAgg's use of tuplestores

When WindowAgg finished one partition of a PARTITION BY, it previously
would call tuplestore_end() to purge all the stored tuples before again
calling tuplestore_begin_heap() and carefully setting up all of the
tuplestore read pointers exactly as required for the given frameOptions.
Since the frameOptions don't change between partitions, this part does
not make much sense.  For queries that had very few rows per partition,
the overhead of this was very large.

It seems much better to create the tuplestore and the read pointers once
and simply call tuplestore_clear() at the end of each partition.
tuplestore_clear() moves all of the read pointers back to the start
position and deletes all the previously stored tuples.

A simple test query with 1 million partitions and 1 tuple per partition
has been shown to run around 40% faster than without this change.  The
additional effort seems to have mostly been spent in malloc/free.

Making this work required adding a new bool field to WindowAggState
which had the unfortunate effect of being the 9th bool field in a group
resulting in the struct being enlarged.  Here we shuffle the fields
around a little so that the two bool fields for runcondition relating
stuff fit into existing padding.  Also, move the "runcondition" field to
be near those.  This frees up enough space with the other bool fields so
that the newly added one fits into the padding bytes.  This was done to
address a very small but apparent performance regression with queries
containing a large number of rows per partition.

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Discussion: https://postgr.es/m/CAHoyFK9n-QCXKTUWT_xxtXninSMEv%2BgbJN66-y6prM3f4WkEHw%40mail.gmail.com

1 files changed, 106 insertions, 56 deletions

diff --git a/src/backend/executor/nodeWindowAgg.c b/src/backend/executor/nodeWindowAgg.c
index 88a85f556b6..51a6708a392 100644
--- a/src/backend/executor/nodeWindowAgg.c
+++ b/src/backend/executor/nodeWindowAgg.c
@@ -1074,57 +1074,24 @@ eval_windowfunction(WindowAggState *winstate, WindowStatePerFunc perfuncstate,
 }
 
 /*
- * begin_partition
- * Start buffering rows of the next partition.
+ * prepare_tuplestore
+ *		Prepare the tuplestore and all of the required read pointers for the
+ *		WindowAggState's frameOptions.
+ *
+ * Note: We use pg_noinline to avoid bloating the calling function with code
+ * which is only called once.
  */
-static void
-begin_partition(WindowAggState *winstate)
+static pg_noinline void
+prepare_tuplestore(WindowAggState *winstate)
 {
 	WindowAgg  *node = (WindowAgg *) winstate->ss.ps.plan;
-	PlanState  *outerPlan = outerPlanState(winstate);
 	int			frameOptions = winstate->frameOptions;
 	int			numfuncs = winstate->numfuncs;
-	int			i;
-
-	winstate->partition_spooled = false;
-	winstate->framehead_valid = false;
-	winstate->frametail_valid = false;
-	winstate->grouptail_valid = false;
-	winstate->spooled_rows = 0;
-	winstate->currentpos = 0;
-	winstate->frameheadpos = 0;
-	winstate->frametailpos = 0;
-	winstate->currentgroup = 0;
-	winstate->frameheadgroup = 0;
-	winstate->frametailgroup = 0;
-	winstate->groupheadpos = 0;
-	winstate->grouptailpos = -1;	/* see update_grouptailpos */
-	ExecClearTuple(winstate->agg_row_slot);
-	if (winstate->framehead_slot)
-		ExecClearTuple(winstate->framehead_slot);
-	if (winstate->frametail_slot)
-		ExecClearTuple(winstate->frametail_slot);
-
-	/*
-	 * If this is the very first partition, we need to fetch the first input
-	 * row to store in first_part_slot.
-	 */
-	if (TupIsNull(winstate->first_part_slot))
-	{
-		TupleTableSlot *outerslot = ExecProcNode(outerPlan);
 
-		if (!TupIsNull(outerslot))
-			ExecCopySlot(winstate->first_part_slot, outerslot);
-		else
-		{
-			/* outer plan is empty, so we have nothing to do */
-			winstate->partition_spooled = true;
-			winstate->more_partitions = false;
-			return;
-		}
-	}
+	/* we shouldn't be called if this was done already */
+	Assert(winstate->buffer == NULL);
 
-	/* Create new tuplestore for this partition */
+	/* Create new tuplestore */
 	winstate->buffer = tuplestore_begin_heap(false, false, work_mem);
 
 	/*
@@ -1158,16 +1125,10 @@ begin_partition(WindowAggState *winstate)
 
 		agg_winobj->readptr = tuplestore_alloc_read_pointer(winstate->buffer,
 															readptr_flags);
-		agg_winobj->markpos = -1;
-		agg_winobj->seekpos = -1;
-
-		/* Also reset the row counters for aggregates */
-		winstate->aggregatedbase = 0;
-		winstate->aggregatedupto = 0;
 	}
 
 	/* create mark and read pointers for each real window function */
-	for (i = 0; i < numfuncs; i++)
+	for (int i = 0; i < numfuncs; i++)
 	{
 		WindowStatePerFunc perfuncstate = &(winstate->perfunc[i]);
 
@@ -1179,8 +1140,6 @@ begin_partition(WindowAggState *winstate)
 															0);
 			winobj->readptr = tuplestore_alloc_read_pointer(winstate->buffer,
 															EXEC_FLAG_BACKWARD);
-			winobj->markpos = -1;
-			winobj->seekpos = -1;
 		}
 	}
 
@@ -1224,6 +1183,88 @@ begin_partition(WindowAggState *winstate)
 		winstate->grouptail_ptr =
 			tuplestore_alloc_read_pointer(winstate->buffer, 0);
 	}
+}
+
+/*
+ * begin_partition
+ * Start buffering rows of the next partition.
+ */
+static void
+begin_partition(WindowAggState *winstate)
+{
+	PlanState  *outerPlan = outerPlanState(winstate);
+	int			numfuncs = winstate->numfuncs;
+
+	winstate->partition_spooled = false;
+	winstate->framehead_valid = false;
+	winstate->frametail_valid = false;
+	winstate->grouptail_valid = false;
+	winstate->spooled_rows = 0;
+	winstate->currentpos = 0;
+	winstate->frameheadpos = 0;
+	winstate->frametailpos = 0;
+	winstate->currentgroup = 0;
+	winstate->frameheadgroup = 0;
+	winstate->frametailgroup = 0;
+	winstate->groupheadpos = 0;
+	winstate->grouptailpos = -1;	/* see update_grouptailpos */
+	ExecClearTuple(winstate->agg_row_slot);
+	if (winstate->framehead_slot)
+		ExecClearTuple(winstate->framehead_slot);
+	if (winstate->frametail_slot)
+		ExecClearTuple(winstate->frametail_slot);
+
+	/*
+	 * If this is the very first partition, we need to fetch the first input
+	 * row to store in first_part_slot.
+	 */
+	if (TupIsNull(winstate->first_part_slot))
+	{
+		TupleTableSlot *outerslot = ExecProcNode(outerPlan);
+
+		if (!TupIsNull(outerslot))
+			ExecCopySlot(winstate->first_part_slot, outerslot);
+		else
+		{
+			/* outer plan is empty, so we have nothing to do */
+			winstate->partition_spooled = true;
+			winstate->more_partitions = false;
+			return;
+		}
+	}
+
+	/* Create new tuplestore if not done already. */
+	if (unlikely(winstate->buffer == NULL))
+		prepare_tuplestore(winstate);
+
+	winstate->next_partition = false;
+
+	if (winstate->numaggs > 0)
+	{
+		WindowObject agg_winobj = winstate->agg_winobj;
+
+		/* reset mark and see positions for aggregate functions */
+		agg_winobj->markpos = -1;
+		agg_winobj->seekpos = -1;
+
+		/* Also reset the row counters for aggregates */
+		winstate->aggregatedbase = 0;
+		winstate->aggregatedupto = 0;
+	}
+
+	/* reset mark and seek positions for each real window function */
+	for (int i = 0; i < numfuncs; i++)
+	{
+		WindowStatePerFunc perfuncstate = &(winstate->perfunc[i]);
+
+		if (!perfuncstate->plain_agg)
+		{
+			WindowObject winobj = perfuncstate->winobj;
+
+			winobj->markpos = -1;
+			winobj->seekpos = -1;
+		}
+	}
 
 	/*
 	 * Store the first tuple into the tuplestore (it's always available now;
@@ -1360,9 +1401,9 @@ release_partition(WindowAggState *winstate)
 	}
 
 	if (winstate->buffer)
-		tuplestore_end(winstate->buffer);
-	winstate->buffer = NULL;
+		tuplestore_clear(winstate->buffer);
 	winstate->partition_spooled = false;
+	winstate->next_partition = true;
 }
 
 /*
@@ -2143,7 +2184,7 @@ ExecWindowAgg(PlanState *pstate)
 	/* We need to loop as the runCondition or qual may filter out tuples */
 	for (;;)
 	{
-		if (winstate->buffer == NULL)
+		if (winstate->next_partition)
 		{
 			/* Initialize for first partition and set current row = 0 */
 			begin_partition(winstate);
@@ -2686,6 +2727,7 @@ ExecInitWindowAgg(WindowAgg *node, EState *estate, int eflags)
 	winstate->all_first = true;
 	winstate->partition_spooled = false;
 	winstate->more_partitions = false;
+	winstate->next_partition = true;
 
 	return winstate;
 }
@@ -2700,6 +2742,14 @@ ExecEndWindowAgg(WindowAggState *node)
 	PlanState  *outerPlan;
 	int			i;
 
+	if (node->buffer != NULL)
+	{
+		tuplestore_end(node->buffer);
+
+		/* nullify so that release_partition skips the tuplestore_clear() */
+		node->buffer = NULL;
+	}
+
 	release_partition(node);
 
 	for (i = 0; i < node->numaggs; i++)