Teach tuplestore.c to throw away data before the "mark" point when the caller

is using mark/restore but not rewind or backward-scan capability.  Insert a
materialize plan node between a mergejoin and its inner child if the inner
child is a sort that is expected to spill to disk.  The materialize shields
the sort from the need to do mark/restore and thereby allows it to perform
its final merge pass on-the-fly; while the materialize itself is normally
cheap since it won't spill to disk unless the number of tuples with equal
key values exceeds work_mem.

Greg Stark, with some kibitzing from Tom Lane.

src/backend/executor/nodeMaterial.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeMaterial.c,v 1.58 2007/01/05 22:19:28 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeMaterial.c,v 1.59 2007/05/21 17:57:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -56,10 +56,10 @@ ExecMaterial(MaterialState *node)
 	/*
 	 * If first time through, and we need a tuplestore, initialize it.
 	 */
-	if (tuplestorestate == NULL && node->randomAccess)
+	if (tuplestorestate == NULL && node->eflags != 0)
 	{
 		tuplestorestate = tuplestore_begin_heap(true, false, work_mem);
-
+		tuplestore_set_eflags(tuplestorestate, node->eflags);
 		node->tuplestorestate = (void *) tuplestorestate;
 	}
 
@@ -162,14 +162,14 @@ ExecInitMaterial(Material *node, EState *estate, int eflags)
 	matstate->ss.ps.state = estate;
 
 	/*
-	 * We must have random access to the subplan output to do backward scan or
-	 * mark/restore.  We also prefer to materialize the subplan output if we
-	 * might be called on to rewind and replay it many times. However, if none
-	 * of these cases apply, we can skip storing the data.
+	 * We must have a tuplestore buffering the subplan output to do backward
+	 * scan or mark/restore.  We also prefer to materialize the subplan output
+	 * if we might be called on to rewind and replay it many times. However,
+	 * if none of these cases apply, we can skip storing the data.
 	 */
-	matstate->randomAccess = (eflags & (EXEC_FLAG_REWIND |
-										EXEC_FLAG_BACKWARD |
-										EXEC_FLAG_MARK)) != 0;
+	matstate->eflags = (eflags & (EXEC_FLAG_REWIND |
+								  EXEC_FLAG_BACKWARD |
+								  EXEC_FLAG_MARK));
 
 	matstate->eof_underlying = false;
 	matstate->tuplestorestate = NULL;
@@ -255,7 +255,7 @@ ExecEndMaterial(MaterialState *node)
 void
 ExecMaterialMarkPos(MaterialState *node)
 {
-	Assert(node->randomAccess);
+	Assert(node->eflags & EXEC_FLAG_MARK);
 
 	/*
 	 * if we haven't materialized yet, just return.
@@ -275,7 +275,7 @@ ExecMaterialMarkPos(MaterialState *node)
 void
 ExecMaterialRestrPos(MaterialState *node)
 {
-	Assert(node->randomAccess);
+	Assert(node->eflags & EXEC_FLAG_MARK);
 
 	/*
 	 * if we haven't materialized yet, just return.
@@ -300,7 +300,7 @@ ExecMaterialReScan(MaterialState *node, ExprContext *exprCtxt)
 {
 	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
 
-	if (node->randomAccess)
+	if (node->eflags != 0)
 	{
 		/*
 		 * If we haven't materialized yet, just return. If outerplan' chgParam
@@ -312,15 +312,21 @@ ExecMaterialReScan(MaterialState *node, ExprContext *exprCtxt)
 
 		/*
 		 * If subnode is to be rescanned then we forget previous stored
-		 * results; we have to re-read the subplan and re-store.
+		 * results; we have to re-read the subplan and re-store.  Also,
+		 * if we told tuplestore it needn't support rescan, we lose and
+		 * must re-read.  (This last should not happen in common cases;
+		 * else our caller lied by not passing EXEC_FLAG_REWIND to us.)
 		 *
 		 * Otherwise we can just rewind and rescan the stored output. The
 		 * state of the subnode does not change.
 		 */
-		if (((PlanState *) node)->lefttree->chgParam != NULL)
+		if (((PlanState *) node)->lefttree->chgParam != NULL ||
+			(node->eflags & EXEC_FLAG_REWIND) == 0)
 		{
 			tuplestore_end((Tuplestorestate *) node->tuplestorestate);
 			node->tuplestorestate = NULL;
+			if (((PlanState *) node)->lefttree->chgParam == NULL)
+				ExecReScan(((PlanState *) node)->lefttree, exprCtxt);
 			node->eof_underlying = false;
 		}
 		else

src/backend/executor/nodeMergejoin.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeMergejoin.c,v 1.87 2007/02/02 00:07:03 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeMergejoin.c,v 1.88 2007/05/21 17:57:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -706,6 +706,9 @@ ExecMergeJoin(MergeJoinState *node)
 				}
 				else
 				{
+					/* Mark before advancing, if wanted */
+					if (node->mj_ExtraMarks)
+						ExecMarkPos(innerPlan);
 					/* Stay in same state to fetch next inner tuple */
 					if (doFillInner)
 					{
@@ -830,6 +833,9 @@ ExecMergeJoin(MergeJoinState *node)
 				 * now we get the next inner tuple, if any.  If there's none,
 				 * advance to next outer tuple (which may be able to join to
 				 * previously marked tuples).
+				 *
+				 * NB: must NOT do "extraMarks" here, since we may need to
+				 * return to previously marked tuples.
 				 */
 				innerTupleSlot = ExecProcNode(innerPlan);
 				node->mj_InnerTupleSlot = innerTupleSlot;
@@ -1140,6 +1146,9 @@ ExecMergeJoin(MergeJoinState *node)
 				break;
 
 				/*
+				 * SKIPOUTER_ADVANCE: advance over an outer tuple that is
+				 * known not to join to any inner tuple.
+				 *
 				 * Before advancing, we check to see if we must emit an
 				 * outer-join fill tuple for this outer tuple.
 				 */
@@ -1204,6 +1213,9 @@ ExecMergeJoin(MergeJoinState *node)
 				break;
 
 				/*
+				 * SKIPINNER_ADVANCE: advance over an inner tuple that is
+				 * known not to join to any outer tuple.
+				 *
 				 * Before advancing, we check to see if we must emit an
 				 * outer-join fill tuple for this inner tuple.
 				 */
@@ -1225,6 +1237,10 @@ ExecMergeJoin(MergeJoinState *node)
 						return result;
 				}
 
+				/* Mark before advancing, if wanted */
+				if (node->mj_ExtraMarks)
+					ExecMarkPos(innerPlan);
+
 				/*
 				 * now we get the next inner tuple, if any
 				 */
@@ -1295,6 +1311,10 @@ ExecMergeJoin(MergeJoinState *node)
 						return result;
 				}
 
+				/* Mark before advancing, if wanted */
+				if (node->mj_ExtraMarks)
+					ExecMarkPos(innerPlan);
+
 				/*
 				 * now we get the next inner tuple, if any
 				 */
@@ -1425,6 +1445,22 @@ ExecInitMergeJoin(MergeJoin *node, EState *estate, int eflags)
 	innerPlanState(mergestate) = ExecInitNode(innerPlan(node), estate,
 											  eflags | EXEC_FLAG_MARK);
 
+	/*
+	 * For certain types of inner child nodes, it is advantageous to issue
+	 * MARK every time we advance past an inner tuple we will never return
+	 * to.  For other types, MARK on a tuple we cannot return to is a waste
+	 * of cycles.  Detect which case applies and set mj_ExtraMarks if we
+	 * want to issue "unnecessary" MARK calls.
+	 *
+	 * Currently, only Material wants the extra MARKs, and it will be helpful
+	 * only if eflags doesn't specify REWIND.
+	 */
+	if (IsA(innerPlan(node), Material) &&
+		(eflags & EXEC_FLAG_REWIND) == 0)
+		mergestate->mj_ExtraMarks = true;
+	else
+		mergestate->mj_ExtraMarks = false;
+
 #define MERGEJOIN_NSLOTS 4
 
 	/*

src/backend/optimizer/path/costsize.c

@@ -54,7 +54,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.182 2007/05/04 01:13:44 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.183 2007/05/21 17:57:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1038,6 +1038,23 @@ cost_sort(Path *path, PlannerInfo *root,
 	path->total_cost = startup_cost + run_cost;
 }
 
+/*
+ * sort_exceeds_work_mem
+ *	  Given a finished Sort plan node, detect whether it is expected to
+ *	  spill to disk (ie, will need more than work_mem workspace)
+ *
+ * This assumes there will be no available LIMIT.
+ */
+bool
+sort_exceeds_work_mem(Sort *sort)
+{
+	double		input_bytes = relation_byte_size(sort->plan.plan_rows,
+												 sort->plan.plan_width);
+	long		work_mem_bytes = work_mem * 1024L;
+
+	return (input_bytes > work_mem_bytes);
+}
+
 /*
  * cost_material
  *	  Determines and returns the cost of materializing a relation, including

src/backend/optimizer/plan/createplan.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.230 2007/05/04 01:13:44 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.231 2007/05/21 17:57:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1600,6 +1600,30 @@ create_mergejoin_plan(PlannerInfo *root,
 	else
 		innerpathkeys = best_path->jpath.innerjoinpath->pathkeys;
 
+	/*
+	 * If inner plan is a sort that is expected to spill to disk, add a
+	 * materialize node to shield it from the need to handle mark/restore.
+	 * This will allow it to perform the last merge pass on-the-fly, while
+	 * in most cases not requiring the materialize to spill to disk.
+	 *
+	 * XXX really, Sort oughta do this for itself, probably, to avoid the
+	 * overhead of a separate plan node.
+	 */
+	if (IsA(inner_plan, Sort) &&
+		sort_exceeds_work_mem((Sort *) inner_plan))
+	{
+		Plan	   *matplan = (Plan *) make_material(inner_plan);
+
+		/*
+		 * We assume the materialize will not spill to disk, and therefore
+		 * charge just cpu_tuple_cost per tuple.
+		 */
+		copy_plan_costsize(matplan, inner_plan);
+		matplan->total_cost += cpu_tuple_cost * matplan->plan_rows;
+
+		inner_plan = matplan;
+	}
+
 	/*
 	 * Compute the opfamily/strategy/nullsfirst arrays needed by the executor.
 	 * The information is in the pathkeys for the two inputs, but we need to

src/include/nodes/execnodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.174 2007/05/17 19:35:08 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.175 2007/05/21 17:57:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1180,6 +1180,7 @@ typedef struct NestLoopState
  *		NumClauses		   number of mergejoinable join clauses
  *		Clauses			   info for each mergejoinable clause
  *		JoinState		   current "state" of join.  see execdefs.h
+ *		ExtraMarks		   true to issue extra Mark operations on inner scan
  *		FillOuter		   true if should emit unjoined outer tuples anyway
  *		FillInner		   true if should emit unjoined inner tuples anyway
  *		MatchedOuter	   true if found a join match for current outer tuple
@@ -1202,6 +1203,7 @@ typedef struct MergeJoinState
 	int			mj_NumClauses;
 	MergeJoinClause mj_Clauses; /* array of length mj_NumClauses */
 	int			mj_JoinState;
+	bool		mj_ExtraMarks;
 	bool		mj_FillOuter;
 	bool		mj_FillInner;
 	bool		mj_MatchedOuter;
@@ -1281,7 +1283,7 @@ typedef struct HashJoinState
 typedef struct MaterialState
 {
 	ScanState	ss;				/* its first field is NodeTag */
-	bool		randomAccess;	/* need random access to subplan output? */
+	int			eflags;			/* capability flags to pass to tuplestore */
 	bool		eof_underlying; /* reached end of underlying plan? */
 	void	   *tuplestorestate;	/* private state of tuplestore.c */
 } MaterialState;

src/include/optimizer/cost.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.86 2007/05/04 01:13:45 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.87 2007/05/21 17:57:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -75,6 +75,7 @@ extern void cost_valuesscan(Path *path, PlannerInfo *root,
 extern void cost_sort(Path *path, PlannerInfo *root,
 		  List *pathkeys, Cost input_cost, double tuples, int width,
 		  double limit_tuples);
+extern bool sort_exceeds_work_mem(Sort *sort);
 extern void cost_material(Path *path,
 			  Cost input_cost, double tuples, int width);
 extern void cost_agg(Path *path, PlannerInfo *root,

src/include/utils/tuplestore.h

@@ -22,7 +22,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/tuplestore.h,v 1.20 2007/01/05 22:20:00 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/tuplestore.h,v 1.21 2007/05/21 17:57:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -46,6 +46,8 @@ extern Tuplestorestate *tuplestore_begin_heap(bool randomAccess,
 					  bool interXact,
 					  int maxKBytes);
 
+extern void tuplestore_set_eflags(Tuplestorestate *state, int eflags);
+
 extern void tuplestore_puttupleslot(Tuplestorestate *state,
 						TupleTableSlot *slot);
 extern void tuplestore_puttuple(Tuplestorestate *state, HeapTuple tuple);
@@ -53,7 +55,6 @@ extern void tuplestore_puttuple(Tuplestorestate *state, HeapTuple tuple);
 /* tuplestore_donestoring() used to be required, but is no longer used */
 #define tuplestore_donestoring(state)	((void) 0)
 
-/* backwards scan is only allowed if randomAccess was specified 'true' */
 extern bool tuplestore_gettupleslot(Tuplestorestate *state, bool forward,
 						TupleTableSlot *slot);
 extern bool tuplestore_advance(Tuplestorestate *state, bool forward);