Be more tense about not creating tuplestores with randomAccess = true unless

backwards scan could actually happen.  In particular, pass a flag to
materialize-mode SRFs that tells them whether they need to require random
access.  In passing, also suppress unneeded backward-scan overhead for a
Portal's holdStore tuplestore.  Per my proposal about reducing I/O costs for
tuplestores.

contrib/tablefunc/tablefunc.c

 /*
- * $PostgreSQL: pgsql/contrib/tablefunc/tablefunc.c,v 1.54 2008/10/28 22:02:05 tgl Exp $ 
+ * $PostgreSQL: pgsql/contrib/tablefunc/tablefunc.c,v 1.55 2008/10/29 00:00:38 tgl Exp $ 
  *
  *
  * tablefunc
@@ -51,7 +51,8 @@ static HTAB *load_categories_hash(char *cats_sql, MemoryContext per_query_ctx);
 static Tuplestorestate *get_crosstab_tuplestore(char *sql,
 						HTAB *crosstab_hash,
 						TupleDesc tupdesc,
-						MemoryContext per_query_ctx);
+						MemoryContext per_query_ctx,
+						bool randomAccess);
 static void validateConnectbyTupleDesc(TupleDesc tupdesc, bool show_branch, bool show_serial);
 static bool compatCrosstabTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2);
 static bool compatConnectbyTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2);
@@ -66,6 +67,7 @@ static Tuplestorestate *connectby(char *relname,
 		  bool show_branch,
 		  bool show_serial,
 		  MemoryContext per_query_ctx,
+		  bool randomAccess,
 		  AttInMetadata *attinmeta);
 static Tuplestorestate *build_tuplestore_recursively(char *key_fld,
 							 char *parent_key_fld,
@@ -745,7 +747,8 @@ crosstab_hash(PG_FUNCTION_ARGS)
 	rsinfo->setResult = get_crosstab_tuplestore(sql,
 												crosstab_hash,
 												tupdesc,
-												per_query_ctx);
+												per_query_ctx,
+												rsinfo->allowedModes & SFRM_Materialize_Random);
 
 	/*
 	 * SFRM_Materialize mode expects us to return a NULL Datum. The actual
@@ -852,7 +855,8 @@ static Tuplestorestate *
 get_crosstab_tuplestore(char *sql,
 						HTAB *crosstab_hash,
 						TupleDesc tupdesc,
-						MemoryContext per_query_ctx)
+						MemoryContext per_query_ctx,
+						bool randomAccess)
 {
 	Tuplestorestate *tupstore;
 	int			num_categories = hash_get_num_entries(crosstab_hash);
@@ -863,8 +867,8 @@ get_crosstab_tuplestore(char *sql,
 	int			proc;
 	MemoryContext SPIcontext;
 
-	/* initialize our tuplestore */
-	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	/* initialize our tuplestore (while still in query context!) */
+	tupstore = tuplestore_begin_heap(randomAccess, false, work_mem);
 
 	/* Connect to SPI manager */
 	if ((ret = SPI_connect()) < 0)
@@ -1113,6 +1117,7 @@ connectby_text(PG_FUNCTION_ARGS)
 								  show_branch,
 								  show_serial,
 								  per_query_ctx,
+								  rsinfo->allowedModes & SFRM_Materialize_Random,
 								  attinmeta);
 	rsinfo->setDesc = tupdesc;
 
@@ -1192,6 +1197,7 @@ connectby_text_serial(PG_FUNCTION_ARGS)
 								  show_branch,
 								  show_serial,
 								  per_query_ctx,
+								  rsinfo->allowedModes & SFRM_Materialize_Random,
 								  attinmeta);
 	rsinfo->setDesc = tupdesc;
 
@@ -1222,6 +1228,7 @@ connectby(char *relname,
 		  bool show_branch,
 		  bool show_serial,
 		  MemoryContext per_query_ctx,
+		  bool randomAccess,
 		  AttInMetadata *attinmeta)
 {
 	Tuplestorestate *tupstore = NULL;
@@ -1239,7 +1246,7 @@ connectby(char *relname,
 	oldcontext = MemoryContextSwitchTo(per_query_ctx);
 
 	/* initialize our tuplestore */
-	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	tupstore = tuplestore_begin_heap(randomAccess, false, work_mem);
 
 	MemoryContextSwitchTo(oldcontext);
 

contrib/xml2/xpath.c

 /*
- * $PostgreSQL: pgsql/contrib/xml2/xpath.c,v 1.20 2008/05/17 01:28:22 adunstan Exp $ 
+ * $PostgreSQL: pgsql/contrib/xml2/xpath.c,v 1.21 2008/10/29 00:00:38 tgl Exp $ 
  *
  * Parser interface for DOM-based parser (libxml) rather than
    stream-based SAX-type parser */
@@ -688,7 +688,9 @@ xpath_table(PG_FUNCTION_ARGS)
 	 * Create the tuplestore - work_mem is the max in-memory size before a
 	 * file is created on disk to hold it.
 	 */
-	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	tupstore =
+		tuplestore_begin_heap(rsinfo->allowedModes & SFRM_Materialize_Random,
+							  false, work_mem);
 
 	MemoryContextSwitchTo(oldcontext);
 

src/backend/commands/prepare.c

@@ -10,7 +10,7 @@
  * Copyright (c) 2002-2008, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.91 2008/08/28 23:09:45 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.92 2008/10/29 00:00:38 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -766,7 +766,9 @@ pg_prepared_statement(PG_FUNCTION_ARGS)
 	 * We put all the tuples into a tuplestore in one scan of the hashtable.
 	 * This avoids any issue of the hashtable possibly changing between calls.
 	 */
-	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	tupstore =
+		tuplestore_begin_heap(rsinfo->allowedModes & SFRM_Materialize_Random,
+							  false, work_mem);
 
 	/* hash table might be uninitialized */
 	if (prepared_queries)

src/backend/executor/execQual.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execQual.c,v 1.234 2008/10/28 22:02:05 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execQual.c,v 1.235 2008/10/29 00:00:38 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1429,6 +1429,7 @@ restart:
 		rsinfo.econtext = econtext;
 		rsinfo.expectedDesc = fcache->funcResultDesc;
 		rsinfo.allowedModes = (int) (SFRM_ValuePerCall | SFRM_Materialize);
+		/* note we do not set SFRM_Materialize_Random */
 		rsinfo.returnMode = SFRM_ValuePerCall;
 		/* isDone is filled below */
 		rsinfo.setResult = NULL;
@@ -1702,7 +1703,8 @@ ExecMakeFunctionResultNoSets(FuncExprState *fcache,
 Tuplestorestate *
 ExecMakeTableFunctionResult(ExprState *funcexpr,
 							ExprContext *econtext,
-							TupleDesc expectedDesc)
+							TupleDesc expectedDesc,
+							bool randomAccess)
 {
 	Tuplestorestate *tupstore = NULL;
 	TupleDesc	tupdesc = NULL;
@@ -1736,6 +1738,8 @@ ExecMakeTableFunctionResult(ExprState *funcexpr,
 	rsinfo.econtext = econtext;
 	rsinfo.expectedDesc = expectedDesc;
 	rsinfo.allowedModes = (int) (SFRM_ValuePerCall | SFRM_Materialize);
+	if (randomAccess)
+		rsinfo.allowedModes |= (int) SFRM_Materialize_Random;
 	rsinfo.returnMode = SFRM_ValuePerCall;
 	/* isDone is filled below */
 	rsinfo.setResult = NULL;
@@ -1909,7 +1913,7 @@ ExecMakeTableFunctionResult(ExprState *funcexpr,
 									   -1,
 									   0);
 				}
-				tupstore = tuplestore_begin_heap(true, false, work_mem);
+				tupstore = tuplestore_begin_heap(randomAccess, false, work_mem);
 				MemoryContextSwitchTo(oldcontext);
 				rsinfo.setResult = tupstore;
 				rsinfo.setDesc = tupdesc;
@@ -1976,7 +1980,7 @@ no_function_result:
 	if (rsinfo.setResult == NULL)
 	{
 		MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
-		tupstore = tuplestore_begin_heap(true, false, work_mem);
+		tupstore = tuplestore_begin_heap(randomAccess, false, work_mem);
 		rsinfo.setResult = tupstore;
 		if (!returnsSet)
 		{

src/backend/executor/nodeFunctionscan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeFunctionscan.c,v 1.48 2008/10/28 22:02:05 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeFunctionscan.c,v 1.49 2008/10/29 00:00:38 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -64,7 +64,8 @@ FunctionNext(FunctionScanState *node)
 		node->tuplestorestate = tuplestorestate =
 			ExecMakeTableFunctionResult(node->funcexpr,
 										node->ss.ps.ps_ExprContext,
-										node->tupdesc);
+										node->tupdesc,
+										node->eflags & EXEC_FLAG_BACKWARD);
 	}
 
 	/*
@@ -123,6 +124,7 @@ ExecInitFunctionScan(FunctionScan *node, EState *estate, int eflags)
 	scanstate = makeNode(FunctionScanState);
 	scanstate->ss.ps.plan = (Plan *) node;
 	scanstate->ss.ps.state = estate;
+	scanstate->eflags = eflags;
 
 	/*
 	 * Miscellaneous initialization

src/backend/utils/fmgr/README

-$PostgreSQL: pgsql/src/backend/utils/fmgr/README,v 1.14 2008/10/28 22:02:05 tgl Exp $
+$PostgreSQL: pgsql/src/backend/utils/fmgr/README,v 1.15 2008/10/29 00:00:38 tgl Exp $
 
 Function Manager
 ================
@@ -432,6 +432,10 @@ function is called in).  The function stores pointers to the Tuplestore and
 TupleDesc into ReturnSetInfo, sets returnMode to indicate materialize mode,
 and returns null.  isDone is not used and should be left at ExprSingleResult.
 
+The Tuplestore must be created with randomAccess = true if
+SFRM_Materialize_Random is set in allowedModes, but it can (and preferably
+should) be created with randomAccess = false if not.
+
 If available, the expected tuple descriptor is passed in ReturnSetInfo;
 in other contexts the expectedDesc field will be NULL.  The function need
 not pay attention to expectedDesc, but it may be useful in special cases.

src/backend/utils/mmgr/portalmem.c

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/portalmem.c,v 1.111 2008/07/18 20:26:06 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/portalmem.c,v 1.112 2008/10/29 00:00:38 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -354,11 +354,17 @@ PortalCreateHoldStore(Portal portal)
 							  ALLOCSET_DEFAULT_INITSIZE,
 							  ALLOCSET_DEFAULT_MAXSIZE);
 
-	/* Create the tuple store, selecting cross-transaction temp files. */
+	/*
+	 * Create the tuple store, selecting cross-transaction temp files, and
+	 * enabling random access only if cursor requires scrolling.
+	 *
+	 * XXX: Should maintenance_work_mem be used for the portal size?
+	 */
 	oldcxt = MemoryContextSwitchTo(portal->holdContext);
 
-	/* XXX: Should maintenance_work_mem be used for the portal size? */
-	portal->holdStore = tuplestore_begin_heap(true, true, work_mem);
+	portal->holdStore =
+		tuplestore_begin_heap(portal->cursorOptions & CURSOR_OPT_SCROLL,
+							  true, work_mem);
 
 	MemoryContextSwitchTo(oldcxt);
 }
@@ -913,7 +919,9 @@ pg_cursor(PG_FUNCTION_ARGS)
 	 * We put all the tuples into a tuplestore in one scan of the hashtable.
 	 * This avoids any issue of the hashtable possibly changing between calls.
 	 */
-	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	tupstore =
+		tuplestore_begin_heap(rsinfo->allowedModes & SFRM_Materialize_Random,
+							  false, work_mem);
 
 	hash_seq_init(&hash_seq, PortalHashTable);
 	while ((hentry = hash_seq_search(&hash_seq)) != NULL)

src/include/executor/executor.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/executor.h,v 1.150 2008/10/28 22:02:05 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/executor/executor.h,v 1.151 2008/10/29 00:00:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -178,7 +178,8 @@ extern Datum GetAttributeByName(HeapTupleHeader tuple, const char *attname,
 				   bool *isNull);
 extern Tuplestorestate *ExecMakeTableFunctionResult(ExprState *funcexpr,
 							ExprContext *econtext,
-							TupleDesc expectedDesc);
+							TupleDesc expectedDesc,
+							bool randomAccess);
 extern Datum ExecEvalExprSwitchContext(ExprState *expression, ExprContext *econtext,
 						  bool *isNull, ExprDoneCond *isDone);
 extern ExprState *ExecInitExpr(Expr *node, PlanState *parent);

src/include/nodes/execnodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.192 2008/10/28 22:02:05 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.193 2008/10/29 00:00:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -156,7 +156,8 @@ typedef enum
 typedef enum
 {
 	SFRM_ValuePerCall = 0x01,	/* one value returned per call */
-	SFRM_Materialize = 0x02		/* result set instantiated in Tuplestore */
+	SFRM_Materialize = 0x02,	/* result set instantiated in Tuplestore */
+	SFRM_Materialize_Random = 0x04		/* Tuplestore needs randomAccess */
 } SetFunctionReturnMode;
 
 /*
@@ -1180,6 +1181,7 @@ typedef struct SubqueryScanState
  *		Function nodes are used to scan the results of a
  *		function appearing in FROM (typically a function returning set).
  *
+ *		eflags				node's capability flags
  *		tupdesc				expected return tuple description
  *		tuplestorestate		private state of tuplestore.c
  *		funcexpr			state for function expression being evaluated
@@ -1188,6 +1190,7 @@ typedef struct SubqueryScanState
 typedef struct FunctionScanState
 {
 	ScanState	ss;				/* its first field is NodeTag */
+	int			eflags;
 	TupleDesc	tupdesc;
 	Tuplestorestate *tuplestorestate;
 	ExprState  *funcexpr;

src/pl/plperl/plperl.c

 /**********************************************************************
  * plperl.c - perl as a procedural language for PostgreSQL
  *
- *	  $PostgreSQL: pgsql/src/pl/plperl/plperl.c,v 1.140 2008/10/09 17:24:05 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plperl/plperl.c,v 1.141 2008/10/29 00:00:39 tgl Exp $
  *
  **********************************************************************/
 
@@ -1922,7 +1922,8 @@ plperl_return_next(SV *sv)
 
 		current_call_data->ret_tdesc = CreateTupleDescCopy(tupdesc);
 		current_call_data->tuple_store =
-			tuplestore_begin_heap(true, false, work_mem);
+			tuplestore_begin_heap(rsi->allowedModes & SFRM_Materialize_Random,
+								  false, work_mem);
 		if (prodesc->fn_retistuple)
 		{
 			current_call_data->attinmeta =

src/pl/plpgsql/src/pl_exec.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.221 2008/09/24 14:40:00 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.222 2008/10/29 00:00:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2357,7 +2357,9 @@ exec_init_tuple_store(PLpgSQL_execstate *estate)
 	estate->tuple_store_cxt = rsi->econtext->ecxt_per_query_memory;
 
 	oldcxt = MemoryContextSwitchTo(estate->tuple_store_cxt);
-	estate->tuple_store = tuplestore_begin_heap(true, false, work_mem);
+	estate->tuple_store =
+		tuplestore_begin_heap(rsi->allowedModes & SFRM_Materialize_Random,
+							  false, work_mem);
 	MemoryContextSwitchTo(oldcxt);
 
 	estate->rettupdesc = rsi->expectedDesc;