Tweak portal (cursor) code so that it will not call the executor again

when user does another FETCH after reaching end of data, or another
FETCH backwards after reaching start.  This is needed because some plan
nodes are not very robust about being called again after they've already
returned NULL; for example, MergeJoin will crash in some states but not
others.  While the ideal approach would be for them all to handle this
correctly, it seems foolish to assume that no such bugs would creep in
again once cleaned up.  Therefore, the most robust answer is to prevent
the situation from arising at all.

src/backend/commands/command.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/Attic/command.c,v 1.121 2001/02/14 21:35:00 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/Attic/command.c,v 1.122 2001/02/27 22:07:34 tgl Exp $
  *
  * NOTES
  *	  The PerformAddAttribute() code, like most of the relation
@@ -107,8 +107,8 @@ PerformPortalFetch(char *name,
 				   CommandDest dest)
 {
 	Portal		portal;
-	int			feature;
 	QueryDesc  *queryDesc;
+	EState	   *estate;
 	MemoryContext oldcontext;
 
 	/* ----------------
@@ -140,19 +140,14 @@ PerformPortalFetch(char *name,
 	oldcontext = MemoryContextSwitchTo(PortalGetHeapMemory(portal));
 
 	/* ----------------
-	 *	setup "feature" to tell the executor which direction to go in.
-	 * ----------------
-	 */
-	if (forward)
-		feature = EXEC_FOR;
-	else
-		feature = EXEC_BACK;
-
-	/* ----------------
-	 *	tell the destination to prepare to receive some tuples
+	 *	tell the destination to prepare to receive some tuples.
+	 *
+	 *	If we've been asked for a MOVE, make a temporary QueryDesc
+	 *	with the appropriate dummy destination.
 	 * ----------------
 	 */
 	queryDesc = PortalGetQueryDesc(portal);
+	estate = PortalGetState(portal);
 
 	if (dest == None)			/* MOVE */
 	{
@@ -165,7 +160,7 @@ PerformPortalFetch(char *name,
 
 	BeginCommand(name,
 				 queryDesc->operation,
-				 portal->attinfo, /* QueryDescGetTypeInfo(queryDesc) */
+				 PortalGetTupleDesc(portal),
 				 false,			/* portal fetches don't end up in
 								 * relations */
 				 false,			/* this is a portal fetch, not a "retrieve
@@ -174,18 +169,45 @@ PerformPortalFetch(char *name,
 				 dest);
 
 	/* ----------------
-	 *	execute the portal fetch operation
+	 *	Determine which direction to go in, and check to see if we're already
+	 *	at the end of the available tuples in that direction.  If so, do
+	 *	nothing.  (This check exists because not all plan node types are
+	 *	robust about being called again if they've already returned NULL
+	 *	once.)  If it's OK to do the fetch, call the executor.  Then,
+	 *	update the atStart/atEnd state depending on the number of tuples
+	 *	that were retrieved.
 	 * ----------------
 	 */
-	ExecutorRun(queryDesc, PortalGetState(portal), feature, (long) count);
-
-	if (dest == None)			/* MOVE */
-		pfree(queryDesc);
+	if (forward)
+	{
+		if (! portal->atEnd)
+		{
+			ExecutorRun(queryDesc, estate, EXEC_FOR, (long) count);
+			if (estate->es_processed > 0)
+				portal->atStart = false; /* OK to back up now */
+			if (count <= 0 || (int) estate->es_processed < count)
+				portal->atEnd = true; /* we retrieved 'em all */
+		}
+	}
+	else
+	{
+		if (! portal->atStart)
+		{
+			ExecutorRun(queryDesc, estate, EXEC_BACK, (long) count);
+			if (estate->es_processed > 0)
+				portal->atEnd = false; /* OK to go forward now */
+			if (count <= 0 || (int) estate->es_processed < count)
+				portal->atStart = true; /* we retrieved 'em all */
+		}
+	}
 
 	/* ----------------
-	 * Switch back to old context.
+	 *	Clean up and switch back to old context.
 	 * ----------------
 	 */
+	if (dest == None)			/* MOVE */
+		pfree(queryDesc);
+
 	MemoryContextSwitchTo(oldcontext);
 
 	/* ----------------

src/backend/tcop/pquery.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/tcop/pquery.c,v 1.41 2001/01/24 19:43:09 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/tcop/pquery.c,v 1.42 2001/02/27 22:07:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -248,7 +248,7 @@ ProcessQuery(Query *parsetree,
 	 * ----------------
 	 */
 	if (isRetrieveIntoRelation)
-		queryDesc->dest = (int) None;
+		queryDesc->dest = None;
 
 	/* ----------------
 	 *	create a default executor state.

src/backend/utils/mmgr/portalmem.c

@@ -8,44 +8,26 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/mmgr/portalmem.c,v 1.39 2001/01/24 19:43:17 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/mmgr/portalmem.c,v 1.40 2001/02/27 22:07:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 /*
  * NOTES
- *		Do not confuse "Portal" with "PortalEntry" (or "PortalBuffer").
- *		When a PQexec() routine is run, the resulting tuples
- *		find their way into a "PortalEntry".  The contents of the resulting
- *		"PortalEntry" can then be inspected by other PQxxx functions.
+ *		A "Portal" is a structure used to keep track of cursor queries.
  *
- *		A "Portal" is a structure used to keep track of queries of the
- *		form:
- *				retrieve portal FOO ( blah... ) where blah...
- *
- *		When the backend sees a "retrieve portal" query, it allocates
- *		a "PortalD" structure, plans the query and then stores the query
+ *		When the backend sees a "declare cursor" query, it allocates a
+ *		"PortalData" structure, plans the query and then stores the query
  *		in the portal without executing it.  Later, when the backend
  *		sees a
- *				fetch 1 into FOO
- *
+ *				fetch 1 from FOO
  *		the system looks up the portal named "FOO" in the portal table,
  *		gets the planned query and then calls the executor with a feature of
  *		'(EXEC_FOR 1).  The executor then runs the query and returns a single
  *		tuple.	The problem is that we have to hold onto the state of the
- *		portal query until we see a "close p".	This means we have to be
+ *		portal query until we see a "close".  This means we have to be
  *		careful about memory management.
  *
- *		Having said all that, here is what a PortalD currently looks like:
- *
- * struct PortalD {
- *		char*							name;
- *		classObj(MemoryContext)			heap;
- *		List							queryDesc;
- *		EState							state;
- *		void							(*cleanup) ARGS((Portal portal));
- * };
- *
  *		I hope this makes things clearer to whoever reads this -cim 2/22/91
  */
 
@@ -188,43 +170,13 @@ PortalSetQuery(Portal portal,
 	AssertArg(IsA((Node *) state, EState));
 
 	portal->queryDesc = queryDesc;
-	portal->state = state;
 	portal->attinfo = attinfo;
+	portal->state = state;
+	portal->atStart = true;		/* Allow fetch forward only */
+	portal->atEnd = false;
 	portal->cleanup = cleanup;
 }
 
-/*
- * PortalGetQueryDesc
- *		Returns query attached to portal.
- *
- * Exceptions:
- *		BadState if called when disabled.
- *		BadArg if portal is invalid.
- */
-QueryDesc  *
-PortalGetQueryDesc(Portal portal)
-{
-	AssertArg(PortalIsValid(portal));
-
-	return portal->queryDesc;
-}
-
-/*
- * PortalGetState
- *		Returns state attached to portal.
- *
- * Exceptions:
- *		BadState if called when disabled.
- *		BadArg if portal is invalid.
- */
-EState *
-PortalGetState(Portal portal)
-{
-	AssertArg(PortalIsValid(portal));
-
-	return portal->state;
-}
-
 /*
  * CreatePortal
  *		Returns a new portal given a name.
@@ -265,7 +217,8 @@ CreatePortal(char *name)
 	portal->queryDesc = NULL;
 	portal->attinfo = NULL;
 	portal->state = NULL;
-
+	portal->atStart = true;		/* disallow fetches until query is set */
+	portal->atEnd = true;
 	portal->cleanup = NULL;
 
 	/* put portal in table */
@@ -315,13 +268,3 @@ AtEOXact_portals(void)
 {
 	HashTableWalk(PortalHashTable, (HashtFunc) PortalDrop, 0);
 }
-
-/*
- * PortalGetHeapMemory
- *		Returns heap memory context for a given portal.
- */
-MemoryContext
-PortalGetHeapMemory(Portal portal)
-{
-	return portal->heap;
-}

src/include/utils/portal.h

@@ -7,19 +7,14 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: portal.h,v 1.25 2001/01/24 19:43:28 momjian Exp $
+ * $Id: portal.h,v 1.26 2001/02/27 22:07:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 /*
  * Note:
  *		A portal is an abstraction which represents the execution state of
- * a running query (or a fixed sequence of queries).
- *
- * Note:
- *		now that PQ calls can be made from within a backend, a portal
- *		may also be used to keep track of the tuples resulting
- *		from the execution of a query.	In this case, entryIndex
+ * a running query (specifically, a CURSOR).
  */
 #ifndef PORTAL_H
 #define PORTAL_H
@@ -28,17 +23,19 @@
 #include "nodes/memnodes.h"
 
 
-typedef struct PortalD *Portal;
+typedef struct PortalData *Portal;
 
-typedef struct PortalD
+typedef struct PortalData
 {
 	char	   *name;			/* Portal's name */
 	MemoryContext heap;			/* subsidiary memory */
 	QueryDesc  *queryDesc;		/* Info about query associated with portal */
 	TupleDesc	attinfo;
-	EState	   *state;
+	EState	   *state;			/* Execution state of query */
+	bool		atStart;		/* T => fetch backwards is not allowed */
+	bool		atEnd;			/* T => fetch forwards is not allowed */
 	void		(*cleanup) (Portal); /* Cleanup routine (optional) */
-} PortalD;
+} PortalData;
 
 /*
  * PortalIsValid
@@ -46,6 +43,21 @@ typedef struct PortalD
  */
 #define PortalIsValid(p) PointerIsValid(p)
 
+/*
+ * Access macros for Portal ... use these in preference to field access.
+ */
+#define PortalGetQueryDesc(portal)  ((portal)->queryDesc)
+#define PortalGetTupleDesc(portal)  ((portal)->attinfo)
+#define PortalGetState(portal)  ((portal)->state)
+#define PortalGetHeapMemory(portal)  ((portal)->heap)
+
+/*
+ * estimate of the maximum number of open portals a user would have,
+ * used in initially sizing the PortalHashTable in EnablePortalManager()
+ */
+#define PORTALS_PER_USER	   10
+
+
 extern void EnablePortalManager(void);
 extern void AtEOXact_portals(void);
 extern Portal CreatePortal(char *name);
@@ -54,14 +66,6 @@ extern Portal GetPortalByName(char *name);
 extern void PortalSetQuery(Portal portal, QueryDesc *queryDesc,
 			   TupleDesc attinfo, EState *state,
 			   void (*cleanup) (Portal portal));
-extern QueryDesc *PortalGetQueryDesc(Portal portal);
-extern EState *PortalGetState(Portal portal);
-extern MemoryContext PortalGetHeapMemory(Portal portal);
-
-/* estimate of the maximum number of open portals a user would have,
- * used in initially sizing the PortalHashTable in EnablePortalManager()
- */
-#define PORTALS_PER_USER	   10
 
 
 #endif	 /* PORTAL_H */