diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index e2f8078b23..13b04a06b1 100644
--- a/src/backend/access/index/indexam.c
+++ b/src/backend/access/index/indexam.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.80 2005/04/14 20:03:23 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.81 2005/05/15 21:19:54 tgl Exp $
  *
  * INTERFACE ROUTINES
  *		index_open		- open an index relation by relation OID
@@ -411,6 +411,10 @@ index_markpos(IndexScanDesc scan)
 
 /* ----------------
  *		index_restrpos	- restore a scan position
+ *
+ * NOTE: this only restores the internal scan state of the index AM.
+ * The current result tuple (scan->xs_ctup) doesn't change.  See comments
+ * for ExecRestrPos().
  * ----------------
  */
 void
diff --git a/src/backend/executor/execAmi.c b/src/backend/executor/execAmi.c
index ddcd37ae28..c2cb4b6883 100644
--- a/src/backend/executor/execAmi.c
+++ b/src/backend/executor/execAmi.c
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *	$PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.83 2005/04/19 22:35:11 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.84 2005/05/15 21:19:54 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -248,6 +248,14 @@ ExecMarkPos(PlanState *node)
  * ExecRestrPos
  *
  * restores the scan position previously saved with ExecMarkPos()
+ *
+ * NOTE: the semantics of this are that the first ExecProcNode following
+ * the restore operation will yield the same tuple as the first one following
+ * the mark operation.  It is unspecified what happens to the plan node's
+ * result TupleTableSlot.  (In most cases the result slot is unchanged by
+ * a restore, but the node may choose to clear it or to load it with the
+ * restored-to tuple.)  Hence the caller should discard any previously
+ * returned TupleTableSlot after doing a restore.
  */
 void
 ExecRestrPos(PlanState *node)
@@ -290,6 +298,11 @@ ExecRestrPos(PlanState *node)
  * XXX Ideally, all plan node types would support mark/restore, and this
  * wouldn't be needed.  For now, this had better match the routines above.
  * But note the test is on Plan nodetype, not PlanState nodetype.
+ *
+ * (However, since the only present use of mark/restore is in mergejoin,
+ * there is no need to support mark/restore in any plan type that is not
+ * capable of generating ordered output.  So the seqscan, tidscan, and
+ * functionscan support is actually useless code at present.)
  */
 bool
 ExecSupportsMarkRestore(NodeTag plantype)
diff --git a/src/backend/executor/nodeMergejoin.c b/src/backend/executor/nodeMergejoin.c
index 6eb2bcc364..fb279e8b68 100644
--- a/src/backend/executor/nodeMergejoin.c
+++ b/src/backend/executor/nodeMergejoin.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeMergejoin.c,v 1.73 2005/05/14 21:29:23 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeMergejoin.c,v 1.74 2005/05/15 21:19:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1134,8 +1134,10 @@ ExecMergeJoin(MergeJoinState *node)
 					ExecRestrPos(innerPlan);
 
 					/*
-					 * ExecRestrPos really should give us back a new Slot,
-					 * but since it doesn't, use the marked slot.
+					 * ExecRestrPos probably should give us back a new Slot,
+					 * but since it doesn't, use the marked slot.  (The
+					 * previously returned mj_InnerTupleSlot cannot be
+					 * assumed to hold the required tuple.)
 					 */
 					node->mj_InnerTupleSlot = innerTupleSlot;
 					/* we need not do MJEvalInnerValues again */
diff --git a/src/backend/executor/nodeSeqscan.c b/src/backend/executor/nodeSeqscan.c
index f5a284a26b..fab526f399 100644
--- a/src/backend/executor/nodeSeqscan.c
+++ b/src/backend/executor/nodeSeqscan.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeSeqscan.c,v 1.52 2005/03/16 21:38:07 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeSeqscan.c,v 1.53 2005/05/15 21:19:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -342,9 +342,8 @@ ExecSeqReScan(SeqScanState *node, ExprContext *exprCtxt)
 void
 ExecSeqMarkPos(SeqScanState *node)
 {
-	HeapScanDesc scan;
+	HeapScanDesc scan = node->ss_currentScanDesc;
 
-	scan = node->ss_currentScanDesc;
 	heap_markpos(scan);
 }
 
@@ -357,8 +356,15 @@ ExecSeqMarkPos(SeqScanState *node)
 void
 ExecSeqRestrPos(SeqScanState *node)
 {
-	HeapScanDesc scan;
+	HeapScanDesc scan = node->ss_currentScanDesc;
+
+	/*
+	 * Clear any reference to the previously returned tuple.  This is
+	 * needed because the slot is simply pointing at scan->rs_cbuf, which
+	 * heap_restrpos will change; we'd have an internally inconsistent
+	 * slot if we didn't do this.
+	 */
+	ExecClearTuple(node->ss_ScanTupleSlot);
 
-	scan = node->ss_currentScanDesc;
 	heap_restrpos(scan);
 }