diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index c3a45b98db..166fed7233 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.249 2008/01/30 18:35:55 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.250 2008/03/04 19:54:06 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -868,6 +868,10 @@ relation_open(Oid relationId, LOCKMODE lockmode)
 	if (!RelationIsValid(r))
 		elog(ERROR, "could not open relation with OID %u", relationId);
 
+	/* Make note that we've accessed a temporary relation */
+	if (r->rd_istemp)
+		MyXactAccessedTempRel = true;
+
 	pgstat_initstats(r);
 
 	return r;
@@ -912,6 +916,10 @@ try_relation_open(Oid relationId, LOCKMODE lockmode)
 	if (!RelationIsValid(r))
 		elog(ERROR, "could not open relation with OID %u", relationId);
 
+	/* Make note that we've accessed a temporary relation */
+	if (r->rd_istemp)
+		MyXactAccessedTempRel = true;
+
 	pgstat_initstats(r);
 
 	return r;
@@ -958,6 +966,10 @@ relation_open_nowait(Oid relationId, LOCKMODE lockmode)
 	if (!RelationIsValid(r))
 		elog(ERROR, "could not open relation with OID %u", relationId);
 
+	/* Make note that we've accessed a temporary relation */
+	if (r->rd_istemp)
+		MyXactAccessedTempRel = true;
+
 	pgstat_initstats(r);
 
 	return r;
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 13c3a0378b..65611089c4 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.257 2008/01/15 18:56:59 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.258 2008/03/04 19:54:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -62,6 +62,13 @@ bool		XactSyncCommit = true;
 int			CommitDelay = 0;	/* precommit delay in microseconds */
 int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
 
+/*
+ * MyXactAccessedTempRel is set when a temporary relation is accessed.
+ * We don't allow PREPARE TRANSACTION in that case.  (This is global
+ * so that it can be set from heapam.c.)
+ */
+bool		MyXactAccessedTempRel = false;
+
 
 /*
  *	transaction states - transaction state from server perspective
@@ -1445,6 +1452,7 @@ StartTransaction(void)
 	XactIsoLevel = DefaultXactIsoLevel;
 	XactReadOnly = DefaultXactReadOnly;
 	forceSyncCommit = false;
+	MyXactAccessedTempRel = false;
 
 	/*
 	 * reinitialize within-transaction counters
@@ -1770,6 +1778,26 @@ PrepareTransaction(void)
 
 	/* NOTIFY and flatfiles will be handled below */
 
+	/*
+	 * Don't allow PREPARE TRANSACTION if we've accessed a temporary table
+	 * in this transaction.  Having the prepared xact hold locks on another
+	 * backend's temp table seems a bad idea --- for instance it would prevent
+	 * the backend from exiting.  There are other problems too, such as how
+	 * to clean up the source backend's local buffers and ON COMMIT state
+	 * if the prepared xact includes a DROP of a temp table.
+	 *
+	 * We must check this after executing any ON COMMIT actions, because
+	 * they might still access a temp relation.
+	 *
+	 * XXX In principle this could be relaxed to allow some useful special
+	 * cases, such as a temp table created and dropped all within the
+	 * transaction.  That seems to require much more bookkeeping though.
+	 */
+	if (MyXactAccessedTempRel)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot PREPARE a transaction that has operated on temporary tables")));
+
 	/* Prevent cancel/die interrupt while cleaning up */
 	HOLD_INTERRUPTS();
 
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index a882f4e432..67bb2ba9fb 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.96 2008/01/08 23:18:50 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.97 2008/03/04 19:54:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -19,12 +19,10 @@
 #include "access/transam.h"
 #include "access/xact.h"
 #include "catalog/catalog.h"
-#include "catalog/namespace.h"
 #include "miscadmin.h"
 #include "storage/lmgr.h"
 #include "storage/procarray.h"
 #include "utils/inval.h"
-#include "utils/lsyscache.h"
 
 
 /*
@@ -663,44 +661,6 @@ UnlockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 }
 
 
-/*
- * LockTagIsTemp
- *		Determine whether a locktag is for a lock on a temporary object
- *
- * We need this because 2PC cannot deal with temp objects
- */
-bool
-LockTagIsTemp(const LOCKTAG *tag)
-{
-	switch ((LockTagType) tag->locktag_type)
-	{
-		case LOCKTAG_RELATION:
-		case LOCKTAG_RELATION_EXTEND:
-		case LOCKTAG_PAGE:
-		case LOCKTAG_TUPLE:
-			/* check for lock on a temp relation */
-			/* field1 is dboid, field2 is reloid for all of these */
-			if ((Oid) tag->locktag_field1 == InvalidOid)
-				return false;	/* shared, so not temp */
-			if (isTempOrToastNamespace(get_rel_namespace((Oid) tag->locktag_field2)))
-				return true;
-			break;
-		case LOCKTAG_TRANSACTION:
-		case LOCKTAG_VIRTUALTRANSACTION:
-			/* there are no temp transactions */
-			break;
-		case LOCKTAG_OBJECT:
-			/* there are currently no non-table temp objects */
-			break;
-		case LOCKTAG_USERLOCK:
-		case LOCKTAG_ADVISORY:
-			/* assume these aren't temp */
-			break;
-	}
-	return false;				/* default case */
-}
-
-
 /*
  * Append a description of a lockable object to buf.
  *
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index 24f9b7782e..c8ba3538cf 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.181 2008/02/02 22:26:17 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.182 2008/03/04 19:54:06 tgl Exp $
  *
  * NOTES
  *	  A lock table is a shared memory hash table.  When
@@ -37,7 +37,6 @@
 #include "access/twophase_rmgr.h"
 #include "miscadmin.h"
 #include "pgstat.h"
-#include "storage/lmgr.h"
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
 #include "utils/resowner.h"
@@ -1871,12 +1870,6 @@ AtPrepare_Locks(void)
 				elog(ERROR, "cannot PREPARE when session locks exist");
 		}
 
-		/* Can't handle it if the lock is on a temporary object */
-		if (LockTagIsTemp(&locallock->tag.lock))
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("cannot PREPARE a transaction that has operated on temporary tables")));
-
 		/*
 		 * Create a 2PC record.
 		 */
diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index c0020c8bcd..dff05c73db 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.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/access/xact.h,v 1.93 2008/01/01 19:45:56 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/access/xact.h,v 1.94 2008/03/04 19:54:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -44,6 +44,9 @@ extern bool XactReadOnly;
 /* Asynchronous commits */
 extern bool XactSyncCommit;
 
+/* Kluge for 2PC support */
+extern bool MyXactAccessedTempRel;
+
 /*
  *	start- and end-of-transaction callbacks for dynamically loaded modules
  */
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index b89d0c6c8d..05150afbac 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.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/storage/lmgr.h,v 1.60 2008/01/01 19:45:59 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/storage/lmgr.h,v 1.61 2008/03/04 19:54:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -72,9 +72,6 @@ extern void LockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 extern void UnlockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 				   LOCKMODE lockmode);
 
-/* Knowledge about which locktags describe temp objects */
-extern bool LockTagIsTemp(const LOCKTAG *tag);
-
 /* Describe a locktag for error messages */
 extern void DescribeLockTag(StringInfo buf, const LOCKTAG *tag);