Tweak code so that pg_subtrans is never consulted for XIDs older than

RecentXmin (== MyProc->xmin).  This ensures that it will be safe to
truncate pg_subtrans at RecentGlobalXmin, which should largely eliminate
any fear of bloat.  Along the way, eliminate SubTransXidsHaveCommonAncestor,
which isn't really needed and could not give a trustworthy result anyway
under the lookback restriction.
In an unrelated but nearby change, #ifdef out GetUndoRecPtr, which has
been dead code since 2001 and seems unlikely to ever be resurrected.

src/backend/access/transam/subtrans.c

@@ -15,7 +15,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.1 2004/07/01 00:49:42 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.2 2004/08/22 02:41:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -30,6 +30,7 @@
 #include "access/subtrans.h"
 #include "miscadmin.h"
 #include "storage/lwlock.h"
+#include "utils/tqual.h"
 
 
 /*
@@ -113,6 +114,9 @@ SubTransGetParent(TransactionId xid)
 	TransactionId *ptr;
 	TransactionId	parent;
 
+	/* Can't ask about stuff that might not be around anymore */
+	Assert(TransactionIdFollowsOrEquals(xid, RecentXmin));
+
 	/* Bootstrap and frozen XIDs have no parent */
 	if (!TransactionIdIsNormal(xid))
 		return InvalidTransactionId;
@@ -133,6 +137,13 @@ SubTransGetParent(TransactionId xid)
  * SubTransGetTopmostTransaction
  *
  * Returns the topmost transaction of the given transaction id.
+ *
+ * Because we cannot look back further than RecentXmin, it is possible
+ * that this function will lie and return an intermediate subtransaction ID
+ * instead of the true topmost parent ID.  This is OK, because in practice
+ * we only care about detecting whether the topmost parent is still running
+ * or is part of a current snapshot's list of still-running transactions.
+ * Therefore, any XID before RecentXmin is as good as any other.
  */
 TransactionId
 SubTransGetTopmostTransaction(TransactionId xid)
@@ -140,9 +151,14 @@ SubTransGetTopmostTransaction(TransactionId xid)
 	TransactionId parentXid = xid,
 				  previousXid = xid;
 
+	/* Can't ask about stuff that might not be around anymore */
+	Assert(TransactionIdFollowsOrEquals(xid, RecentXmin));
+
 	while (TransactionIdIsValid(parentXid))
 	{
 		previousXid = parentXid;
+		if (TransactionIdPrecedes(parentXid, RecentXmin))
+			break;
 		parentXid = SubTransGetParent(parentXid);
 	}
 
@@ -151,30 +167,6 @@ SubTransGetTopmostTransaction(TransactionId xid)
 	return previousXid;
 }
 
-/*
- * SubTransXidsHaveCommonAncestor
- *
- * Returns true iff the Xids have a common ancestor
- */
-bool
-SubTransXidsHaveCommonAncestor(TransactionId xid1, TransactionId xid2)
-{
-	if (TransactionIdEquals(xid1, xid2))
-		return true;
-
-	while (TransactionIdIsValid(xid1) && TransactionIdIsValid(xid2))
-	{
-		if (TransactionIdPrecedes(xid2, xid1))
-			xid1 = SubTransGetParent(xid1);
-		else
-			xid2 = SubTransGetParent(xid2);
-
-		if (TransactionIdEquals(xid1, xid2))
-			return true;
-	}
-
-	return false;
-}
 
 /*
  * Initialization of shared memory for Subtrans

src/backend/access/transam/transam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.57 2004/07/01 00:49:42 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.58 2004/08/22 02:41:57 tgl Exp $
  *
  * NOTES
  *	  This file contains the high level access-method interface to the
@@ -22,6 +22,7 @@
 #include "access/clog.h"
 #include "access/subtrans.h"
 #include "access/transam.h"
+#include "utils/tqual.h"
 
 
 /* ----------------
@@ -199,11 +200,15 @@ TransactionIdDidCommit(TransactionId transactionId)
 
 	/*
 	 * If it's marked subcommitted, we have to check the parent recursively.
+	 * However, if it's older than RecentXmin, we can't look at pg_subtrans;
+	 * instead assume that the parent crashed without cleaning up its children.
 	 */
 	if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
 	{
 		TransactionId parentXid;
-	   
+
+		if (TransactionIdPrecedes(transactionId, RecentXmin))
+			return false;
 		parentXid = SubTransGetParent(transactionId);
 		Assert(TransactionIdIsValid(parentXid));
 		return TransactionIdDidCommit(parentXid);
@@ -243,24 +248,17 @@ TransactionIdDidAbort(TransactionId transactionId)
 
 	/*
 	 * If it's marked subcommitted, we have to check the parent recursively.
-	 * 
-	 * If we detect that the parent has aborted, update pg_clog to show the
-	 * subtransaction as aborted.  This is only needed when the parent
-	 * crashed before either committing or aborting.  We want to clean up
-	 * pg_clog so future visitors don't need to make this check again.
+	 * However, if it's older than RecentXmin, we can't look at pg_subtrans;
+	 * instead assume that the parent crashed without cleaning up its children.
 	 */
 	if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
 	{
 		TransactionId parentXid;
-		bool parentAborted;
-	   
-		parentXid = SubTransGetParent(transactionId);
-		parentAborted = TransactionIdDidAbort(parentXid);
 
-		if (parentAborted)
-			TransactionIdAbort(transactionId);
-
-		return parentAborted;
+		if (TransactionIdPrecedes(transactionId, RecentXmin))
+			return true;
+		parentXid = SubTransGetParent(transactionId);
+		return TransactionIdDidAbort(parentXid);
 	}
 
 	/*

src/backend/storage/ipc/sinval.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/ipc/sinval.c,v 1.68 2004/08/15 17:03:36 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/ipc/sinval.c,v 1.69 2004/08/22 02:41:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -526,7 +526,7 @@ TransactionIdIsInProgress(TransactionId xid)
 	/*
 	 * Don't bother checking a very old transaction.
 	 */
-	if (TransactionIdPrecedes(xid, RecentGlobalXmin))
+	if (TransactionIdPrecedes(xid, RecentXmin))
 	{
 		xc_by_recent_xmin_inc();
 		return false;
@@ -912,6 +912,7 @@ CountActiveBackends(void)
 	return count;
 }
 
+#ifdef NOT_USED
 /*
  * GetUndoRecPtr -- returns oldest PGPROC->logRec.
  */
@@ -947,6 +948,7 @@ GetUndoRecPtr(void)
 
 	return (urec);
 }
+#endif /* NOT_USED */
 
 /*
  * BackendIdGetProc - given a BackendId, find its PGPROC structure

src/backend/storage/lmgr/lmgr.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.66 2004/07/28 14:23:29 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.67 2004/08/22 02:41:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -345,7 +345,7 @@ XactLockTableWait(TransactionId xid)
 	LOCKTAG		tag;
 	TransactionId myxid = GetTopTransactionId();
 
-	Assert(!SubTransXidsHaveCommonAncestor(xid, myxid));
+	Assert(!TransactionIdEquals(xid, myxid));
 
 	MemSet(&tag, 0, sizeof(tag));
 	tag.relId = XactLockTableId;

src/backend/utils/time/tqual.c

@@ -16,7 +16,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/time/tqual.c,v 1.74 2004/07/28 14:23:30 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/time/tqual.c,v 1.75 2004/08/22 02:41:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -933,8 +933,6 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId OldestXmin)
 		{
 			if (tuple->t_infomask & HEAP_XMAX_INVALID)	/* xid invalid */
 				return HEAPTUPLE_INSERT_IN_PROGRESS;
-			Assert(SubTransXidsHaveCommonAncestor(HeapTupleHeaderGetXmin(tuple),
-						HeapTupleHeaderGetXmax(tuple)));
 			if (tuple->t_infomask & HEAP_MARKED_FOR_UPDATE)
 				return HEAPTUPLE_INSERT_IN_PROGRESS;
 			/* inserted and then deleted by same xact */
@@ -1008,7 +1006,7 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId OldestXmin)
 	 * Deleter committed, but check special cases.
 	 */
 
-	if (SubTransXidsHaveCommonAncestor(HeapTupleHeaderGetXmin(tuple),
+	if (TransactionIdEquals(HeapTupleHeaderGetXmin(tuple),
 							HeapTupleHeaderGetXmax(tuple)))
 	{
 		/*

src/include/access/subtrans.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/subtrans.h,v 1.1 2004/07/01 00:51:38 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/subtrans.h,v 1.2 2004/08/22 02:41:58 tgl Exp $
  */
 #ifndef SUBTRANS_H
 #define SUBTRANS_H
@@ -20,7 +20,6 @@
 extern void SubTransSetParent(TransactionId xid, TransactionId parent);
 extern TransactionId SubTransGetParent(TransactionId xid);
 extern TransactionId SubTransGetTopmostTransaction(TransactionId xid);
-extern bool SubTransXidsHaveCommonAncestor(TransactionId xid1, TransactionId xid2);
 
 extern int	SUBTRANSShmemSize(void);
 extern void SUBTRANSShmemInit(void);

src/include/access/xlog.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.54 2004/07/21 22:31:25 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.55 2004/08/22 02:41:58 tgl Exp $
  */
 #ifndef XLOG_H
 #define XLOG_H
@@ -135,9 +135,4 @@ extern void CreateCheckPoint(bool shutdown, bool force);
 extern void XLogPutNextOid(Oid nextOid);
 extern XLogRecPtr GetRedoRecPtr(void);
 
-/* in storage/ipc/sinval.c, but don't want to declare in sinval.h because
- * we'd have to include xlog.h into that ...
- */
-extern XLogRecPtr GetUndoRecPtr(void);
-
 #endif   /* XLOG_H */