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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.
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7 files changed

+40
-56
lines changed

7 files changed

+40
-56
lines changed

‎src/backend/access/transam/subtrans.c

Lines changed: 17 additions & 25 deletions
Original file line numberDiff line numberDiff line change
@@ -15,7 +15,7 @@
1515
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
1616
* Portions Copyright (c) 1994, Regents of the University of California
1717
*
18-
* $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.1 2004/07/01 00:49:42 tgl Exp $
18+
* $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.2 2004/08/22 02:41:57 tgl Exp $
1919
*
2020
*-------------------------------------------------------------------------
2121
*/
@@ -30,6 +30,7 @@
3030
#include "access/subtrans.h"
3131
#include "miscadmin.h"
3232
#include "storage/lwlock.h"
33+
#include "utils/tqual.h"
3334

3435

3536
/*
@@ -113,6 +114,9 @@ SubTransGetParent(TransactionId xid)
113114
TransactionId *ptr;
114115
TransactionId parent;
115116

117+
/* Can't ask about stuff that might not be around anymore */
118+
Assert(TransactionIdFollowsOrEquals(xid, RecentXmin));
119+
116120
/* Bootstrap and frozen XIDs have no parent */
117121
if (!TransactionIdIsNormal(xid))
118122
return InvalidTransactionId;
@@ -133,16 +137,28 @@ SubTransGetParent(TransactionId xid)
133137
* SubTransGetTopmostTransaction
134138
*
135139
* Returns the topmost transaction of the given transaction id.
140+
*
141+
* Because we cannot look back further than RecentXmin, it is possible
142+
* that this function will lie and return an intermediate subtransaction ID
143+
* instead of the true topmost parent ID. This is OK, because in practice
144+
* we only care about detecting whether the topmost parent is still running
145+
* or is part of a current snapshot's list of still-running transactions.
146+
* Therefore, any XID before RecentXmin is as good as any other.
136147
*/
137148
TransactionId
138149
SubTransGetTopmostTransaction(TransactionId xid)
139150
{
140151
TransactionId parentXid = xid,
141152
previousXid = xid;
142153

154+
/* Can't ask about stuff that might not be around anymore */
155+
Assert(TransactionIdFollowsOrEquals(xid, RecentXmin));
156+
143157
while (TransactionIdIsValid(parentXid))
144158
{
145159
previousXid = parentXid;
160+
if (TransactionIdPrecedes(parentXid, RecentXmin))
161+
break;
146162
parentXid = SubTransGetParent(parentXid);
147163
}
148164

@@ -151,30 +167,6 @@ SubTransGetTopmostTransaction(TransactionId xid)
151167
return previousXid;
152168
}
153169

154-
/*
155-
* SubTransXidsHaveCommonAncestor
156-
*
157-
* Returns true iff the Xids have a common ancestor
158-
*/
159-
bool
160-
SubTransXidsHaveCommonAncestor(TransactionId xid1, TransactionId xid2)
161-
{
162-
if (TransactionIdEquals(xid1, xid2))
163-
return true;
164-
165-
while (TransactionIdIsValid(xid1) && TransactionIdIsValid(xid2))
166-
{
167-
if (TransactionIdPrecedes(xid2, xid1))
168-
xid1 = SubTransGetParent(xid1);
169-
else
170-
xid2 = SubTransGetParent(xid2);
171-
172-
if (TransactionIdEquals(xid1, xid2))
173-
return true;
174-
}
175-
176-
return false;
177-
}
178170

179171
/*
180172
* Initialization of shared memory for Subtrans

‎src/backend/access/transam/transam.c

Lines changed: 13 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.57 2004/07/01 00:49:42 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/access/transam/transam.c,v 1.58 2004/08/22 02:41:57 tgl Exp $
1212
*
1313
* NOTES
1414
* This file contains the high level access-method interface to the
@@ -22,6 +22,7 @@
2222
#include "access/clog.h"
2323
#include "access/subtrans.h"
2424
#include "access/transam.h"
25+
#include "utils/tqual.h"
2526

2627

2728
/* ----------------
@@ -199,11 +200,15 @@ TransactionIdDidCommit(TransactionId transactionId)
199200

200201
/*
201202
* If it's marked subcommitted, we have to check the parent recursively.
203+
* However, if it's older than RecentXmin, we can't look at pg_subtrans;
204+
* instead assume that the parent crashed without cleaning up its children.
202205
*/
203206
if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
204207
{
205208
TransactionId parentXid;
206-
209+
210+
if (TransactionIdPrecedes(transactionId, RecentXmin))
211+
return false;
207212
parentXid = SubTransGetParent(transactionId);
208213
Assert(TransactionIdIsValid(parentXid));
209214
return TransactionIdDidCommit(parentXid);
@@ -243,24 +248,17 @@ TransactionIdDidAbort(TransactionId transactionId)
243248

244249
/*
245250
* If it's marked subcommitted, we have to check the parent recursively.
246-
*
247-
* If we detect that the parent has aborted, update pg_clog to show the
248-
* subtransaction as aborted. This is only needed when the parent
249-
* crashed before either committing or aborting. We want to clean up
250-
* pg_clog so future visitors don't need to make this check again.
251+
* However, if it's older than RecentXmin, we can't look at pg_subtrans;
252+
* instead assume that the parent crashed without cleaning up its children.
251253
*/
252254
if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
253255
{
254256
TransactionId parentXid;
255-
bool parentAborted;
256-
257-
parentXid = SubTransGetParent(transactionId);
258-
parentAborted = TransactionIdDidAbort(parentXid);
259257

260-
if (parentAborted)
261-
TransactionIdAbort(transactionId);
262-
263-
return parentAborted;
258+
if (TransactionIdPrecedes(transactionId, RecentXmin))
259+
return true;
260+
parentXid = SubTransGetParent(transactionId);
261+
return TransactionIdDidAbort(parentXid);
264262
}
265263

266264
/*

‎src/backend/storage/ipc/sinval.c

Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/storage/ipc/sinval.c,v 1.68 2004/08/15 17:03:36 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/storage/ipc/sinval.c,v 1.69 2004/08/22 02:41:57 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -526,7 +526,7 @@ TransactionIdIsInProgress(TransactionId xid)
526526
/*
527527
* Don't bother checking a very old transaction.
528528
*/
529-
if (TransactionIdPrecedes(xid, RecentGlobalXmin))
529+
if (TransactionIdPrecedes(xid, RecentXmin))
530530
{
531531
xc_by_recent_xmin_inc();
532532
return false;
@@ -912,6 +912,7 @@ CountActiveBackends(void)
912912
return count;
913913
}
914914

915+
#ifdef NOT_USED
915916
/*
916917
* GetUndoRecPtr -- returns oldest PGPROC->logRec.
917918
*/
@@ -947,6 +948,7 @@ GetUndoRecPtr(void)
947948

948949
return (urec);
949950
}
951+
#endif /* NOT_USED */
950952

951953
/*
952954
* BackendIdGetProc - given a BackendId, find its PGPROC structure

‎src/backend/storage/lmgr/lmgr.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.66 2004/07/28 14:23:29 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/storage/lmgr/lmgr.c,v 1.67 2004/08/22 02:41:57 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -345,7 +345,7 @@ XactLockTableWait(TransactionId xid)
345345
LOCKTAG tag;
346346
TransactionId myxid = GetTopTransactionId();
347347

348-
Assert(!SubTransXidsHaveCommonAncestor(xid, myxid));
348+
Assert(!TransactionIdEquals(xid, myxid));
349349

350350
MemSet(&tag, 0, sizeof(tag));
351351
tag.relId = XactLockTableId;

‎src/backend/utils/time/tqual.c

Lines changed: 2 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -16,7 +16,7 @@
1616
* Portions Copyright (c) 1994, Regents of the University of California
1717
*
1818
* IDENTIFICATION
19-
* $PostgreSQL: pgsql/src/backend/utils/time/tqual.c,v 1.74 2004/07/28 14:23:30 tgl Exp $
19+
* $PostgreSQL: pgsql/src/backend/utils/time/tqual.c,v 1.75 2004/08/22 02:41:57 tgl Exp $
2020
*
2121
*-------------------------------------------------------------------------
2222
*/
@@ -933,8 +933,6 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId OldestXmin)
933933
{
934934
if (tuple->t_infomask & HEAP_XMAX_INVALID) /* xid invalid */
935935
return HEAPTUPLE_INSERT_IN_PROGRESS;
936-
Assert(SubTransXidsHaveCommonAncestor(HeapTupleHeaderGetXmin(tuple),
937-
HeapTupleHeaderGetXmax(tuple)));
938936
if (tuple->t_infomask & HEAP_MARKED_FOR_UPDATE)
939937
return HEAPTUPLE_INSERT_IN_PROGRESS;
940938
/* inserted and then deleted by same xact */
@@ -1008,7 +1006,7 @@ HeapTupleSatisfiesVacuum(HeapTupleHeader tuple, TransactionId OldestXmin)
10081006
* Deleter committed, but check special cases.
10091007
*/
10101008

1011-
if (SubTransXidsHaveCommonAncestor(HeapTupleHeaderGetXmin(tuple),
1009+
if (TransactionIdEquals(HeapTupleHeaderGetXmin(tuple),
10121010
HeapTupleHeaderGetXmax(tuple)))
10131011
{
10141012
/*

‎src/include/access/subtrans.h

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -6,7 +6,7 @@
66
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
77
* Portions Copyright (c) 1994, Regents of the University of California
88
*
9-
* $PostgreSQL: pgsql/src/include/access/subtrans.h,v 1.1 2004/07/01 00:51:38 tgl Exp $
9+
* $PostgreSQL: pgsql/src/include/access/subtrans.h,v 1.2 2004/08/22 02:41:58 tgl Exp $
1010
*/
1111
#ifndef SUBTRANS_H
1212
#define SUBTRANS_H
@@ -20,7 +20,6 @@
2020
extern void SubTransSetParent(TransactionId xid, TransactionId parent);
2121
extern TransactionId SubTransGetParent(TransactionId xid);
2222
extern TransactionId SubTransGetTopmostTransaction(TransactionId xid);
23-
extern bool SubTransXidsHaveCommonAncestor(TransactionId xid1, TransactionId xid2);
2423

2524
extern int SUBTRANSShmemSize(void);
2625
extern void SUBTRANSShmemInit(void);

‎src/include/access/xlog.h

Lines changed: 1 addition & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -6,7 +6,7 @@
66
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
77
* Portions Copyright (c) 1994, Regents of the University of California
88
*
9-
* $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.54 2004/07/21 22:31:25 tgl Exp $
9+
* $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.55 2004/08/22 02:41:58 tgl Exp $
1010
*/
1111
#ifndef XLOG_H
1212
#define XLOG_H
@@ -135,9 +135,4 @@ extern void CreateCheckPoint(bool shutdown, bool force);
135135
extern void XLogPutNextOid(Oid nextOid);
136136
extern XLogRecPtr GetRedoRecPtr(void);
137137

138-
/* in storage/ipc/sinval.c, but don't want to declare in sinval.h because
139-
* we'd have to include xlog.h into that ...
140-
*/
141-
extern XLogRecPtr GetUndoRecPtr(void);
142-
143138
#endif /* XLOG_H */

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