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Improve snapshot manager by keeping explicit track of snapshots.
There are two ways to track a snapshot: there's the "registered" list, which is used for arbitrary long-lived snapshots; and there's the "active stack", which is used for the snapshot that is considered "active" at any time. This also allows users of snapshots to stop worrying about snapshot memory allocation and freeing, and about using PG_TRY blocks around ActiveSnapshot assignment. This is all done automatically now. As a consequence, this allows us to reset MyProc->xmin when there are no more snapshots registered in the current backend, reducing the impact that long-running transactions have on VACUUM.
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27 files changed

+981
-537
lines changed

27 files changed

+981
-537
lines changed

‎src/backend/access/transam/xact.c

Lines changed: 10 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -10,7 +10,7 @@
1010
*
1111
*
1212
* IDENTIFICATION
13-
* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.263 2008/05/12 00:00:46 alvherre Exp $
13+
* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.264 2008/05/12 20:01:58 alvherre Exp $
1414
*
1515
*-------------------------------------------------------------------------
1616
*/
@@ -622,12 +622,9 @@ CommandCounterIncrement(void)
622622
}
623623
currentCommandIdUsed = false;
624624

625-
/* Propagate new command ID into static snapshots, if set */
626-
if (SerializableSnapshot)
627-
SerializableSnapshot->curcid = currentCommandId;
628-
if (LatestSnapshot)
629-
LatestSnapshot->curcid = currentCommandId;
630-
625+
/* Propagate new command ID into static snapshots */
626+
SnapshotSetCommandId(currentCommandId);
627+
631628
/*
632629
* Make any catalog changes done by the just-completed command
633630
* visible in the local syscache. We obviously don't need to do
@@ -1509,9 +1506,8 @@ StartTransaction(void)
15091506
s->transactionId = InvalidTransactionId; /* until assigned */
15101507

15111508
/*
1512-
* Make sure we've freed any old snapshot, and reset xact state variables
1509+
* Make sure we've reset xact state variables
15131510
*/
1514-
FreeXactSnapshot();
15151511
XactIsoLevel = DefaultXactIsoLevel;
15161512
XactReadOnly = DefaultXactReadOnly;
15171513
forceSyncCommit = false;
@@ -1753,6 +1749,7 @@ CommitTransaction(void)
17531749
AtEOXact_ComboCid();
17541750
AtEOXact_HashTables(true);
17551751
AtEOXact_PgStat(true);
1752+
AtEOXact_Snapshot(true);
17561753
pgstat_report_xact_timestamp(0);
17571754

17581755
CurrentResourceOwner = NULL;
@@ -1985,6 +1982,7 @@ PrepareTransaction(void)
19851982
AtEOXact_ComboCid();
19861983
AtEOXact_HashTables(true);
19871984
/* don't call AtEOXact_PgStat here */
1985+
AtEOXact_Snapshot(true);
19881986

19891987
CurrentResourceOwner = NULL;
19901988
ResourceOwnerDelete(TopTransactionResourceOwner);
@@ -2129,6 +2127,7 @@ AbortTransaction(void)
21292127
AtEOXact_ComboCid();
21302128
AtEOXact_HashTables(false);
21312129
AtEOXact_PgStat(false);
2130+
AtEOXact_Snapshot(false);
21322131
pgstat_report_xact_timestamp(0);
21332132

21342133
/*
@@ -3844,6 +3843,7 @@ CommitSubTransaction(void)
38443843
s->parent->subTransactionId);
38453844
AtEOSubXact_HashTables(true, s->nestingLevel);
38463845
AtEOSubXact_PgStat(true, s->nestingLevel);
3846+
AtSubCommit_Snapshot(s->nestingLevel);
38473847

38483848
/*
38493849
* We need to restore the upper transaction's read-only state, in case the
@@ -3963,6 +3963,7 @@ AbortSubTransaction(void)
39633963
s->parent->subTransactionId);
39643964
AtEOSubXact_HashTables(false, s->nestingLevel);
39653965
AtEOSubXact_PgStat(false, s->nestingLevel);
3966+
AtSubAbort_Snapshot(s->nestingLevel);
39663967
}
39673968

39683969
/*

‎src/backend/catalog/index.c

Lines changed: 7 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/catalog/index.c,v 1.298 2008/05/12 00:00:47 alvherre Exp $
11+
* $PostgreSQL: pgsql/src/backend/catalog/index.c,v 1.299 2008/05/12 20:01:59 alvherre Exp $
1212
*
1313
*
1414
* INTERFACE ROUTINES
@@ -1507,7 +1507,7 @@ IndexBuildHeapScan(Relation heapRelation,
15071507
}
15081508
else if (indexInfo->ii_Concurrent)
15091509
{
1510-
snapshot = CopySnapshot(GetTransactionSnapshot());
1510+
snapshot = RegisterSnapshot(GetTransactionSnapshot());
15111511
OldestXmin = InvalidTransactionId; /* not used */
15121512
}
15131513
else
@@ -1517,10 +1517,7 @@ IndexBuildHeapScan(Relation heapRelation,
15171517
OldestXmin = GetOldestXmin(heapRelation->rd_rel->relisshared, true);
15181518
}
15191519

1520-
scan = heap_beginscan(heapRelation, /* relation */
1521-
snapshot, /* seeself */
1522-
0, /* number of keys */
1523-
NULL); /* scan key */
1520+
scan = heap_beginscan(heapRelation, snapshot, 0, NULL);
15241521

15251522
reltuples = 0;
15261523

@@ -1793,6 +1790,10 @@ IndexBuildHeapScan(Relation heapRelation,
17931790

17941791
heap_endscan(scan);
17951792

1793+
/* we can now forget our snapshot, if set */
1794+
if (indexInfo->ii_Concurrent)
1795+
UnregisterSnapshot(snapshot);
1796+
17961797
ExecDropSingleTupleTableSlot(slot);
17971798

17981799
FreeExecutorState(estate);

‎src/backend/commands/cluster.c

Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -11,7 +11,7 @@
1111
*
1212
*
1313
* IDENTIFICATION
14-
* $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.175 2008/05/12 00:00:47 alvherre Exp $
14+
* $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.176 2008/05/12 20:01:59 alvherre Exp $
1515
*
1616
*-------------------------------------------------------------------------
1717
*/
@@ -212,6 +212,7 @@ cluster(ClusterStmt *stmt, bool isTopLevel)
212212
rvs = get_tables_to_cluster(cluster_context);
213213

214214
/* Commit to get out of starting transaction */
215+
PopActiveSnapshot();
215216
CommitTransactionCommand();
216217

217218
/* Ok, now that we've got them all, cluster them one by one */
@@ -222,8 +223,9 @@ cluster(ClusterStmt *stmt, bool isTopLevel)
222223
/* Start a new transaction for each relation. */
223224
StartTransactionCommand();
224225
/* functions in indexes may want a snapshot set */
225-
ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
226+
PushActiveSnapshot(GetTransactionSnapshot());
226227
cluster_rel(rvtc, true);
228+
PopActiveSnapshot();
227229
CommitTransactionCommand();
228230
}
229231

‎src/backend/commands/copy.c

Lines changed: 8 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/commands/copy.c,v 1.298 2008/03/26 18:48:59 alvherre Exp $
11+
* $PostgreSQL: pgsql/src/backend/commands/copy.c,v 1.299 2008/05/12 20:01:59 alvherre Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -1044,21 +1044,18 @@ DoCopy(const CopyStmt *stmt, const char *queryString)
10441044
plan = planner(query, 0, NULL);
10451045

10461046
/*
1047-
* Update snapshot command ID to ensure this query sees results of any
1048-
* previously executed queries. (It's a bit cheesy to modify
1049-
* ActiveSnapshot without making a copy, but for the limited ways in
1050-
* which COPY can be invoked, I think it's OK, because the active
1051-
* snapshot shouldn't be shared with anything else anyway.)
1047+
* Use a snapshot with an updated command ID to ensure this query sees
1048+
* results of any previously executed queries.
10521049
*/
1053-
ActiveSnapshot->curcid = GetCurrentCommandId(false);
1050+
PushUpdatedSnapshot(GetActiveSnapshot());
10541051

10551052
/* Create dest receiver for COPY OUT */
10561053
dest = CreateDestReceiver(DestCopyOut, NULL);
10571054
((DR_copy *) dest)->cstate = cstate;
10581055

10591056
/* Create a QueryDesc requesting no output */
1060-
cstate->queryDesc = CreateQueryDesc(plan,
1061-
ActiveSnapshot, InvalidSnapshot,
1057+
cstate->queryDesc = CreateQueryDesc(plan, GetActiveSnapshot(),
1058+
InvalidSnapshot,
10621059
dest, NULL, false);
10631060

10641061
/*
@@ -1161,6 +1158,7 @@ DoCopy(const CopyStmt *stmt, const char *queryString)
11611158
/* Close down the query and free resources. */
11621159
ExecutorEnd(cstate->queryDesc);
11631160
FreeQueryDesc(cstate->queryDesc);
1161+
PopActiveSnapshot();
11641162
}
11651163

11661164
/* Clean up storage (probably not really necessary) */
@@ -1390,7 +1388,7 @@ CopyTo(CopyState cstate)
13901388
values = (Datum *) palloc(num_phys_attrs * sizeof(Datum));
13911389
nulls = (bool *) palloc(num_phys_attrs * sizeof(bool));
13921390

1393-
scandesc = heap_beginscan(cstate->rel, ActiveSnapshot, 0, NULL);
1391+
scandesc = heap_beginscan(cstate->rel, GetActiveSnapshot(), 0, NULL);
13941392

13951393
while ((tuple = heap_getnext(scandesc, ForwardScanDirection)) != NULL)
13961394
{

‎src/backend/commands/explain.c

Lines changed: 7 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
* Portions Copyright (c) 1994-5, Regents of the University of California
88
*
99
* IDENTIFICATION
10-
* $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.173 2008/04/18 01:42:17 tgl Exp $
10+
* $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.174 2008/05/12 20:01:59 alvherre Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -229,17 +229,14 @@ ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
229229
int eflags;
230230

231231
/*
232-
* Update snapshot command ID to ensure this query sees results of any
233-
* previously executed queries. (It's a bit cheesy to modify
234-
* ActiveSnapshot without making a copy, but for the limited ways in which
235-
* EXPLAIN can be invoked, I think it's OK, because the active snapshot
236-
* shouldn't be shared with anything else anyway.)
232+
* Use a snapshot with an updated command ID to ensure this query sees
233+
* results of any previously executed queries.
237234
*/
238-
ActiveSnapshot->curcid = GetCurrentCommandId(false);
235+
PushUpdatedSnapshot(GetActiveSnapshot());
239236

240237
/* Create a QueryDesc requesting no output */
241238
queryDesc = CreateQueryDesc(plannedstmt,
242-
ActiveSnapshot, InvalidSnapshot,
239+
GetActiveSnapshot(), InvalidSnapshot,
243240
None_Receiver, params,
244241
stmt->analyze);
245242

@@ -324,6 +321,8 @@ ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
324321

325322
FreeQueryDesc(queryDesc);
326323

324+
PopActiveSnapshot();
325+
327326
/* We need a CCI just in case query expanded to multiple plans */
328327
if (stmt->analyze)
329328
CommandCounterIncrement();

‎src/backend/commands/indexcmds.c

Lines changed: 18 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/commands/indexcmds.c,v 1.175 2008/05/12 00:00:47 alvherre Exp $
11+
* $PostgreSQL: pgsql/src/backend/commands/indexcmds.c,v 1.176 2008/05/12 20:01:59 alvherre Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -484,6 +484,7 @@ DefineIndex(RangeVar *heapRelation,
484484
*/
485485
LockRelationIdForSession(&heaprelid, ShareUpdateExclusiveLock);
486486

487+
PopActiveSnapshot();
487488
CommitTransactionCommand();
488489
StartTransactionCommand();
489490

@@ -542,7 +543,7 @@ DefineIndex(RangeVar *heapRelation,
542543
indexRelation = index_open(indexRelationId, RowExclusiveLock);
543544

544545
/* Set ActiveSnapshot since functions in the indexes may need it */
545-
ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
546+
PushActiveSnapshot(GetTransactionSnapshot());
546547

547548
/* We have to re-build the IndexInfo struct, since it was lost in commit */
548549
indexInfo = BuildIndexInfo(indexRelation);
@@ -581,6 +582,9 @@ DefineIndex(RangeVar *heapRelation,
581582

582583
heap_close(pg_index, RowExclusiveLock);
583584

585+
/* we can do away with our snapshot */
586+
PopActiveSnapshot();
587+
584588
/*
585589
* Commit this transaction to make the indisready update visible.
586590
*/
@@ -616,8 +620,8 @@ DefineIndex(RangeVar *heapRelation,
616620
* We also set ActiveSnapshot to this snap, since functions in indexes may
617621
* need a snapshot.
618622
*/
619-
snapshot = CopySnapshot(GetTransactionSnapshot());
620-
ActiveSnapshot = snapshot;
623+
snapshot = RegisterSnapshot(GetTransactionSnapshot());
624+
PushActiveSnapshot(snapshot);
621625

622626
/*
623627
* Scan the index and the heap, insert any missing index entries.
@@ -645,7 +649,7 @@ DefineIndex(RangeVar *heapRelation,
645649
* Also, GetCurrentVirtualXIDs never reports our own vxid, so we need not
646650
* check for that.
647651
*/
648-
old_snapshots = GetCurrentVirtualXIDs(ActiveSnapshot->xmax, false,
652+
old_snapshots = GetCurrentVirtualXIDs(snapshot->xmax, false,
649653
PROC_IS_AUTOVACUUM | PROC_IN_VACUUM);
650654

651655
while (VirtualTransactionIdIsValid(*old_snapshots))
@@ -686,6 +690,12 @@ DefineIndex(RangeVar *heapRelation,
686690
*/
687691
CacheInvalidateRelcacheByRelid(heaprelid.relId);
688692

693+
/* we can now do away with our active snapshot */
694+
PopActiveSnapshot();
695+
696+
/* And we can remove the validating snapshot too */
697+
UnregisterSnapshot(snapshot);
698+
689699
/*
690700
* Last thing to do is release the session-level lock on the parent table.
691701
*/
@@ -1453,18 +1463,20 @@ ReindexDatabase(const char *databaseName, bool do_system, bool do_user)
14531463
heap_close(relationRelation, AccessShareLock);
14541464

14551465
/* Now reindex each rel in a separate transaction */
1466+
PopActiveSnapshot();
14561467
CommitTransactionCommand();
14571468
foreach(l, relids)
14581469
{
14591470
Oid relid = lfirst_oid(l);
14601471

14611472
StartTransactionCommand();
14621473
/* functions in indexes may want a snapshot set */
1463-
ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
1474+
PushActiveSnapshot(GetTransactionSnapshot());
14641475
if (reindex_relation(relid, true))
14651476
ereport(NOTICE,
14661477
(errmsg("table \"%s\" was reindexed",
14671478
get_rel_name(relid))));
1479+
PopActiveSnapshot();
14681480
CommitTransactionCommand();
14691481
}
14701482
StartTransactionCommand();

‎src/backend/commands/portalcmds.c

Lines changed: 6 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -14,7 +14,7 @@
1414
*
1515
*
1616
* IDENTIFICATION
17-
* $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.73 2008/04/02 18:31:50 tgl Exp $
17+
* $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.74 2008/05/12 20:01:59 alvherre Exp $
1818
*
1919
*-------------------------------------------------------------------------
2020
*/
@@ -121,7 +121,7 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
121121
/*
122122
* Start execution, inserting parameters if any.
123123
*/
124-
PortalStart(portal, params, ActiveSnapshot);
124+
PortalStart(portal, params, GetActiveSnapshot());
125125

126126
Assert(portal->strategy == PORTAL_ONE_SELECT);
127127

@@ -293,7 +293,6 @@ PersistHoldablePortal(Portal portal)
293293
{
294294
QueryDesc *queryDesc = PortalGetQueryDesc(portal);
295295
Portal saveActivePortal;
296-
Snapshot saveActiveSnapshot;
297296
ResourceOwner saveResourceOwner;
298297
MemoryContext savePortalContext;
299298
MemoryContext oldcxt;
@@ -334,18 +333,18 @@ PersistHoldablePortal(Portal portal)
334333
* Set up global portal context pointers.
335334
*/
336335
saveActivePortal = ActivePortal;
337-
saveActiveSnapshot = ActiveSnapshot;
338336
saveResourceOwner = CurrentResourceOwner;
339337
savePortalContext = PortalContext;
340338
PG_TRY();
341339
{
342340
ActivePortal = portal;
343-
ActiveSnapshot = queryDesc->snapshot;
344341
CurrentResourceOwner = portal->resowner;
345342
PortalContext = PortalGetHeapMemory(portal);
346343

347344
MemoryContextSwitchTo(PortalContext);
348345

346+
PushActiveSnapshot(queryDesc->snapshot);
347+
349348
/*
350349
* Rewind the executor: we need to store the entire result set in the
351350
* tuplestore, so that subsequent backward FETCHs can be processed.
@@ -411,7 +410,6 @@ PersistHoldablePortal(Portal portal)
411410

412411
/* Restore global vars and propagate error */
413412
ActivePortal = saveActivePortal;
414-
ActiveSnapshot = saveActiveSnapshot;
415413
CurrentResourceOwner = saveResourceOwner;
416414
PortalContext = savePortalContext;
417415

@@ -425,10 +423,11 @@ PersistHoldablePortal(Portal portal)
425423
portal->status = PORTAL_READY;
426424

427425
ActivePortal = saveActivePortal;
428-
ActiveSnapshot = saveActiveSnapshot;
429426
CurrentResourceOwner = saveResourceOwner;
430427
PortalContext = savePortalContext;
431428

429+
PopActiveSnapshot();
430+
432431
/*
433432
* We can now release any subsidiary memory of the portal's heap context;
434433
* we'll never use it again. The executor already dropped its context,

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