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Commit6c2003f

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Don't force-assign transaction id when exporting a snapshot.
Previously we required every exported transaction to have an xidassigned. That was used to check that the exporting transaction isstill running, which in turn is needed to guarantee that thatnecessary rows haven't been removed in between exporting and importingthe snapshot.The exported xid caused unnecessary problems with logical decoding,because slot creation has to wait for all concurrent xid to finish,which in turn serializes concurrent slot creation. It alsoprohibited snapshots to be exported on hot-standby replicas.Instead export the virtual transactionid, which avoids the unnecessaryserialization and the inability to export snapshots on standbys. Thischanges the file name of the exported snapshot, but since we neverdocumented what that one means, that seems ok.Author: Petr Jelinek, slightly editorialized by meReviewed-By: Andres FreundDiscussion:https://postgr.es/m/f598b4b8-8cd7-0d54-0939-adda763d8c34@2ndquadrant.com
1 parentb6966d4 commit6c2003f

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7 files changed

+115
-67
lines changed

7 files changed

+115
-67
lines changed

‎doc/src/sgml/ref/set_transaction.sgml

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -222,8 +222,8 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
222222
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
223223
SELECT pg_export_snapshot();
224224
pg_export_snapshot
225-
--------------------
226-
000003A1-1
225+
---------------------
226+
00000003-0000001B-1
227227
(1 row)
228228
</programlisting>
229229

@@ -233,7 +233,7 @@ SELECT pg_export_snapshot();
233233

234234
<programlisting>
235235
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
236-
SET TRANSACTION SNAPSHOT '000003A1-1';
236+
SET TRANSACTION SNAPSHOT '00000003-0000001B-1';
237237
</programlisting></para>
238238
</refsect1>
239239

‎src/backend/replication/logical/snapbuild.c

Lines changed: 7 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -262,7 +262,7 @@ static bool ExportInProgress = false;
262262
staticvoidSnapBuildPurgeCommittedTxn(SnapBuild*builder);
263263

264264
/* snapshot building/manipulation/distribution functions */
265-
staticSnapshotSnapBuildBuildSnapshot(SnapBuild*builder,TransactionIdxid);
265+
staticSnapshotSnapBuildBuildSnapshot(SnapBuild*builder);
266266

267267
staticvoidSnapBuildFreeSnapshot(Snapshotsnap);
268268

@@ -463,7 +463,7 @@ SnapBuildSnapDecRefcount(Snapshot snap)
463463
* and ->subxip/subxcnt values.
464464
*/
465465
staticSnapshot
466-
SnapBuildBuildSnapshot(SnapBuild*builder,TransactionIdxid)
466+
SnapBuildBuildSnapshot(SnapBuild*builder)
467467
{
468468
Snapshotsnapshot;
469469
Sizessize;
@@ -562,7 +562,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
562562
if (TransactionIdIsValid(MyPgXact->xmin))
563563
elog(ERROR,"cannot build an initial slot snapshot when MyPgXact->xmin already is valid");
564564

565-
snap=SnapBuildBuildSnapshot(builder,GetTopTransactionId());
565+
snap=SnapBuildBuildSnapshot(builder);
566566

567567
/*
568568
* We know that snap->xmin is alive, enforced by the logical xmin
@@ -679,7 +679,7 @@ SnapBuildGetOrBuildSnapshot(SnapBuild *builder, TransactionId xid)
679679
/* only build a new snapshot if we don't have a prebuilt one */
680680
if (builder->snapshot==NULL)
681681
{
682-
builder->snapshot=SnapBuildBuildSnapshot(builder,xid);
682+
builder->snapshot=SnapBuildBuildSnapshot(builder);
683683
/* increase refcount for the snapshot builder */
684684
SnapBuildSnapIncRefcount(builder->snapshot);
685685
}
@@ -743,7 +743,7 @@ SnapBuildProcessChange(SnapBuild *builder, TransactionId xid, XLogRecPtr lsn)
743743
/* only build a new snapshot if we don't have a prebuilt one */
744744
if (builder->snapshot==NULL)
745745
{
746-
builder->snapshot=SnapBuildBuildSnapshot(builder,xid);
746+
builder->snapshot=SnapBuildBuildSnapshot(builder);
747747
/* increase refcount for the snapshot builder */
748748
SnapBuildSnapIncRefcount(builder->snapshot);
749749
}
@@ -1061,7 +1061,7 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
10611061
if (builder->snapshot)
10621062
SnapBuildSnapDecRefcount(builder->snapshot);
10631063

1064-
builder->snapshot=SnapBuildBuildSnapshot(builder,xid);
1064+
builder->snapshot=SnapBuildBuildSnapshot(builder);
10651065

10661066
/* we might need to execute invalidations, add snapshot */
10671067
if (!ReorderBufferXidHasBaseSnapshot(builder->reorder,xid))
@@ -1831,7 +1831,7 @@ SnapBuildRestore(SnapBuild *builder, XLogRecPtr lsn)
18311831
{
18321832
SnapBuildSnapDecRefcount(builder->snapshot);
18331833
}
1834-
builder->snapshot=SnapBuildBuildSnapshot(builder,InvalidTransactionId);
1834+
builder->snapshot=SnapBuildBuildSnapshot(builder);
18351835
SnapBuildSnapIncRefcount(builder->snapshot);
18361836

18371837
ReorderBufferSetRestartPoint(builder->reorder,lsn);

‎src/backend/storage/ipc/procarray.c

Lines changed: 7 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1793,14 +1793,15 @@ GetSnapshotData(Snapshot snapshot)
17931793
* Returns TRUE if successful, FALSE if source xact is no longer running.
17941794
*/
17951795
bool
1796-
ProcArrayInstallImportedXmin(TransactionIdxmin,TransactionIdsourcexid)
1796+
ProcArrayInstallImportedXmin(TransactionIdxmin,
1797+
VirtualTransactionId*sourcevxid)
17971798
{
17981799
boolresult= false;
17991800
ProcArrayStruct*arrayP=procArray;
18001801
intindex;
18011802

18021803
Assert(TransactionIdIsNormal(xmin));
1803-
if (!TransactionIdIsNormal(sourcexid))
1804+
if (!sourcevxid)
18041805
return false;
18051806

18061807
/* Get lock so source xact can't end while we're doing this */
@@ -1817,8 +1818,10 @@ ProcArrayInstallImportedXmin(TransactionId xmin, TransactionId sourcexid)
18171818
if (pgxact->vacuumFlags&PROC_IN_VACUUM)
18181819
continue;
18191820

1820-
xid=pgxact->xid;/* fetch just once */
1821-
if (xid!=sourcexid)
1821+
/* We are only interested in the specific virtual transaction. */
1822+
if (proc->backendId!=sourcevxid->backendId)
1823+
continue;
1824+
if (proc->lxid!=sourcevxid->localTransactionId)
18221825
continue;
18231826

18241827
/*

‎src/backend/storage/lmgr/predicate.c

Lines changed: 15 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -148,7 +148,7 @@
148148
* predicate lock maintenance
149149
*GetSerializableTransactionSnapshot(Snapshot snapshot)
150150
*SetSerializableTransactionSnapshot(Snapshot snapshot,
151-
*TransactionId sourcexid)
151+
*VirtualTransactionId *sourcevxid)
152152
*RegisterPredicateLockingXid(void)
153153
*PredicateLockRelation(Relation relation, Snapshot snapshot)
154154
*PredicateLockPage(Relation relation, BlockNumber blkno,
@@ -434,7 +434,8 @@ static uint32 predicatelock_hash(const void *key, Size keysize);
434434
staticvoidSummarizeOldestCommittedSxact(void);
435435
staticSnapshotGetSafeSnapshot(Snapshotsnapshot);
436436
staticSnapshotGetSerializableTransactionSnapshotInt(Snapshotsnapshot,
437-
TransactionIdsourcexid);
437+
VirtualTransactionId*sourcevxid,
438+
intsourcepid);
438439
staticboolPredicateLockExists(constPREDICATELOCKTARGETTAG*targettag);
439440
staticboolGetParentPredicateLockTag(constPREDICATELOCKTARGETTAG*tag,
440441
PREDICATELOCKTARGETTAG*parent);
@@ -1510,7 +1511,7 @@ GetSafeSnapshot(Snapshot origSnapshot)
15101511
* one passed to it, but we avoid assuming that here.
15111512
*/
15121513
snapshot=GetSerializableTransactionSnapshotInt(origSnapshot,
1513-
InvalidTransactionId);
1514+
NULL,InvalidPid);
15141515

15151516
if (MySerializableXact==InvalidSerializableXact)
15161517
returnsnapshot;/* no concurrent r/w xacts; it's safe */
@@ -1643,7 +1644,7 @@ GetSerializableTransactionSnapshot(Snapshot snapshot)
16431644
returnGetSafeSnapshot(snapshot);
16441645

16451646
returnGetSerializableTransactionSnapshotInt(snapshot,
1646-
InvalidTransactionId);
1647+
NULL,InvalidPid);
16471648
}
16481649

16491650
/*
@@ -1658,7 +1659,8 @@ GetSerializableTransactionSnapshot(Snapshot snapshot)
16581659
*/
16591660
void
16601661
SetSerializableTransactionSnapshot(Snapshotsnapshot,
1661-
TransactionIdsourcexid)
1662+
VirtualTransactionId*sourcevxid,
1663+
intsourcepid)
16621664
{
16631665
Assert(IsolationIsSerializable());
16641666

@@ -1673,7 +1675,8 @@ SetSerializableTransactionSnapshot(Snapshot snapshot,
16731675
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
16741676
errmsg("a snapshot-importing transaction must not be READ ONLY DEFERRABLE")));
16751677

1676-
(void)GetSerializableTransactionSnapshotInt(snapshot,sourcexid);
1678+
(void)GetSerializableTransactionSnapshotInt(snapshot,sourcevxid,
1679+
sourcepid);
16771680
}
16781681

16791682
/*
@@ -1687,7 +1690,8 @@ SetSerializableTransactionSnapshot(Snapshot snapshot,
16871690
*/
16881691
staticSnapshot
16891692
GetSerializableTransactionSnapshotInt(Snapshotsnapshot,
1690-
TransactionIdsourcexid)
1693+
VirtualTransactionId*sourcevxid,
1694+
intsourcepid)
16911695
{
16921696
PGPROC*proc;
16931697
VirtualTransactionIdvxid;
@@ -1741,17 +1745,17 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
17411745
}while (!sxact);
17421746

17431747
/* Get the snapshot, or check that it's safe to use */
1744-
if (!TransactionIdIsValid(sourcexid))
1748+
if (!sourcevxid)
17451749
snapshot=GetSnapshotData(snapshot);
1746-
elseif (!ProcArrayInstallImportedXmin(snapshot->xmin,sourcexid))
1750+
elseif (!ProcArrayInstallImportedXmin(snapshot->xmin,sourcevxid))
17471751
{
17481752
ReleasePredXact(sxact);
17491753
LWLockRelease(SerializableXactHashLock);
17501754
ereport(ERROR,
17511755
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
17521756
errmsg("could not import the requested snapshot"),
1753-
errdetail("The sourcetransaction %u is not running anymore.",
1754-
sourcexid)));
1757+
errdetail("The sourceprocess with pid %d is not running anymore.",
1758+
sourcepid)));
17551759
}
17561760

17571761
/*

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