snapshot scalability: Move PGXACT->xmin back to PGPROC.
Now that xmin isn't needed for GetSnapshotData() anymore, it leads to unnecessary cacheline ping-pong to have it in PGXACT, as it is updated considerably more frequently than the other PGXACT members. After the changes in dc7420c2c92, this is a very straight-forward change. For highly concurrent, snapshot acquisition heavy, workloads this change alone can significantly increase scalability. E.g. plain pgbench on a smaller 2 socket machine gains 1.07x for read-only pgbench, 1.22x for read-only pgbench when submitting queries in batches of 100, and 2.85x for batches of 100 'SELECT';. The latter numbers are obviously not to be expected in the real-world, but micro-benchmark the snapshot computation scalability (previously spending ~80% of the time in GetSnapshotData()). Author: Andres Freund <andres@anarazel.de> Reviewed-By: Robert Haas <robertmhaas@gmail.com> Reviewed-By: Thomas Munro <thomas.munro@gmail.com> Reviewed-By: David Rowley <dgrowleyml@gmail.com> Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de
This commit is contained in:
Andres Freund
parent
a811ea5bde
commit
1f51c17c68
@ -389,7 +389,7 @@ gistRedoPageReuse(XLogReaderState *record)
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*
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* latestRemovedXid was the page's deleteXid. The
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* GlobalVisIsRemovableFullXid(deleteXid) test in gistPageRecyclable()
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* conceptually mirrors the pgxact->xmin > limitXmin test in
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* conceptually mirrors the PGPROC->xmin > limitXmin test in
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* GetConflictingVirtualXIDs(). Consequently, one XID value achieves the
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* same exclusion effect on primary and standby.
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*/
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@ -2317,7 +2317,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
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* we're in VACUUM and would not otherwise have an XID. Having already
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* updated links to the target, ReadNewTransactionId() suffices as an
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* upper bound. Any scan having retained a now-stale link is advertising
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* in its PGXACT an xmin less than or equal to the value we read here. It
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* in its PGPROC an xmin less than or equal to the value we read here. It
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* will continue to do so, holding back the xmin horizon, for the duration
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* of that scan.
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*/
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@ -296,7 +296,7 @@ ensure that the C compiler does exactly what you tell it to.)
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Another important activity that uses the shared ProcArray is
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ComputeXidHorizons, which must determine a lower bound for the oldest xmin
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of any active MVCC snapshot, system-wide. Each individual backend
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advertises the smallest xmin of its own snapshots in MyPgXact->xmin, or zero
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advertises the smallest xmin of its own snapshots in MyProc->xmin, or zero
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if it currently has no live snapshots (eg, if it's between transactions or
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hasn't yet set a snapshot for a new transaction). ComputeXidHorizons takes
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the MIN() of the valid xmin fields. It does this with only shared lock on
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@ -331,7 +331,7 @@ necessary.
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Note that while it is certain that two concurrent executions of
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GetSnapshotData will compute the same xmin for their own snapshots, there is
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no such guarantee for the horizons computed by ComputeXidHorizons. This is
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because we allow XID-less transactions to clear their MyPgXact->xmin
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because we allow XID-less transactions to clear their MyProc->xmin
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asynchronously (without taking ProcArrayLock), so one execution might see
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what had been the oldest xmin, and another not. This is OK since the
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thresholds need only be a valid lower bound. As noted above, we are already
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@ -464,7 +464,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
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/* We set up the gxact's VXID as InvalidBackendId/XID */
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proc->lxid = (LocalTransactionId) xid;
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pgxact->xid = xid;
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pgxact->xmin = InvalidTransactionId;
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Assert(proc->xmin == InvalidTransactionId);
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proc->delayChkpt = false;
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pgxact->vacuumFlags = 0;
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proc->pid = 0;
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@ -1535,7 +1535,7 @@ DefineIndex(Oid relationId,
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StartTransactionCommand();
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/* We should now definitely not be advertising any xmin. */
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Assert(MyPgXact->xmin == InvalidTransactionId);
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Assert(MyProc->xmin == InvalidTransactionId);
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/*
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* The index is now valid in the sense that it contains all currently
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@ -553,8 +553,8 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
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elog(ERROR, "cannot build an initial slot snapshot, not all transactions are monitored anymore");
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/* so we don't overwrite the existing value */
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if (TransactionIdIsValid(MyPgXact->xmin))
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elog(ERROR, "cannot build an initial slot snapshot when MyPgXact->xmin already is valid");
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if (TransactionIdIsValid(MyProc->xmin))
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elog(ERROR, "cannot build an initial slot snapshot when MyProc->xmin already is valid");
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snap = SnapBuildBuildSnapshot(builder);
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@ -575,7 +575,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
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}
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#endif
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MyPgXact->xmin = snap->xmin;
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MyProc->xmin = snap->xmin;
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/* allocate in transaction context */
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newxip = (TransactionId *)
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@ -1964,7 +1964,7 @@ PhysicalReplicationSlotNewXmin(TransactionId feedbackXmin, TransactionId feedbac
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ReplicationSlot *slot = MyReplicationSlot;
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SpinLockAcquire(&slot->mutex);
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MyPgXact->xmin = InvalidTransactionId;
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MyProc->xmin = InvalidTransactionId;
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/*
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* For physical replication we don't need the interlock provided by xmin
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@ -2093,7 +2093,7 @@ ProcessStandbyHSFeedbackMessage(void)
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if (!TransactionIdIsNormal(feedbackXmin)
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&& !TransactionIdIsNormal(feedbackCatalogXmin))
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{
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MyPgXact->xmin = InvalidTransactionId;
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MyProc->xmin = InvalidTransactionId;
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if (MyReplicationSlot != NULL)
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PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin);
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return;
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@ -2135,7 +2135,7 @@ ProcessStandbyHSFeedbackMessage(void)
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* risk already since a VACUUM could already have determined the horizon.)
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*
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* If we're using a replication slot we reserve the xmin via that,
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* otherwise via the walsender's PGXACT entry. We can only track the
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* otherwise via the walsender's PGPROC entry. We can only track the
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* catalog xmin separately when using a slot, so we store the least of the
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* two provided when not using a slot.
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*
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@ -2148,9 +2148,9 @@ ProcessStandbyHSFeedbackMessage(void)
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{
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if (TransactionIdIsNormal(feedbackCatalogXmin)
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&& TransactionIdPrecedes(feedbackCatalogXmin, feedbackXmin))
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MyPgXact->xmin = feedbackCatalogXmin;
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MyProc->xmin = feedbackCatalogXmin;
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else
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MyPgXact->xmin = feedbackXmin;
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MyProc->xmin = feedbackXmin;
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}
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}
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@ -587,9 +587,9 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
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Assert(!TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
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proc->lxid = InvalidLocalTransactionId;
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pgxact->xmin = InvalidTransactionId;
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/* must be cleared with xid/xmin: */
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pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
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proc->xmin = InvalidTransactionId;
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proc->delayChkpt = false; /* be sure this is cleared in abort */
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proc->recoveryConflictPending = false;
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@ -609,9 +609,9 @@ ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
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{
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pgxact->xid = InvalidTransactionId;
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proc->lxid = InvalidLocalTransactionId;
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pgxact->xmin = InvalidTransactionId;
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/* must be cleared with xid/xmin: */
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pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
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proc->xmin = InvalidTransactionId;
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proc->delayChkpt = false; /* be sure this is cleared in abort */
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proc->recoveryConflictPending = false;
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@ -763,7 +763,7 @@ ProcArrayClearTransaction(PGPROC *proc)
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*/
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pgxact->xid = InvalidTransactionId;
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proc->lxid = InvalidLocalTransactionId;
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pgxact->xmin = InvalidTransactionId;
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proc->xmin = InvalidTransactionId;
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proc->recoveryConflictPending = false;
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/* redundant, but just in case */
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@ -1563,7 +1563,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
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/* Fetch xid just once - see GetNewTransactionId */
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xid = UINT32_ACCESS_ONCE(pgxact->xid);
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xmin = UINT32_ACCESS_ONCE(pgxact->xmin);
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xmin = UINT32_ACCESS_ONCE(proc->xmin);
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/*
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* Consider both the transaction's Xmin, and its Xid.
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@ -1838,7 +1838,7 @@ GetMaxSnapshotSubxidCount(void)
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*
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* We also update the following backend-global variables:
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* TransactionXmin: the oldest xmin of any snapshot in use in the
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* current transaction (this is the same as MyPgXact->xmin).
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* current transaction (this is the same as MyProc->xmin).
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* RecentXmin: the xmin computed for the most recent snapshot. XIDs
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* older than this are known not running any more.
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*
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@ -1899,7 +1899,7 @@ GetSnapshotData(Snapshot snapshot)
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/*
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* It is sufficient to get shared lock on ProcArrayLock, even if we are
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* going to set MyPgXact->xmin.
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* going to set MyProc->xmin.
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*/
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LWLockAcquire(ProcArrayLock, LW_SHARED);
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@ -2051,8 +2051,8 @@ GetSnapshotData(Snapshot snapshot)
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replication_slot_xmin = procArray->replication_slot_xmin;
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replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;
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if (!TransactionIdIsValid(MyPgXact->xmin))
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MyPgXact->xmin = TransactionXmin = xmin;
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if (!TransactionIdIsValid(MyProc->xmin))
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MyProc->xmin = TransactionXmin = xmin;
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LWLockRelease(ProcArrayLock);
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@ -2172,7 +2172,7 @@ GetSnapshotData(Snapshot snapshot)
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}
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/*
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* ProcArrayInstallImportedXmin -- install imported xmin into MyPgXact->xmin
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* ProcArrayInstallImportedXmin -- install imported xmin into MyProc->xmin
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*
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* This is called when installing a snapshot imported from another
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* transaction. To ensure that OldestXmin doesn't go backwards, we must
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@ -2225,7 +2225,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
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/*
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* Likewise, let's just make real sure its xmin does cover us.
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*/
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xid = UINT32_ACCESS_ONCE(pgxact->xmin);
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xid = UINT32_ACCESS_ONCE(proc->xmin);
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if (!TransactionIdIsNormal(xid) ||
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!TransactionIdPrecedesOrEquals(xid, xmin))
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continue;
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@ -2236,7 +2236,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
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* GetSnapshotData first, we'll be overwriting a valid xmin here, so
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* we don't check that.)
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*/
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MyPgXact->xmin = TransactionXmin = xmin;
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MyProc->xmin = TransactionXmin = xmin;
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result = true;
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break;
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@ -2248,7 +2248,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
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}
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/*
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* ProcArrayInstallRestoredXmin -- install restored xmin into MyPgXact->xmin
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* ProcArrayInstallRestoredXmin -- install restored xmin into MyProc->xmin
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*
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* This is like ProcArrayInstallImportedXmin, but we have a pointer to the
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* PGPROC of the transaction from which we imported the snapshot, rather than
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@ -2261,7 +2261,6 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
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{
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bool result = false;
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TransactionId xid;
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PGXACT *pgxact;
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Assert(TransactionIdIsNormal(xmin));
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Assert(proc != NULL);
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@ -2269,20 +2268,18 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
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/* Get lock so source xact can't end while we're doing this */
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LWLockAcquire(ProcArrayLock, LW_SHARED);
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pgxact = &allPgXact[proc->pgprocno];
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/*
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* Be certain that the referenced PGPROC has an advertised xmin which is
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* no later than the one we're installing, so that the system-wide xmin
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* can't go backwards. Also, make sure it's running in the same database,
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* so that the per-database xmin cannot go backwards.
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*/
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xid = UINT32_ACCESS_ONCE(pgxact->xmin);
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xid = UINT32_ACCESS_ONCE(proc->xmin);
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if (proc->databaseId == MyDatabaseId &&
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TransactionIdIsNormal(xid) &&
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TransactionIdPrecedesOrEquals(xid, xmin))
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{
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MyPgXact->xmin = TransactionXmin = xmin;
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MyProc->xmin = TransactionXmin = xmin;
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result = true;
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}
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@ -2908,7 +2905,7 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
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if (allDbs || proc->databaseId == MyDatabaseId)
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{
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/* Fetch xmin just once - might change on us */
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TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin);
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TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin);
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if (excludeXmin0 && !TransactionIdIsValid(pxmin))
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continue;
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@ -2994,7 +2991,6 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
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{
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int pgprocno = arrayP->pgprocnos[index];
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PGPROC *proc = &allProcs[pgprocno];
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PGXACT *pgxact = &allPgXact[pgprocno];
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/* Exclude prepared transactions */
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if (proc->pid == 0)
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@ -3004,7 +3000,7 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
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proc->databaseId == dbOid)
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{
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/* Fetch xmin just once - can't change on us, but good coding */
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TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin);
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TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin);
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/*
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* We ignore an invalid pxmin because this means that backend has
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@ -420,7 +420,7 @@ BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmi
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PGXACT *xact = &ProcGlobal->allPgXact[proc->pgprocno];
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*xid = xact->xid;
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*xmin = xact->xmin;
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*xmin = proc->xmin;
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}
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}
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@ -388,7 +388,7 @@ InitProcess(void)
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MyProc->fpVXIDLock = false;
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MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
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MyPgXact->xid = InvalidTransactionId;
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MyPgXact->xmin = InvalidTransactionId;
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MyProc->xmin = InvalidTransactionId;
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MyProc->pid = MyProcPid;
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/* backendId, databaseId and roleId will be filled in later */
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MyProc->backendId = InvalidBackendId;
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@ -572,7 +572,7 @@ InitAuxiliaryProcess(void)
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MyProc->fpVXIDLock = false;
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MyProc->fpLocalTransactionId = InvalidLocalTransactionId;
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MyPgXact->xid = InvalidTransactionId;
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MyPgXact->xmin = InvalidTransactionId;
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MyProc->xmin = InvalidTransactionId;
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MyProc->backendId = InvalidBackendId;
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MyProc->databaseId = InvalidOid;
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MyProc->roleId = InvalidOid;
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@ -27,11 +27,11 @@
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* their lifetime is managed separately (as they live longer than one xact.c
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* transaction).
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*
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* These arrangements let us reset MyPgXact->xmin when there are no snapshots
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* These arrangements let us reset MyProc->xmin when there are no snapshots
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* referenced by this transaction, and advance it when the one with oldest
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* Xmin is no longer referenced. For simplicity however, only registered
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* snapshots not active snapshots participate in tracking which one is oldest;
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* we don't try to change MyPgXact->xmin except when the active-snapshot
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* we don't try to change MyProc->xmin except when the active-snapshot
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* stack is empty.
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*
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*
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@ -187,7 +187,7 @@ static ActiveSnapshotElt *OldestActiveSnapshot = NULL;
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/*
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* Currently registered Snapshots. Ordered in a heap by xmin, so that we can
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* quickly find the one with lowest xmin, to advance our MyPgXact->xmin.
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* quickly find the one with lowest xmin, to advance our MyProc->xmin.
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*/
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static int xmin_cmp(const pairingheap_node *a, const pairingheap_node *b,
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void *arg);
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@ -475,7 +475,7 @@ GetNonHistoricCatalogSnapshot(Oid relid)
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/*
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* Make sure the catalog snapshot will be accounted for in decisions
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* about advancing PGXACT->xmin. We could apply RegisterSnapshot, but
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* about advancing PGPROC->xmin. We could apply RegisterSnapshot, but
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* that would result in making a physical copy, which is overkill; and
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* it would also create a dependency on some resource owner, which we
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* do not want for reasons explained at the head of this file. Instead
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@ -596,7 +596,7 @@ SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid,
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/* NB: curcid should NOT be copied, it's a local matter */
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/*
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* Now we have to fix what GetSnapshotData did with MyPgXact->xmin and
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* Now we have to fix what GetSnapshotData did with MyProc->xmin and
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* TransactionXmin. There is a race condition: to make sure we are not
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* causing the global xmin to go backwards, we have to test that the
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* source transaction is still running, and that has to be done
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@ -950,13 +950,13 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
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/*
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* SnapshotResetXmin
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*
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* If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid.
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* If there are no more snapshots, we can reset our PGPROC->xmin to InvalidXid.
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* Note we can do this without locking because we assume that storing an Xid
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* is atomic.
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*
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* Even if there are some remaining snapshots, we may be able to advance our
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* PGXACT->xmin to some degree. This typically happens when a portal is
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* dropped. For efficiency, we only consider recomputing PGXACT->xmin when
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* PGPROC->xmin to some degree. This typically happens when a portal is
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* dropped. For efficiency, we only consider recomputing PGPROC->xmin when
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* the active snapshot stack is empty; this allows us not to need to track
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* which active snapshot is oldest.
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*
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@ -977,15 +977,15 @@ SnapshotResetXmin(void)
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if (pairingheap_is_empty(&RegisteredSnapshots))
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{
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MyPgXact->xmin = InvalidTransactionId;
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MyProc->xmin = InvalidTransactionId;
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return;
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}
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minSnapshot = pairingheap_container(SnapshotData, ph_node,
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pairingheap_first(&RegisteredSnapshots));
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if (TransactionIdPrecedes(MyPgXact->xmin, minSnapshot->xmin))
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MyPgXact->xmin = minSnapshot->xmin;
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if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
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MyProc->xmin = minSnapshot->xmin;
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}
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/*
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@ -1132,13 +1132,13 @@ AtEOXact_Snapshot(bool isCommit, bool resetXmin)
|
||||
|
||||
/*
|
||||
* During normal commit processing, we call ProcArrayEndTransaction() to
|
||||
* reset the MyPgXact->xmin. That call happens prior to the call to
|
||||
* reset the MyProc->xmin. That call happens prior to the call to
|
||||
* AtEOXact_Snapshot(), so we need not touch xmin here at all.
|
||||
*/
|
||||
if (resetXmin)
|
||||
SnapshotResetXmin();
|
||||
|
||||
Assert(resetXmin || MyPgXact->xmin == 0);
|
||||
Assert(resetXmin || MyProc->xmin == 0);
|
||||
}
|
||||
|
||||
|
||||
@ -1830,7 +1830,7 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
|
||||
*/
|
||||
if (old_snapshot_threshold == 0)
|
||||
{
|
||||
if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
|
||||
if (TransactionIdPrecedes(latest_xmin, MyProc->xmin)
|
||||
&& TransactionIdFollows(latest_xmin, xlimit))
|
||||
xlimit = latest_xmin;
|
||||
|
||||
|
||||
@ -101,6 +101,11 @@ struct PGPROC
|
||||
|
||||
Latch procLatch; /* generic latch for process */
|
||||
|
||||
TransactionId xmin; /* minimal running XID as it was when we were
|
||||
* starting our xact, excluding LAZY VACUUM:
|
||||
* vacuum must not remove tuples deleted by
|
||||
* xid >= xmin ! */
|
||||
|
||||
LocalTransactionId lxid; /* local id of top-level transaction currently
|
||||
* being executed by this proc, if running;
|
||||
* else InvalidLocalTransactionId */
|
||||
@ -223,11 +228,6 @@ typedef struct PGXACT
|
||||
* executed by this proc, if running and XID
|
||||
* is assigned; else InvalidTransactionId */
|
||||
|
||||
TransactionId xmin; /* minimal running XID as it was when we were
|
||||
* starting our xact, excluding LAZY VACUUM:
|
||||
* vacuum must not remove tuples deleted by
|
||||
* xid >= xmin ! */
|
||||
|
||||
uint8 vacuumFlags; /* vacuum-related flags, see above */
|
||||
bool overflowed;
|
||||
|
||||
|
||||
Loading…
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Reference in New Issue
Block a user