PostgreSQL 14在MVCC上面有所增强，以保证在大批量客户端连接场景下性能不会出现显著下降（详细可参见参考资料中的链接）。本节介绍了提交记录“snapshot scalability: Move PGXACT->xmin back to PGPROC.”中的修改以及涉及的相关数据结构。 ### 一、数据结构 把PGXACT中的xmin移到PGPROC中 **PGPROC** ```diff diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h index 52ff43cabaafa9f3885c5a0f4912724c8782fb32..5e4b028a5f98d104c984c7d17f61266acfc57b65 100644 (file) --- a/src/include/storage/proc.h +++ b/src/include/storage/proc.h @@ -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 */ ``` **PGXACT** ``` @@ -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; ``` ### 二、源码解读 该commit的提交说明如下: ``` 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. 现在,GetSnapshotData()函数已不再需要xmin, xmin的频繁更新会导致PGXACT结构体在CPU缓存中不必要的移动(不同的进程会在不同的核中执行) 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()). 在高并发场景下,快照的请求非常频繁,这个提交会显著改进扩展性. ``` 把xmin从PGXACT中移到PGPROC中,由于xmin频繁的更新,导致进程频繁访问PGXACT数据结构, 在多CPU架构下,该结构体在CPU缓存之间来回ping-pong,影响扩展性 ``` /* 把xmin从PGXACT中移到PGPROC中,由于xmin频繁的更新,导致进程频繁访问PGXACT数据结构, 在多CPU架构下,该结构体在CPU缓存之间来回ping-pong,影响扩展性 */ diff --git a/src/backend/access/gist/gistxlog.c b/src/backend/access/gist/gistxlog.c index a63b05388c5d440124655ad92cdb65dc7135b30d..dcd28f678b3db168c5070d3d7d67f26c8a89f1c7 100644 (file) --- a/src/backend/access/gist/gistxlog.c +++ b/src/backend/access/gist/gistxlog.c @@ -389,7 +389,7 @@ gistRedoPageReuse(XLogReaderState *record) * * latestRemovedXid was the page's deleteXid. The * GlobalVisIsRemovableFullXid(deleteXid) test in gistPageRecyclable() - * conceptually mirrors the pgxact->xmin > limitXmin test in + * conceptually mirrors the PGPROC->xmin > limitXmin test in * GetConflictingVirtualXIDs(). Consequently, one XID value achieves the * same exclusion effect on primary and standby. */ diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c index 74be3807bb7dedf3f7a61b06c9d64d0ed1acea2d..7f392480ac0f2ed21d045ad0934c5e36a1f1237b 100644 (file) --- a/src/backend/access/nbtree/nbtpage.c +++ b/src/backend/access/nbtree/nbtpage.c @@ -2317,7 +2317,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno, * we're in VACUUM and would not otherwise have an XID. Having already * updated links to the target, ReadNewTransactionId() suffices as an * upper bound. Any scan having retained a now-stale link is advertising - * in its PGXACT an xmin less than or equal to the value we read here. It + * in its PGPROC an xmin less than or equal to the value we read here. It * will continue to do so, holding back the xmin horizon, for the duration * of that scan. */ diff --git a/src/backend/access/transam/README b/src/backend/access/transam/README index 98acb429b67e4e71f2293f455e412bc73e277ee4..eab8edd20ec2d9c4871585622ea70e46142b44b9 100644 (file) --- a/src/backend/access/transam/README +++ b/src/backend/access/transam/README @@ -296,7 +296,7 @@ ensure that the C compiler does exactly what you tell it to.) Another important activity that uses the shared ProcArray is ComputeXidHorizons, which must determine a lower bound for the oldest xmin of any active MVCC snapshot, system-wide. Each individual backend -advertises the smallest xmin of its own snapshots in MyPgXact->xmin, or zero +advertises the smallest xmin of its own snapshots in MyProc->xmin, or zero if it currently has no live snapshots (eg, if it's between transactions or hasn't yet set a snapshot for a new transaction). ComputeXidHorizons takes the MIN() of the valid xmin fields. It does this with only shared lock on @@ -331,7 +331,7 @@ necessary. Note that while it is certain that two concurrent executions of GetSnapshotData will compute the same xmin for their own snapshots, there is no such guarantee for the horizons computed by ComputeXidHorizons. This is -because we allow XID-less transactions to clear their MyPgXact->xmin +because we allow XID-less transactions to clear their MyProc->xmin asynchronously (without taking ProcArrayLock), so one execution might see what had been the oldest xmin, and another not. This is OK since the thresholds need only be a valid lower bound. As noted above, we are already diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c index 31f135f5cedc1d6920d1a37cec67553b5b9562ed..eb5f4680a3d9661992ba48f4802a055366f80ee7 100644 (file) --- a/src/backend/access/transam/twophase.c +++ b/src/backend/access/transam/twophase.c @@ -464,7 +464,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid, /* We set up the gxact's VXID as InvalidBackendId/XID */ proc->lxid = (LocalTransactionId) xid; pgxact->xid = xid; - pgxact->xmin = InvalidTransactionId; + Assert(proc->xmin == InvalidTransactionId); proc->delayChkpt = false; pgxact->vacuumFlags = 0; proc->pid = 0; diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c index 7819266a63063563c1c466446794647f7a2fc904..254dbcdce52b95483ea9589294bf3cd4d77ae5ec 100644 (file) --- a/src/backend/commands/indexcmds.c +++ b/src/backend/commands/indexcmds.c @@ -1535,7 +1535,7 @@ DefineIndex(Oid relationId, StartTransactionCommand(); /* We should now definitely not be advertising any xmin. */ - Assert(MyPgXact->xmin == InvalidTransactionId); + Assert(MyProc->xmin == InvalidTransactionId); /* * The index is now valid in the sense that it contains all currently diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 3089f0d5ddcd8db53bb04eceb426b05396b701c9..e9701ea7221549f02fb683766cbf024577066ca7 100644 (file) --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -553,8 +553,8 @@ SnapBuildInitialSnapshot(SnapBuild *builder) elog(ERROR, "cannot build an initial slot snapshot, not all transactions are monitored anymore"); /* so we don't overwrite the existing value */ - if (TransactionIdIsValid(MyPgXact->xmin)) - elog(ERROR, "cannot build an initial slot snapshot when MyPgXact->xmin already is valid"); + if (TransactionIdIsValid(MyProc->xmin)) + elog(ERROR, "cannot build an initial slot snapshot when MyProc->xmin already is valid"); snap = SnapBuildBuildSnapshot(builder); @@ -575,7 +575,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder) } #endif - MyPgXact->xmin = snap->xmin; + MyProc->xmin = snap->xmin; /* allocate in transaction context */ newxip = (TransactionId *) diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c index 460ca3f947f4feac698128963d46930a7d7a33e1..3f756b470af11c9d467874a7544abe30e7b77ee0 100644 (file) --- a/src/backend/replication/walsender.c +++ b/src/backend/replication/walsender.c @@ -1964,7 +1964,7 @@ PhysicalReplicationSlotNewXmin(TransactionId feedbackXmin, TransactionId feedbac ReplicationSlot *slot = MyReplicationSlot; SpinLockAcquire(&slot->mutex); - MyPgXact->xmin = InvalidTransactionId; + MyProc->xmin = InvalidTransactionId; /* * For physical replication we don't need the interlock provided by xmin @@ -2093,7 +2093,7 @@ ProcessStandbyHSFeedbackMessage(void) if (!TransactionIdIsNormal(feedbackXmin) && !TransactionIdIsNormal(feedbackCatalogXmin)) { - MyPgXact->xmin = InvalidTransactionId; + MyProc->xmin = InvalidTransactionId; if (MyReplicationSlot != NULL) PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin); return; @@ -2135,7 +2135,7 @@ ProcessStandbyHSFeedbackMessage(void) * risk already since a VACUUM could already have determined the horizon.) * * If we're using a replication slot we reserve the xmin via that, - * otherwise via the walsender's PGXACT entry. We can only track the + * otherwise via the walsender's PGPROC entry. We can only track the * catalog xmin separately when using a slot, so we store the least of the * two provided when not using a slot. * @@ -2148,9 +2148,9 @@ ProcessStandbyHSFeedbackMessage(void) { if (TransactionIdIsNormal(feedbackCatalogXmin) && TransactionIdPrecedes(feedbackCatalogXmin, feedbackXmin)) - MyPgXact->xmin = feedbackCatalogXmin; + MyProc->xmin = feedbackCatalogXmin; else - MyPgXact->xmin = feedbackXmin; + MyProc->xmin = feedbackXmin; } } diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c index e582d5af4291bd8b17a5d18c8a74569be2c76fc0..185f581c8b6f1c43c79838e9b27226d3890147d5 100644 (file) --- a/src/backend/storage/ipc/procarray.c +++ b/src/backend/storage/ipc/procarray.c @@ -587,9 +587,9 @@ ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid) Assert(!TransactionIdIsValid(allPgXact[proc->pgprocno].xid)); proc->lxid = InvalidLocalTransactionId; - pgxact->xmin = InvalidTransactionId; /* must be cleared with xid/xmin: */ pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK; + proc->xmin = InvalidTransactionId; proc->delayChkpt = false; /* be sure this is cleared in abort */ proc->recoveryConflictPending = false; @@ -609,9 +609,9 @@ ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact, { pgxact->xid = InvalidTransactionId; proc->lxid = InvalidLocalTransactionId; - pgxact->xmin = InvalidTransactionId; /* must be cleared with xid/xmin: */ pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK; + proc->xmin = InvalidTransactionId; proc->delayChkpt = false; /* be sure this is cleared in abort */ proc->recoveryConflictPending = false; @@ -763,7 +763,7 @@ ProcArrayClearTransaction(PGPROC *proc) */ pgxact->xid = InvalidTransactionId; proc->lxid = InvalidLocalTransactionId; - pgxact->xmin = InvalidTransactionId; + proc->xmin = InvalidTransactionId; proc->recoveryConflictPending = false; /* redundant, but just in case */ @@ -1563,7 +1563,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h) /* Fetch xid just once - see GetNewTransactionId */ xid = UINT32_ACCESS_ONCE(pgxact->xid); - xmin = UINT32_ACCESS_ONCE(pgxact->xmin); + xmin = UINT32_ACCESS_ONCE(proc->xmin); /* * Consider both the transaction's Xmin, and its Xid. @@ -1838,7 +1838,7 @@ GetMaxSnapshotSubxidCount(void) * * We also update the following backend-global variables: * TransactionXmin: the oldest xmin of any snapshot in use in the - * current transaction (this is the same as MyPgXact->xmin). + * current transaction (this is the same as MyProc->xmin). * RecentXmin: the xmin computed for the most recent snapshot. XIDs * older than this are known not running any more. * @@ -1899,7 +1899,7 @@ GetSnapshotData(Snapshot snapshot) /* * It is sufficient to get shared lock on ProcArrayLock, even if we are - * going to set MyPgXact->xmin. + * going to set MyProc->xmin. */ LWLockAcquire(ProcArrayLock, LW_SHARED); @@ -2051,8 +2051,8 @@ GetSnapshotData(Snapshot snapshot) replication_slot_xmin = procArray->replication_slot_xmin; replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin; - if (!TransactionIdIsValid(MyPgXact->xmin)) - MyPgXact->xmin = TransactionXmin = xmin; + if (!TransactionIdIsValid(MyProc->xmin)) + MyProc->xmin = TransactionXmin = xmin; LWLockRelease(ProcArrayLock); @@ -2172,7 +2172,7 @@ GetSnapshotData(Snapshot snapshot) } /* - * ProcArrayInstallImportedXmin -- install imported xmin into MyPgXact->xmin + * ProcArrayInstallImportedXmin -- install imported xmin into MyProc->xmin * * This is called when installing a snapshot imported from another * transaction. To ensure that OldestXmin doesn't go backwards, we must @@ -2225,7 +2225,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin, /* * Likewise, let's just make real sure its xmin does cover us. */ - xid = UINT32_ACCESS_ONCE(pgxact->xmin); + xid = UINT32_ACCESS_ONCE(proc->xmin); if (!TransactionIdIsNormal(xid) || !TransactionIdPrecedesOrEquals(xid, xmin)) continue; @@ -2236,7 +2236,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin, * GetSnapshotData first, we'll be overwriting a valid xmin here, so * we don't check that.) */ - MyPgXact->xmin = TransactionXmin = xmin; + MyProc->xmin = TransactionXmin = xmin; result = true; break; @@ -2248,7 +2248,7 @@ ProcArrayInstallImportedXmin(TransactionId xmin, } /* - * ProcArrayInstallRestoredXmin -- install restored xmin into MyPgXact->xmin + * ProcArrayInstallRestoredXmin -- install restored xmin into MyProc->xmin * * This is like ProcArrayInstallImportedXmin, but we have a pointer to the * PGPROC of the transaction from which we imported the snapshot, rather than @@ -2261,7 +2261,6 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc) { bool result = false; TransactionId xid; - PGXACT *pgxact; Assert(TransactionIdIsNormal(xmin)); Assert(proc != NULL); @@ -2269,20 +2268,18 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc) /* Get lock so source xact can't end while we're doing this */ LWLockAcquire(ProcArrayLock, LW_SHARED); - pgxact = &allPgXact[proc->pgprocno]; - /* * Be certain that the referenced PGPROC has an advertised xmin which is * no later than the one we're installing, so that the system-wide xmin * can't go backwards. Also, make sure it's running in the same database, * so that the per-database xmin cannot go backwards. */ - xid = UINT32_ACCESS_ONCE(pgxact->xmin); + xid = UINT32_ACCESS_ONCE(proc->xmin); if (proc->databaseId == MyDatabaseId && TransactionIdIsNormal(xid) && TransactionIdPrecedesOrEquals(xid, xmin)) { - MyPgXact->xmin = TransactionXmin = xmin; + MyProc->xmin = TransactionXmin = xmin; result = true; } @@ -2908,7 +2905,7 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0, if (allDbs || proc->databaseId == MyDatabaseId) { /* Fetch xmin just once - might change on us */ - TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin); + TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin); if (excludeXmin0 && !TransactionIdIsValid(pxmin)) continue; @@ -2994,7 +2991,6 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid) { int pgprocno = arrayP->pgprocnos[index]; PGPROC *proc = &allProcs[pgprocno]; - PGXACT *pgxact = &allPgXact[pgprocno]; /* Exclude prepared transactions */ if (proc->pid == 0) @@ -3004,7 +3000,7 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid) proc->databaseId == dbOid) { /* Fetch xmin just once - can't change on us, but good coding */ - TransactionId pxmin = UINT32_ACCESS_ONCE(pgxact->xmin); + TransactionId pxmin = UINT32_ACCESS_ONCE(proc->xmin); /* * We ignore an invalid pxmin because this means that backend has diff --git a/src/backend/storage/ipc/sinvaladt.c b/src/backend/storage/ipc/sinvaladt.c index e5c115b92f2b98323ecd7140880cf331b06a6409..ad048bc85fab6b7d52caed84f1d5489dafd608e9 100644 (file) --- a/src/backend/storage/ipc/sinvaladt.c +++ b/src/backend/storage/ipc/sinvaladt.c @@ -420,7 +420,7 @@ BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmi PGXACT *xact = &ProcGlobal->allPgXact[proc->pgprocno]; *xid = xact->xid; - *xmin = xact->xmin; + *xmin = proc->xmin; } } diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c index e57fcd253880c31581dccc7b53f026dc6b18f2d2..de346cd87fcded237806c34395b7248d2c5c6f46 100644 (file) --- a/src/backend/storage/lmgr/proc.c +++ b/src/backend/storage/lmgr/proc.c @@ -388,7 +388,7 @@ InitProcess(void) MyProc->fpVXIDLock = false; MyProc->fpLocalTransactionId = InvalidLocalTransactionId; MyPgXact->xid = InvalidTransactionId; - MyPgXact->xmin = InvalidTransactionId; + MyProc->xmin = InvalidTransactionId; MyProc->pid = MyProcPid; /* backendId, databaseId and roleId will be filled in later */ MyProc->backendId = InvalidBackendId; @@ -572,7 +572,7 @@ InitAuxiliaryProcess(void) MyProc->fpVXIDLock = false; MyProc->fpLocalTransactionId = InvalidLocalTransactionId; MyPgXact->xid = InvalidTransactionId; - MyPgXact->xmin = InvalidTransactionId; + MyProc->xmin = InvalidTransactionId; MyProc->backendId = InvalidBackendId; MyProc->databaseId = InvalidOid; MyProc->roleId = InvalidOid; diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 604d823f68613d1a60bd04c3847c85c0197d6c99..752af0c10dfc06375e2a10aa712039ef8c924027 100644 (file) --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -27,11 +27,11 @@ * their lifetime is managed separately (as they live longer than one xact.c * transaction). * - * These arrangements let us reset MyPgXact->xmin when there are no snapshots + * These arrangements let us reset MyProc->xmin when there are no snapshots * referenced by this transaction, and advance it when the one with oldest * Xmin is no longer referenced. For simplicity however, only registered * snapshots not active snapshots participate in tracking which one is oldest; - * we don't try to change MyPgXact->xmin except when the active-snapshot + * we don't try to change MyProc->xmin except when the active-snapshot * stack is empty. * * @@ -187,7 +187,7 @@ static ActiveSnapshotElt *OldestActiveSnapshot = NULL; /* * Currently registered Snapshots. Ordered in a heap by xmin, so that we can - * quickly find the one with lowest xmin, to advance our MyPgXact->xmin. + * quickly find the one with lowest xmin, to advance our MyProc->xmin. */ static int xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg); @@ -475,7 +475,7 @@ GetNonHistoricCatalogSnapshot(Oid relid) /* * Make sure the catalog snapshot will be accounted for in decisions - * about advancing PGXACT->xmin. We could apply RegisterSnapshot, but + * about advancing PGPROC->xmin. We could apply RegisterSnapshot, but * that would result in making a physical copy, which is overkill; and * it would also create a dependency on some resource owner, which we * do not want for reasons explained at the head of this file. Instead @@ -596,7 +596,7 @@ SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid, /* NB: curcid should NOT be copied, it's a local matter */ /* - * Now we have to fix what GetSnapshotData did with MyPgXact->xmin and + * Now we have to fix what GetSnapshotData did with MyProc->xmin and * TransactionXmin. There is a race condition: to make sure we are not * causing the global xmin to go backwards, we have to test that the * source transaction is still running, and that has to be done @@ -950,13 +950,13 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg) /* * SnapshotResetXmin * - * If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid. + * If there are no more snapshots, we can reset our PGPROC->xmin to InvalidXid. * Note we can do this without locking because we assume that storing an Xid * is atomic. * * Even if there are some remaining snapshots, we may be able to advance our - * PGXACT->xmin to some degree. This typically happens when a portal is - * dropped. For efficiency, we only consider recomputing PGXACT->xmin when + * PGPROC->xmin to some degree. This typically happens when a portal is + * dropped. For efficiency, we only consider recomputing PGPROC->xmin when * the active snapshot stack is empty; this allows us not to need to track * which active snapshot is oldest. * @@ -977,15 +977,15 @@ SnapshotResetXmin(void) if (pairingheap_is_empty(&RegisteredSnapshots)) { - MyPgXact->xmin = InvalidTransactionId; + MyProc->xmin = InvalidTransactionId; return; } minSnapshot = pairingheap_container(SnapshotData, ph_node, pairingheap_first(&RegisteredSnapshots)); - if (TransactionIdPrecedes(MyPgXact->xmin, minSnapshot->xmin)) - MyPgXact->xmin = minSnapshot->xmin; + if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin)) + MyProc->xmin = minSnapshot->xmin; } /* @@ -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; ``` ### 三、跟踪分析 N/A ### 四、参考资料 1.[Improving connection scalability: GetSnapshotData()](https://www.postgresql.org/message-id/20200301083601.ews6hz5dduc3w2se%40alap3.anarazel.de) 2.[snapshot scalability: Don't compute global horizons while building snapshots.](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=dc7420c2c9274a283779ec19718d2d16323640c0)