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“Don't compute global horizons while building snapshots.”中walreceiver.c、vacuum.c、analyze.c、xlog.c、nbtxlog.c的修改以及涉及的相关数据结构。 ### 一、数据结构 N/A ### 二、源码解读 **walreceiver.c** ```diff --- a/src/backend/replication/walreceiver.c +++ b/src/backend/replication/walreceiver.c @@ -1181,22 +1181,7 @@ XLogWalRcvSendHSFeedback(bool immed) /* XLogWalRcvSendHSFeedback : 发送备节点反馈消息外加当前时间给主节点 修改： 改用GetReplicationHorizons获取xmin&catalog_xmin */ */ if (hot_standby_feedback) { - TransactionId slot_xmin; - - /* - * Usually GetOldestXmin() would include both global replication slot - * xmin and catalog_xmin in its calculations, but we want to derive - * separate values for each of those. So we ask for an xmin that - * excludes the catalog_xmin. - */ - xmin = GetOldestXmin(NULL, - PROCARRAY_FLAGS_DEFAULT | PROCARRAY_SLOTS_XMIN); - - ProcArrayGetReplicationSlotXmin(&slot_xmin, &catalog_xmin); - - if (TransactionIdIsValid(slot_xmin) && - TransactionIdPrecedes(slot_xmin, xmin)) - xmin = slot_xmin; + GetReplicationHorizons(&xmin, &catalog_xmin); } else { ``` **vacuum.c** ```diff --- a/src/backend/commands/vacuum.c +++ b/src/backend/commands/vacuum.c @@ -955,8 +955,25 @@ vacuum_set_xid_limits(Relation rel, /* vacuum_set_xid_limits : 计算oldestXmin并冻结截断点 修改： 1.oldestXmin值改为通过GetOldestNonRemovableTransactionId 2.新增如启用快照过旧特性，则设置设置时间&xmin */ * working on a particular table at any time, and that each vacuum is * always an independent transaction. */ - *oldestXmin = - TransactionIdLimitedForOldSnapshots(GetOldestXmin(rel, PROCARRAY_FLAGS_VACUUM), rel); + *oldestXmin = GetOldestNonRemovableTransactionId(rel); + + if (OldSnapshotThresholdActive()) + { + TransactionId limit_xmin; + TimestampTz limit_ts; + + if (TransactionIdLimitedForOldSnapshots(*oldestXmin, rel, &limit_xmin, &limit_ts)) + { + /* + * TODO: We should only set the threshold if we are pruning on the + * basis of the increased limits. Not as crucial here as it is for + * opportunistic pruning (which often happens at a much higher + * frequency), but would still be a significant improvement. + */ + SetOldSnapshotThresholdTimestamp(limit_ts, limit_xmin); + *oldestXmin = limit_xmin; + } + } Assert(TransactionIdIsNormal(*oldestXmin)); @@ -1345,12 +1362,13 @@ vac_update_datfrozenxid(void) /* vac_update_datfrozenxid : 更新pg_database.datfrozenxid 修改： 1.改用GetOldestNonRemovableTransactionId计算最新的冻结ID 2.修正注释 */ bool dirty = false; /* - * Initialize the "min" calculation with GetOldestXmin, which is a - * reasonable approximation to the minimum relfrozenxid for not-yet- - * committed pg_class entries for new tables; see AddNewRelationTuple(). - * So we cannot produce a wrong minimum by starting with this. + * Initialize the "min" calculation with + * GetOldestNonRemovableTransactionId(), which is a reasonable + * approximation to the minimum relfrozenxid for not-yet-committed + * pg_class entries for new tables; see AddNewRelationTuple(). So we + * cannot produce a wrong minimum by starting with this. */ - newFrozenXid = GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM); + newFrozenXid = GetOldestNonRemovableTransactionId(NULL); /* * Similarly, initialize the MultiXact "min" with the value that would be @@ -1681,8 +1699,9 @@ vacuum_rel(Oid relid, RangeVar *relation, VacuumParams *params) StartTransactionCommand(); /* - * Functions in indexes may want a snapshot set. Also, setting a snapshot - * ensures that RecentGlobalXmin is kept truly recent. + * Need to acquire a snapshot to prevent pg_subtrans from being truncated, + * cutoff xids in local memory wrapping around, and to have updated xmin + * horizons. */ PushActiveSnapshot(GetTransactionSnapshot()); @@ -1705,8 +1724,8 @@ vacuum_rel(Oid relid, RangeVar *relation, VacuumParams *params) * * Note: these flags remain set until CommitTransaction or * AbortTransaction. We don't want to clear them until we reset - * MyPgXact->xid/xmin, else OldestXmin might appear to go backwards, - * which is probably Not Good. + * MyPgXact->xid/xmin, otherwise GetOldestNonRemovableTransactionId() + * might appear to go backwards, which is probably Not Good. */ LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE); MyPgXact->vacuumFlags |= PROC_IN_VACUUM; ``` **analyze.c** ```diff --- a/src/backend/commands/analyze.c +++ b/src/backend/commands/analyze.c @@ -1045,7 +1045,7 @@ acquire_sample_rows(Relation onerel, int elevel, /* acquire_sample_rows : 获取样本行 修改 ： GetOldestXmin函数改为GetOldestNonRemovableTransactionId */ totalblocks = RelationGetNumberOfBlocks(onerel); /* Need a cutoff xmin for HeapTupleSatisfiesVacuum */ - OldestXmin = GetOldestXmin(onerel, PROCARRAY_FLAGS_VACUUM); + OldestXmin = GetOldestNonRemovableTransactionId(onerel); /* Prepare for sampling block numbers */ nblocks = BlockSampler_Init(&bs, totalblocks, targrows, random()); ``` **xlog.c** ```diff --- a/src/backend/access/transam/xlog.c +++ b/src/backend/access/transam/xlog.c @@ -9096,7 +9096,7 @@ CreateCheckPoint(int flags) /* CreateCheckPoint : 创建检查点 修改： GetOldestXmin->GetOldestTransactionIdConsideredRunning 该函数返回最旧的事务id，该id是被其他正在运行的backend视为仍为活动事务的id */ * StartupSUBTRANS hasn't been called yet. */ if (!RecoveryInProgress()) - TruncateSUBTRANS(GetOldestXmin(NULL, PROCARRAY_FLAGS_DEFAULT)); + TruncateSUBTRANS(GetOldestTransactionIdConsideredRunning()); /* Real work is done, but log and update stats before releasing lock. */ LogCheckpointEnd(false); @@ -9456,7 +9456,7 @@ CreateRestartPoint(int flags) * this because StartupSUBTRANS hasn't been called yet. */ if (EnableHotStandby) - TruncateSUBTRANS(GetOldestXmin(NULL, PROCARRAY_FLAGS_DEFAULT)); + TruncateSUBTRANS(GetOldestTransactionIdConsideredRunning()); /* Real work is done, but log and update before releasing lock. */ LogCheckpointEnd(true); ``` **nbtxlog.c** ```diff --- a/src/backend/access/nbtree/nbtxlog.c +++ b/src/backend/access/nbtree/nbtxlog.c @@ -948,11 +948,11 @@ btree_xlog_reuse_page(XLogReaderState *record) /* 这里只是简单的改了注释而已 */ * Btree reuse_page records exist to provide a conflict point when we * reuse pages in the index via the FSM. That's all they do though. * - * latestRemovedXid was the page's btpo.xact. The btpo.xact < - * RecentGlobalXmin test in _bt_page_recyclable() conceptually mirrors the - * pgxact->xmin > limitXmin test in GetConflictingVirtualXIDs(). - * Consequently, one XID value achieves the same exclusion effect on - * primary and standby. + * latestRemovedXid was the page's btpo.xact. The + * GlobalVisCheckRemovableXid test in _bt_page_recyclable() conceptually + * mirrors the pgxact->xmin > limitXmin test in + * GetConflictingVirtualXIDs(). Consequently, one XID value achieves the + * same exclusion effect on primary and standby. */ if (InHotStandby) { ``` ### 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)