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Without a few entries beyond old_snapshot_threshold, the lookup would often fail, resulting in the more aggressive pruning or vacuum being skipped often enough to matter. This was very clearly shown by a python test script posted by Ants Aasma, and was likely a factor in an earlier but somewhat less clear-cut test case posted by Jeff Janes. This patch makes no change to the logic, per se -- it just makes the array of mapping entries big enough to make lookup misses based on timing much less likely. An occasional miss is still possible if a thread stalls for more than 10 minutes, but that does not create any problem with correctness of behavior. Besides, if things are so busy that a thread is stalling for more than 10 minutes, it is probably OK to skip the more aggressive cleanup at that particular point in time.
98 lines
3.6 KiB
C
98 lines
3.6 KiB
C
/*-------------------------------------------------------------------------
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*
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* snapmgr.h
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* POSTGRES snapshot manager
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*
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* Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/utils/snapmgr.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef SNAPMGR_H
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#define SNAPMGR_H
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#include "fmgr.h"
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#include "utils/relcache.h"
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#include "utils/resowner.h"
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#include "utils/snapshot.h"
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/*
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* The structure used to map times to TransactionId values for the "snapshot
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* too old" feature must have a few entries at the tail to hold old values;
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* otherwise the lookup will often fail and the expected early pruning or
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* vacuum will not usually occur. It is best if this padding is for a number
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* of minutes greater than a thread would normally be stalled, but it's OK if
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* early vacuum opportunities are occasionally missed, so there's no need to
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* use an extreme value or get too fancy. 10 minutes seems plenty.
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*/
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#define OLD_SNAPSHOT_PADDING_ENTRIES 10
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#define OLD_SNAPSHOT_TIME_MAP_ENTRIES (old_snapshot_threshold + OLD_SNAPSHOT_PADDING_ENTRIES)
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/* GUC variables */
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extern PGDLLIMPORT int old_snapshot_threshold;
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extern Size SnapMgrShmemSize(void);
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extern void SnapMgrInit(void);
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extern int64 GetSnapshotCurrentTimestamp(void);
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extern int64 GetOldSnapshotThresholdTimestamp(void);
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extern bool FirstSnapshotSet;
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extern TransactionId TransactionXmin;
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extern TransactionId RecentXmin;
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extern TransactionId RecentGlobalXmin;
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extern TransactionId RecentGlobalDataXmin;
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extern Snapshot GetTransactionSnapshot(void);
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extern Snapshot GetLatestSnapshot(void);
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extern void SnapshotSetCommandId(CommandId curcid);
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extern Snapshot GetCatalogSnapshot(Oid relid);
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extern Snapshot GetNonHistoricCatalogSnapshot(Oid relid);
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extern void InvalidateCatalogSnapshot(void);
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extern void PushActiveSnapshot(Snapshot snapshot);
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extern void PushCopiedSnapshot(Snapshot snapshot);
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extern void UpdateActiveSnapshotCommandId(void);
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extern void PopActiveSnapshot(void);
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extern Snapshot GetActiveSnapshot(void);
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extern bool ActiveSnapshotSet(void);
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extern Snapshot RegisterSnapshot(Snapshot snapshot);
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extern void UnregisterSnapshot(Snapshot snapshot);
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extern Snapshot RegisterSnapshotOnOwner(Snapshot snapshot, ResourceOwner owner);
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extern void UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner);
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extern void AtSubCommit_Snapshot(int level);
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extern void AtSubAbort_Snapshot(int level);
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extern void AtEOXact_Snapshot(bool isCommit);
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extern Datum pg_export_snapshot(PG_FUNCTION_ARGS);
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extern void ImportSnapshot(const char *idstr);
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extern bool XactHasExportedSnapshots(void);
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extern void DeleteAllExportedSnapshotFiles(void);
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extern bool ThereAreNoPriorRegisteredSnapshots(void);
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extern TransactionId TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
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Relation relation);
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extern void MaintainOldSnapshotTimeMapping(int64 whenTaken, TransactionId xmin);
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extern char *ExportSnapshot(Snapshot snapshot);
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/* Support for catalog timetravel for logical decoding */
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struct HTAB;
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extern struct HTAB *HistoricSnapshotGetTupleCids(void);
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extern void SetupHistoricSnapshot(Snapshot snapshot_now, struct HTAB *tuplecids);
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extern void TeardownHistoricSnapshot(bool is_error);
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extern bool HistoricSnapshotActive(void);
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extern Size EstimateSnapshotSpace(Snapshot snapshot);
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extern void SerializeSnapshot(Snapshot snapshot, char *start_address);
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extern Snapshot RestoreSnapshot(char *start_address);
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extern void RestoreTransactionSnapshot(Snapshot snapshot, void *master_pgproc);
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#endif /* SNAPMGR_H */
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