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During vacuum's first and third phases, we examine tuples' visibility to determine if we can set the page all-visible in the visibility map. Previously, this check compared tuple xmins against a single XID chosen at the start of vacuum (OldestXmin). We now use GlobalVisState, which enables future work to set the VM during on-access pruning, since ordinary queries have access to GlobalVisState but not OldestXmin. This also benefits vacuum: in some cases, GlobalVisState may advance during a vacuum, allowing more pages to become considered all-visible. And, in the future, we could easily add a heuristic to update GlobalVisState more frequently during vacuums of large tables. OldestXmin is still used for freezing and as a backstop to ensure we don't freeze a dead tuple that wasn't yet prunable according to GlobalVisState in the rare occurrences where GlobalVisState moves backwards. Because comparing a transaction ID against GlobalVisState is more expensive than comparing against a single XID, we defer this check until after scanning all tuples on the page. Therefore, we perform the GlobalVisState check only once per page. This is safe because visibility_cutoff_xid records the newest live xmin on the page; if it is globally visible, then the entire page is all-visible. Using GlobalVisState means on-access pruning can also maintain visibility_cutoff_xid, which is required to set the visibility map on-access in the future. Author: Melanie Plageman <melanieplageman@gmail.com> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Chao Li <li.evan.chao@gmail.com> Reviewed-by: Kirill Reshke <reshkekirill@gmail.com> Discussion: https://postgr.es/m/flat/bqc4kh5midfn44gnjiqez3bjqv4zogydguvdn446riw45jcf3y%404ez66il7ebvk#c755ef151507aba58471ffaca607e493
148 lines
5.4 KiB
C
148 lines
5.4 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) 1996-2026, 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 "access/transam.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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extern PGDLLIMPORT bool FirstSnapshotSet;
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extern PGDLLIMPORT TransactionId TransactionXmin;
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extern PGDLLIMPORT TransactionId RecentXmin;
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/* Variables representing various special snapshot semantics */
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extern PGDLLIMPORT SnapshotData SnapshotSelfData;
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extern PGDLLIMPORT SnapshotData SnapshotAnyData;
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extern PGDLLIMPORT SnapshotData SnapshotToastData;
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#define SnapshotSelf (&SnapshotSelfData)
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#define SnapshotAny (&SnapshotAnyData)
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/* Use get_toast_snapshot() for the TOAST snapshot */
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/*
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* We don't provide a static SnapshotDirty variable because it would be
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* non-reentrant. Instead, users of that snapshot type should declare a
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* local variable of type SnapshotData, and initialize it with this macro.
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*/
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#define InitDirtySnapshot(snapshotdata) \
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((snapshotdata).snapshot_type = SNAPSHOT_DIRTY)
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/*
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* Similarly, some initialization is required for a NonVacuumable snapshot.
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* The caller must supply the visibility cutoff state to use (c.f.
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* GlobalVisTestFor()).
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*/
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#define InitNonVacuumableSnapshot(snapshotdata, vistestp) \
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((snapshotdata).snapshot_type = SNAPSHOT_NON_VACUUMABLE, \
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(snapshotdata).vistest = (vistestp))
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/*
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* Is the snapshot implemented as an MVCC snapshot (i.e. it uses
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* SNAPSHOT_MVCC)? If so, there will be at most one visible tuple in a chain
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* of updated tuples, and each visible tuple will be seen exactly once.
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*/
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#define IsMVCCSnapshot(snapshot) \
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((snapshot)->snapshot_type == SNAPSHOT_MVCC)
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/*
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* Special kind of MVCC snapshot, used during logical decoding. The visibility
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* is checked from the perspective of an already committed transaction that is
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* being decoded.
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*/
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#define IsHistoricMVCCSnapshot(snapshot) \
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((snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
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/*
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* Is the snapshot either an MVCC snapshot or has equivalent visibility
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* semantics (see IsMVCCSnapshot())?
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*/
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#define IsMVCCLikeSnapshot(snapshot) \
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(IsMVCCSnapshot(snapshot) || IsHistoricMVCCSnapshot(snapshot))
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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 InvalidateCatalogSnapshotConditionally(void);
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extern void PushActiveSnapshot(Snapshot snapshot);
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extern void PushActiveSnapshotWithLevel(Snapshot snapshot, int snap_level);
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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, bool resetXmin);
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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 void WaitForOlderSnapshots(TransactionId limitXmin, bool progress);
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extern bool ThereAreNoPriorRegisteredSnapshots(void);
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extern bool HaveRegisteredOrActiveSnapshot(void);
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extern char *ExportSnapshot(Snapshot snapshot);
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/*
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* These live in procarray.c because they're intimately linked to the
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* procarray contents, but thematically they better fit into snapmgr.h.
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*/
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typedef struct GlobalVisState GlobalVisState;
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extern GlobalVisState *GlobalVisTestFor(Relation rel);
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extern bool GlobalVisTestIsRemovableXid(GlobalVisState *state,
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TransactionId xid,
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bool allow_update);
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extern bool GlobalVisTestIsRemovableFullXid(GlobalVisState *state,
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FullTransactionId fxid,
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bool allow_update);
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extern bool GlobalVisCheckRemovableXid(Relation rel, TransactionId xid);
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extern bool GlobalVisCheckRemovableFullXid(Relation rel, FullTransactionId fxid);
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extern bool GlobalVisTestXidConsideredRunning(GlobalVisState *state,
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TransactionId xid,
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bool allow_update);
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/*
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* Utility functions for implementing visibility routines in table AMs.
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*/
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extern bool XidInMVCCSnapshot(TransactionId xid, 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 historic_snapshot, 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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struct PGPROC;
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extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc);
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#endif /* SNAPMGR_H */
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