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/*------------------------------------------------------------------------- * * snapshot.h * POSTGRES snapshot definition * * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * * src/include/utils/snapshot.h * *------------------------------------------------------------------------- */ #ifndef SNAPSHOT_H #define SNAPSHOT_H #include "access/htup.h" #include "access/xlogdefs.h" #include "lib/pairingheap.h" #include "storage/buf.h" typedef struct SnapshotData *Snapshot; #define InvalidSnapshot ((Snapshot) NULL) /* * We use SnapshotData structures to represent both "regular" (MVCC) * snapshots and "special" snapshots that have non-MVCC semantics. * The specific semantics of a snapshot are encoded by the "satisfies" * function. */ typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup, Snapshot snapshot, Buffer buffer); /* * Struct representing all kind of possible snapshots. * * There are several different kinds of snapshots: * * Normal MVCC snapshots * * MVCC snapshots taken during recovery (in Hot-Standby mode) * * Historic MVCC snapshots used during logical decoding * * snapshots passed to HeapTupleSatisfiesDirty() * * snapshots used for SatisfiesAny, Toast, Self where no members are * accessed. * * TODO: It's probably a good idea to split this struct using a NodeTag * similar to how parser and executor nodes are handled, with one type for * each different kind of snapshot to avoid overloading the meaning of * individual fields. */ typedef struct SnapshotData { SnapshotSatisfiesFunc satisfies; /* tuple test function */ /* * The remaining fields are used only for MVCC snapshots, and are normally * just zeroes in special snapshots. (But xmin and xmax are used * specially by HeapTupleSatisfiesDirty.) * * An MVCC snapshot can never see the effects of XIDs >= xmax. It can see * the effects of all older XIDs except those listed in the snapshot. xmin * is stored as an optimization to avoid needing to search the XID arrays * for most tuples. */ TransactionId xmin; /* all XID < xmin are visible to me */ TransactionId xmax; /* all XID >= xmax are invisible to me */ /* * For normal MVCC snapshot this contains the all xact IDs that are in * progress, unless the snapshot was taken during recovery in which case * it's empty. For historic MVCC snapshots, the meaning is inverted, i.e. * it contains *committed* transactions between xmin and xmax. * * note: all ids in xip[] satisfy xmin <= xip[i] < xmax */ TransactionId *xip; uint32 xcnt; /* # of xact ids in xip[] */ /* * For non-historic MVCC snapshots, this contains subxact IDs that are in * progress (and other transactions that are in progress if taken during * recovery). For historic snapshot it contains *all* xids assigned to the * replayed transaction, including the toplevel xid. * * note: all ids in subxip[] are >= xmin, but we don't bother filtering * out any that are >= xmax */ TransactionId *subxip; int32 subxcnt; /* # of xact ids in subxip[] */ bool suboverflowed; /* has the subxip array overflowed? */ bool takenDuringRecovery; /* recovery-shaped snapshot? */ bool copied; /* false if it's a static snapshot */ CommandId curcid; /* in my xact, CID < curcid are visible */ /* * An extra return value for HeapTupleSatisfiesDirty, not used in MVCC * snapshots. */ uint32 speculativeToken; /* * Book-keeping information, used by the snapshot manager */ uint32 active_count; /* refcount on ActiveSnapshot stack */ uint32 regd_count; /* refcount on RegisteredSnapshots */ pairingheap_node ph_node; /* link in the RegisteredSnapshots heap */ int64 whenTaken; /* timestamp when snapshot was taken */ XLogRecPtr lsn; /* position in the WAL stream when taken */ } SnapshotData; /* * Result codes for HeapTupleSatisfiesUpdate. This should really be in * tqual.h, but we want to avoid including that file elsewhere. */ typedef enum { HeapTupleMayBeUpdated, HeapTupleInvisible, HeapTupleSelfUpdated, HeapTupleUpdated, HeapTupleBeingUpdated, HeapTupleWouldBlock /* can be returned by heap_tuple_lock */ } HTSU_Result; #endif /* SNAPSHOT_H */