snapshot.h

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/*-------------------------------------------------------------------------
 *
 * snapshot.h
 *    POSTGRES snapshot definition
 *
 * Portions Copyright (c) 1996-2013, 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 "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) (HeapTupleHeader tuple,
                                           Snapshot snapshot, Buffer buffer);

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 */
    TransactionId *xip;         /* array of xact IDs in progress */
    uint32      xcnt;           /* # of xact ids in xip[] */
    /* note: all ids in xip[] satisfy xmin <= xip[i] < xmax */
    int32       subxcnt;        /* # of xact ids in subxip[] */
    TransactionId *subxip;      /* array of subxact IDs in progress */
    bool        suboverflowed;  /* has the subxip array overflowed? */
    bool        takenDuringRecovery;    /* recovery-shaped snapshot? */
    bool        copied;         /* false if it's a static snapshot */

    /*
     * note: all ids in subxip[] are >= xmin, but we don't bother filtering
     * out any that are >= xmax
     */
    CommandId   curcid;         /* in my xact, CID < curcid are visible */
    uint32      active_count;   /* refcount on ActiveSnapshot stack */
    uint32      regd_count;     /* refcount on RegisteredSnapshotList */
} 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
} HTSU_Result;

#endif   /* SNAPSHOT_H */