src/include/utils/snapshot.h

   1 /*-------------------------------------------------------------------------
   2  *
   3  * snapshot.h
   4  *        POSTGRES snapshot definition
   5  *
   6  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
   7  * Portions Copyright (c) 1994, Regents of the University of California
   8  *
   9  * $PostgreSQL$
  10  *
  11  *-------------------------------------------------------------------------
  12  */
  13 #ifndef SNAPSHOT_H
  14 #define SNAPSHOT_H
  15
  16 #include "access/htup.h"
  17 #include "storage/buf.h"
  18
  19
  20 typedef struct SnapshotData *Snapshot;
  21
  22 #define InvalidSnapshot         ((Snapshot) NULL)
  23
  24 /*
  25  * We use SnapshotData structures to represent both "regular" (MVCC)
  26  * snapshots and "special" snapshots that have non-MVCC semantics.
  27  * The specific semantics of a snapshot are encoded by the "satisfies"
  28  * function.
  29  */
  30 typedef bool (*SnapshotSatisfiesFunc) (HeapTupleHeader tuple,
  31                                                                                    Snapshot snapshot, Buffer buffer);
  32
  33 typedef struct SnapshotData
  34 {
  35         SnapshotSatisfiesFunc satisfies;        /* tuple test function */
  36
  37         /*
  38          * The remaining fields are used only for MVCC snapshots, and are normally
  39          * just zeroes in special snapshots.  (But xmin and xmax are used
  40          * specially by HeapTupleSatisfiesDirty.)
  41          *
  42          * An MVCC snapshot can never see the effects of XIDs >= xmax. It can see
  43          * the effects of all older XIDs except those listed in the snapshot. xmin
  44          * is stored as an optimization to avoid needing to search the XID arrays
  45          * for most tuples.
  46          */
  47         TransactionId xmin;                     /* all XID < xmin are visible to me */
  48         TransactionId xmax;                     /* all XID >= xmax are invisible to me */
  49         uint32          xcnt;                   /* # of xact ids in xip[] */
  50         TransactionId *xip;                     /* array of xact IDs in progress */
  51         /* note: all ids in xip[] satisfy xmin <= xip[i] < xmax */
  52         int32           subxcnt;                /* # of xact ids in subxip[], -1 if overflow */
  53         TransactionId *subxip;          /* array of subxact IDs in progress */
  54
  55         /*
  56          * note: all ids in subxip[] are >= xmin, but we don't bother filtering
  57          * out any that are >= xmax
  58          */
  59         CommandId       curcid;                 /* in my xact, CID < curcid are visible */
  60         uint32          active_count;   /* refcount on ActiveSnapshot stack */
  61         uint32          regd_count;             /* refcount on RegisteredSnapshotList */
  62         bool            copied;                 /* false if it's a static snapshot */
  63 } SnapshotData;
  64
  65 /*
  66  * Result codes for HeapTupleSatisfiesUpdate.  This should really be in
  67  * tqual.h, but we want to avoid including that file elsewhere.
  68  */
  69 typedef enum
  70 {
  71         HeapTupleMayBeUpdated,
  72         HeapTupleInvisible,
  73         HeapTupleSelfUpdated,
  74         HeapTupleUpdated,
  75         HeapTupleBeingUpdated
  76 } HTSU_Result;
  77
  78 #endif   /* SNAPSHOT_H */