src/include/utils/old_snapshot.h

   1 /*-------------------------------------------------------------------------
   2  *
   3  * old_snapshot.h
   4  *              Data structures for 'snapshot too old'
   5  *
   6  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
   7  * Portions Copyright (c) 1994, Regents of the University of California
   8  *
   9  * IDENTIFICATION
  10  *        src/include/utils/old_snapshot.h
  11  *
  12  *-------------------------------------------------------------------------
  13  */
  14
  15 #ifndef OLD_SNAPSHOT_H
  16 #define OLD_SNAPSHOT_H
  17
  18 #include "datatype/timestamp.h"
  19 #include "storage/s_lock.h"
  20
  21 /*
  22  * Structure for dealing with old_snapshot_threshold implementation.
  23  */
  24 typedef struct OldSnapshotControlData
  25 {
  26         /*
  27          * Variables for old snapshot handling are shared among processes and are
  28          * only allowed to move forward.
  29          */
  30         slock_t         mutex_current;  /* protect current_timestamp */
  31         TimestampTz current_timestamp;  /* latest snapshot timestamp */
  32         slock_t         mutex_latest_xmin;      /* protect latest_xmin and next_map_update */
  33         TransactionId latest_xmin;      /* latest snapshot xmin */
  34         TimestampTz next_map_update;    /* latest snapshot valid up to */
  35         slock_t         mutex_threshold;        /* protect threshold fields */
  36         TimestampTz threshold_timestamp;        /* earlier snapshot is old */
  37         TransactionId threshold_xid;    /* earlier xid may be gone */
  38
  39         /*
  40          * Keep one xid per minute for old snapshot error handling.
  41          *
  42          * Use a circular buffer with a head offset, a count of entries currently
  43          * used, and a timestamp corresponding to the xid at the head offset.  A
  44          * count_used value of zero means that there are no times stored; a
  45          * count_used value of OLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer
  46          * is full and the head must be advanced to add new entries.  Use
  47          * timestamps aligned to minute boundaries, since that seems less
  48          * surprising than aligning based on the first usage timestamp.  The
  49          * latest bucket is effectively stored within latest_xmin.  The circular
  50          * buffer is updated when we get a new xmin value that doesn't fall into
  51          * the same interval.
  52          *
  53          * It is OK if the xid for a given time slot is from earlier than
  54          * calculated by adding the number of minutes corresponding to the
  55          * (possibly wrapped) distance from the head offset to the time of the
  56          * head entry, since that just results in the vacuuming of old tuples
  57          * being slightly less aggressive.  It would not be OK for it to be off in
  58          * the other direction, since it might result in vacuuming tuples that are
  59          * still expected to be there.
  60          *
  61          * Use of an SLRU was considered but not chosen because it is more
  62          * heavyweight than is needed for this, and would probably not be any less
  63          * code to implement.
  64          *
  65          * Persistence is not needed.
  66          */
  67         int                     head_offset;    /* subscript of oldest tracked time */
  68         TimestampTz head_timestamp; /* time corresponding to head xid */
  69         int                     count_used;             /* how many slots are in use */
  70         TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
  71 } OldSnapshotControlData;
  72
  73 extern PGDLLIMPORT volatile OldSnapshotControlData *oldSnapshotControl;
  74
  75 #endif