old_snapshot.h

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/*-------------------------------------------------------------------------
 *
 * old_snapshot.h
 *      Data structures for 'snapshot too old'
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/include/utils/old_snapshot.h
 *
 *-------------------------------------------------------------------------
 */
 
#ifndef OLD_SNAPSHOT_H
#define OLD_SNAPSHOT_H
 
#include "datatype/timestamp.h"
#include "storage/s_lock.h"
 
/*
 * Structure for dealing with old_snapshot_threshold implementation.
 */
typedef struct OldSnapshotControlData
{
    /*
     * Variables for old snapshot handling are shared among processes and are
     * only allowed to move forward.
     */
    slock_t     mutex_current;  /* protect current_timestamp */
    TimestampTz current_timestamp;  /* latest snapshot timestamp */
    slock_t     mutex_latest_xmin;  /* protect latest_xmin and next_map_update */
    TransactionId latest_xmin;  /* latest snapshot xmin */
    TimestampTz next_map_update;    /* latest snapshot valid up to */
    slock_t     mutex_threshold;    /* protect threshold fields */
    TimestampTz threshold_timestamp;    /* earlier snapshot is old */
    TransactionId threshold_xid;    /* earlier xid may be gone */
 
    /*
     * Keep one xid per minute for old snapshot error handling.
     *
     * Use a circular buffer with a head offset, a count of entries currently
     * used, and a timestamp corresponding to the xid at the head offset.  A
     * count_used value of zero means that there are no times stored; a
     * count_used value of OLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer
     * is full and the head must be advanced to add new entries.  Use
     * timestamps aligned to minute boundaries, since that seems less
     * surprising than aligning based on the first usage timestamp.  The
     * latest bucket is effectively stored within latest_xmin.  The circular
     * buffer is updated when we get a new xmin value that doesn't fall into
     * the same interval.
     *
     * It is OK if the xid for a given time slot is from earlier than
     * calculated by adding the number of minutes corresponding to the
     * (possibly wrapped) distance from the head offset to the time of the
     * head entry, since that just results in the vacuuming of old tuples
     * being slightly less aggressive.  It would not be OK for it to be off in
     * the other direction, since it might result in vacuuming tuples that are
     * still expected to be there.
     *
     * Use of an SLRU was considered but not chosen because it is more
     * heavyweight than is needed for this, and would probably not be any less
     * code to implement.
     *
     * Persistence is not needed.
     */
    int         head_offset;    /* subscript of oldest tracked time */
    TimestampTz head_timestamp; /* time corresponding to head xid */
    int         count_used;     /* how many slots are in use */
    TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
} OldSnapshotControlData;
 
extern PGDLLIMPORT volatile OldSnapshotControlData *oldSnapshotControl;
 
#endif