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