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old_snapshot.h
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1 /*-------------------------------------------------------------------------
2  *
3  * old_snapshot.h
4  * Data structures for 'snapshot too old'
5  *
6  * Portions Copyright (c) 1996-2021, 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 */
72 
74 
75 #endif
#define PGDLLIMPORT
Definition: c.h:1318
#define FLEXIBLE_ARRAY_MEMBER
Definition: c.h:350
uint32 TransactionId
Definition: c.h:587
int64 TimestampTz
Definition: timestamp.h:39
struct OldSnapshotControlData OldSnapshotControlData
PGDLLIMPORT volatile OldSnapshotControlData * oldSnapshotControl
Definition: snapmgr.c:80
int slock_t
Definition: s_lock.h:958
TimestampTz next_map_update
Definition: old_snapshot.h:34
TimestampTz threshold_timestamp
Definition: old_snapshot.h:36
TransactionId latest_xmin
Definition: old_snapshot.h:33
TimestampTz head_timestamp
Definition: old_snapshot.h:68
TimestampTz current_timestamp
Definition: old_snapshot.h:31
TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER]
Definition: old_snapshot.h:70
TransactionId threshold_xid
Definition: old_snapshot.h:37