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 */ 70 TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER]; 71 } OldSnapshotControlData; 72 73 extern PGDLLIMPORT volatile OldSnapshotControlData *oldSnapshotControl; 74 75 #endif 76