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Track the oldest XID that can be safely looked up in CLOG.

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This provides infrastructure for looking up arbitrary, user-supplied
XIDs without a risk of scary-looking failures from within the clog
module.  Normally, the oldest XID that can be safely looked up in CLOG
is the same as the oldest XID that can reused without causing
wraparound, and the latter is already tracked.  However, while
truncation is in progress, the values are different, so we must
keep track of them separately.

Craig Ringer, reviewed by Simon Riggs and by me.

Discussion: http://postgr.es/m/CAMsr+YHQiWNEi0daCTboS40T+V5s_+dst3PYv_8v2wNVH+Xx4g@mail.gmail.com
This commit is contained in:
Robert Haas
2017-03-23 14:08:23 -04:00
parent 50c956add8
commit ea42cc18c3
11 changed files with 102 additions and 18 deletions
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7
src/include/access/transam.h
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@@ -134,6 +134,12 @@ typedef struct VariableCacheData
*/
TransactionId latestCompletedXid; /* newest XID that has committed or
* aborted */
/*
* These fields are protected by CLogTruncationLock
*/
TransactionId oldestClogXid; /* oldest it's safe to look up in clog */
} VariableCacheData;
typedef VariableCacheData *VariableCache;
@@ -173,6 +179,7 @@ extern TransactionId GetNewTransactionId(bool isSubXact);
extern TransactionId ReadNewTransactionId(void);
extern void SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
Oid oldest_datoid);
extern void AdvanceOldestClogXid(TransactionId oldest_datfrozenxid);
extern bool ForceTransactionIdLimitUpdate(void);
extern Oid GetNewObjectId(void);