Force VACUUM to recalculate oldestXmin even when we haven't changed our

own database's datfrozenxid, if the current value is old enough to be
forcing autovacuums or warning messages.  This ensures that a bogus
value is replaced as soon as possible.  Per a comment from Heikki.

3 files changed, 19 insertions, 9 deletions

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index f4dc2df048..f8e3f365a1 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -372,32 +372,41 @@ SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
 
 
 /*
- * TransactionIdLimitIsValid -- is the shared XID wrap-limit data sane?
+ * ForceTransactionIdLimitUpdate -- does the XID wrap-limit data need updating?
  *
  * We primarily check whether oldestXidDB is valid.  The cases we have in
  * mind are that that database was dropped, or the field was reset to zero
  * by pg_resetxlog.  In either case we should force recalculation of the
- * wrap limit.  In future we might add some more sanity checks here.
+ * wrap limit.  Also do it if oldestXid is old enough to be forcing
+ * autovacuums or other actions; this ensures we update our state as soon
+ * as possible once extra overhead is being incurred.
  */
 bool
-TransactionIdLimitIsValid(void)
+ForceTransactionIdLimitUpdate(void)
 {
+	TransactionId nextXid;
+	TransactionId xidVacLimit;
 	TransactionId oldestXid;
 	Oid			oldestXidDB;
 
 	/* Locking is probably not really necessary, but let's be careful */
 	LWLockAcquire(XidGenLock, LW_SHARED);
+	nextXid = ShmemVariableCache->nextXid;
+	xidVacLimit = ShmemVariableCache->xidVacLimit;
 	oldestXid = ShmemVariableCache->oldestXid;
 	oldestXidDB = ShmemVariableCache->oldestXidDB;
 	LWLockRelease(XidGenLock);
 
 	if (!TransactionIdIsNormal(oldestXid))
-		return false;			/* shouldn't happen, but just in case */
+		return true;			/* shouldn't happen, but just in case */
+	if (TransactionIdFollowsOrEquals(nextXid, xidVacLimit) &&
+		TransactionIdIsValid(xidVacLimit))
+		return true;			/* past VacLimit, don't delay updating */
 	if (!SearchSysCacheExists(DATABASEOID,
 							  ObjectIdGetDatum(oldestXidDB),
 							  0, 0, 0))
-		return false;			/* could happen, per comment above */
-	return true;
+		return true;			/* could happen, per comments above */
+	return false;
 }
 
 
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 1349de2f72..c9c9baad54 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -893,9 +893,10 @@ vac_update_datfrozenxid(void)
 
 	/*
 	 * If we were able to advance datfrozenxid, see if we can truncate pg_clog.
-	 * Also do it if the shared XID-wrap-limit info is stale.
+	 * Also do it if the shared XID-wrap-limit info is stale, since this
+	 * action will update that too.
 	 */
-	if (dirty || !TransactionIdLimitIsValid())
+	if (dirty || ForceTransactionIdLimitUpdate())
 		vac_truncate_clog(newFrozenXid);
 }
 
diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index 9b19a754e7..8ce30e3ec4 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -156,7 +156,7 @@ extern TransactionId GetNewTransactionId(bool isSubXact);
 extern TransactionId ReadNewTransactionId(void);
 extern void SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
 					  Oid oldest_datoid);
-extern bool TransactionIdLimitIsValid(void);
+extern bool ForceTransactionIdLimitUpdate(void);
 extern Oid	GetNewObjectId(void);
 
 #endif   /* TRAMSAM_H */