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Add comment explaining why it is safe to use FirstNormalTransactionXid for oldestActiveXid while replica startup #388

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Feb 22, 2024
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Add comment explaining why it is safe to use FirstNormalTransactionXid for oldestActiveXid while replica startup #388

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12 changes: 6 additions & 6 deletions src/backend/access/transam/xlog.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5194,13 +5194,13 @@ StartupXLOG(void)
else if (!TransactionIdIsValid(checkPoint.oldestActiveXid))
{
/*
* It should not actually happen: PS oldestActiveXid
* from running xacts WAL records and include it in checkpoint
* sent in basebackup.
* FirstNormalTransactionId is conservative estimation of oldest active XACT, unless
* current XID is greater than 1^31. So it is also not 100% safe solution but better than assertion failure.
* Pageserver extracts oldestActiveXid from snapshot and running xacts WAL records
* and include it in checkpoint sent in basebackup.
* So oldestActiveXid can be zero only after database initialization when no checkpoints are yet performed
* and not running xacts records was logged.
* In this case it is possible to use FirstNormalTransactionId as safe conservative estimation
* of oldest active transaction XID.
*/
elog(FATAL, "oldestActiveXid=%d", checkPoint.oldestActiveXid);
checkPoint.oldestActiveXid = FirstNormalTransactionId;
}
}
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