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Commit 996d273

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Avoid consuming an XID during vac_truncate_clog().
vac_truncate_clog() uses its own transaction ID as the comparison point in a sanity check that no database's datfrozenxid has already wrapped around "into the future". That was probably fine when written, but in a lazy vacuum we won't have assigned an XID, so calling GetCurrentTransactionId() causes an XID to be assigned when otherwise one would not be. Most of the time that's not a big problem ... but if we are hard up against the wraparound limit, consuming XIDs during antiwraparound vacuums is a very bad thing. Instead, use ReadNewTransactionId(), which not only avoids this problem but is in itself a better comparison point to test whether wraparound has already occurred. Report and patch by Alexander Korotkov. Back-patch to all versions. Report: <CAPpHfdspOkmiQsxh-UZw2chM6dRMwXAJGEmmbmqYR=yvM7-s6A@mail.gmail.com>
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src/backend/commands/vacuum.c

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Original file line numberDiff line numberDiff line change
@@ -1051,7 +1051,7 @@ vac_truncate_clog(TransactionId frozenXID,
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TransactionId lastSaneFrozenXid,
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MultiXactId lastSaneMinMulti)
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{
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TransactionId myXID = GetCurrentTransactionId();
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TransactionId nextXID = ReadNewTransactionId();
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Relation relation;
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HeapScanDesc scan;
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HeapTuple tuple;
@@ -1100,7 +1100,7 @@ vac_truncate_clog(TransactionId frozenXID,
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MultiXactIdPrecedes(lastSaneMinMulti, dbform->datminmxid))
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bogus = true;
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if (TransactionIdPrecedes(myXID, dbform->datfrozenxid))
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if (TransactionIdPrecedes(nextXID, dbform->datfrozenxid))
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frozenAlreadyWrapped = true;
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else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
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{

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