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Commitbf0d21e

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Don't update relfrozenxid if any pages were skipped.
Vacuum recognizes that it can update relfrozenxid by checking whether it hasprocessed all pages of a relation. Unfortunately it performed that checkafter truncating the dead pages at the end of the relation, and used the newnumber of pages to decide whether all pages have been scanned. If the newnumber of pages happened to be smaller or equal to the number of pagesscanned, it incorrectly decided that all pages were scanned.This can lead to relfrozenxid being updated, even though some pages wereskipped that still contain old XIDs. That can lead to data loss due to xidwraparounds with some rows suddenly missing. This likely has escaped noticeso far because it takes a large number (~2^31) of xids being used to see theeffect, while a full-table vacuum before that would fix the issue.The incorrect logic was introduced by commitb4b6923. Backpatch this fix down to 8.4,like that commit.Andres Freund, with some modifications by me.
1 parent9a1c6dd commitbf0d21e

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‎src/backend/commands/vacuumlazy.c‎

Lines changed: 18 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -166,7 +166,8 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
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BlockNumberpossibly_freeable;
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PGRUsageru0;
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TimestampTzstarttime=0;
169-
boolscan_all;
169+
boolscan_all;/* should we scan all pages? */
170+
boolscanned_all;/* did we actually scan all pages? */
170171
TransactionIdfreezeTableLimit;
171172
BlockNumbernew_rel_pages;
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doublenew_rel_tuples;
@@ -209,6 +210,21 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
209210
/* Done with indexes */
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vac_close_indexes(nindexes,Irel,NoLock);
211212

213+
/*
214+
* Compute whether we actually scanned the whole relation. If we did, we
215+
* can adjust relfrozenxid.
216+
*
217+
* NB: We need to check this before truncating the relation, because that
218+
* will change ->rel_pages.
219+
*/
220+
if (vacrelstats->scanned_pages<vacrelstats->rel_pages)
221+
{
222+
Assert(!scan_all);
223+
scanned_all= false;
224+
}
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else
226+
scanned_all= true;
227+
212228
/*
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* Optionally truncate the relation.
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*
@@ -245,9 +261,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
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new_rel_tuples=vacrelstats->old_rel_tuples;
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}
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248-
new_frozen_xid=FreezeLimit;
249-
if (vacrelstats->scanned_pages<vacrelstats->rel_pages)
250-
new_frozen_xid=InvalidTransactionId;
264+
new_frozen_xid=scanned_all ?FreezeLimit :InvalidTransactionId;
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252266
vac_update_relstats(onerel,
253267
new_rel_pages,new_rel_tuples,

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