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Commit988ffc3

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Update "don't truncate with failsafe" rationale.
There is a very good (though non-obvious) reason to avoid relationtruncation during a VACUUM that has triggered the failsafe mechanism,which was missed before now. Update related comments, so this isn'tforgotten.Reported-By: John Naylor <john.naylor@enterprisedb.com>Discussion:https://postgr.es/m/CAFBsxsFiMPxQ-dHZ8tOgktn=+ffeJT3+GinZ4zdOGbmAnCYadA@mail.gmail.com
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‎src/backend/access/heap/vacuumlazy.c

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@@ -2793,8 +2793,13 @@ lazy_cleanup_one_index(Relation indrel, IndexBulkDeleteResult *istat,
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* an AccessExclusive lock must be replayed on any hot standby, where it can
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* be particularly disruptive.
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*
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* Also don't attempt it if wraparound failsafe is in effect. It's hard to
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* predict how long lazy_truncate_heap will take. Don't take any chances.
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* Also don't attempt it if wraparound failsafe is in effect. The entire
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* system might be refusing to allocate new XIDs at this point. The system
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* definitely won't return to normal unless and until VACUUM actually advances
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* the oldest relfrozenxid -- which hasn't happened for target rel just yet.
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* If lazy_truncate_heap attempted to acquire an AccessExclusiveLock to
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* truncate the table under these circumstances, an XID exhaustion error might
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* make it impossible for VACUUM to fix the underlying XID exhaustion problem.
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* There is very little chance of truncation working out when the failsafe is
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* in effect in any case. lazy_scan_prune makes the optimistic assumption
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* that any LP_DEAD items it encounters will always be LP_UNUSED by the time

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