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Commit7136bf3

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Document LP_DEAD accounting issues in VACUUM.
Document VACUUM's soft assumption that any LP_DEAD items encounteredduring pruning will become LP_UNUSED items before VACUUM finishes up.This is integral to the accounting used by VACUUM to generate its finalreport on the table to the stats collector. It also affects how VACUUMdetermines which heap pages are truncatable. In both cases VACUUM isconcerned with the likely contents of the page in the near future, notthe current contents of the page.This state of affairs created the false impression that VACUUM's deadtuple accounting had significant difference with similar accounting usedduring ANALYZE. There were and are no substantive differences, at leastwhen the soft assumption completely works out. This is far clearer now.Also document cases where things don't quite work out for VACUUM's deadtuple accounting. It's possible that a significant number of LP_DEADitems will be left behind by VACUUM, and won't be recorded as remainingdead tuples in VACUUM's statistics collector report. This behaviordates back to commita96c41f, which taught VACUUM to run without indexand heap vacuuming at the user's request. The failsafe mechanism addedto VACUUM more recently by commit1e55e7d takes the same approach todead tuple accounting.Reported-By: Masahiko Sawada <sawada.mshk@gmail.com>Discussion:https://postgr.es/m/CAH2-Wz=Jmtu18PrsYq3EvvZJGOmZqSO2u3bvKpx9xJa5uhNp=Q@mail.gmail.com
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‎src/backend/access/heap/vacuumlazy.c

Lines changed: 42 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -686,7 +686,16 @@ heap_vacuum_rel(Relation rel, VacuumParams *params,
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new_min_multi,
687687
false);
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689-
/* report results to the stats collector, too */
689+
/*
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* Report results to the stats collector, too.
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*
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* Deliberately avoid telling the stats collector about LP_DEAD items that
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* remain in the table due to VACUUM bypassing index and heap vacuuming.
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* ANALYZE will consider the remaining LP_DEAD items to be dead tuples.
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* It seems like a good idea to err on the side of not vacuuming again too
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* soon in cases where the failsafe prevented significant amounts of heap
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* vacuuming.
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*/
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pgstat_report_vacuum(RelationGetRelid(rel),
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rel->rd_rel->relisshared,
692701
Max(new_live_tuples,0),
@@ -1334,6 +1343,9 @@ lazy_scan_heap(LVRelState *vacrel, VacuumParams *params, bool aggressive)
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*/
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lazy_scan_prune(vacrel,buf,blkno,page,vistest,&prunestate);
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1346+
Assert(!prunestate.all_visible|| !prunestate.has_lpdead_items);
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Assert(!all_visible_according_to_vm||prunestate.all_visible);
1348+
13371349
/* Remember the location of the last page with nonremovable tuples */
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if (prunestate.hastup)
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vacrel->nonempty_pages=blkno+1;
@@ -1404,7 +1416,6 @@ lazy_scan_heap(LVRelState *vacrel, VacuumParams *params, bool aggressive)
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* Handle setting visibility map bit based on what the VM said about
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* the page before pruning started, and using prunestate
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*/
1407-
Assert(!prunestate.all_visible|| !prunestate.has_lpdead_items);
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if (!all_visible_according_to_vm&&prunestate.all_visible)
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{
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uint8flags=VISIBILITYMAP_ALL_VISIBLE;
@@ -1786,6 +1797,14 @@ lazy_scan_prune(LVRelState *vacrel,
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* The logic here is a bit simpler than acquire_sample_rows(), as
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* VACUUM can't run inside a transaction block, which makes some cases
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* impossible (e.g. in-progress insert from the same transaction).
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*
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* We treat LP_DEAD items a little differently, too -- we don't count
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* them as dead_tuples at all (we only consider new_dead_tuples). The
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* outcome is no different because we assume that any LP_DEAD items we
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* encounter here will become LP_UNUSED inside lazy_vacuum_heap_page()
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* before we report anything to the stats collector. (Cases where we
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* bypass index vacuuming will violate our assumption, but the overall
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* impact of that should be negligible.)
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*/
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switch (res)
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{
@@ -1901,9 +1920,6 @@ lazy_scan_prune(LVRelState *vacrel,
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* that will need to be vacuumed in indexes later, or a LP_NORMAL tuple
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* that remains and needs to be considered for freezing now (LP_UNUSED and
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* LP_REDIRECT items also remain, but are of no further interest to us).
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*
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* Add page level counters to caller's counts, and then actually process
1906-
* LP_DEAD and LP_NORMAL items.
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*/
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vacrel->offnum=InvalidOffsetNumber;
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@@ -1988,13 +2004,6 @@ lazy_scan_prune(LVRelState *vacrel,
19882004
}
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#endif
19902006

1991-
/* Add page-local counts to whole-VACUUM counts */
1992-
vacrel->tuples_deleted+=tuples_deleted;
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vacrel->lpdead_items+=lpdead_items;
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vacrel->new_dead_tuples+=new_dead_tuples;
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vacrel->num_tuples+=num_tuples;
1996-
vacrel->live_tuples+=live_tuples;
1997-
19982007
/*
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* Now save details of the LP_DEAD items from the page in the dead_tuples
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* array. Also record that page has dead items in per-page prunestate.
@@ -2021,6 +2030,13 @@ lazy_scan_prune(LVRelState *vacrel,
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pgstat_progress_update_param(PROGRESS_VACUUM_NUM_DEAD_TUPLES,
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dead_tuples->num_tuples);
20232032
}
2033+
2034+
/* Finally, add page-local counts to whole-VACUUM counts */
2035+
vacrel->tuples_deleted+=tuples_deleted;
2036+
vacrel->lpdead_items+=lpdead_items;
2037+
vacrel->new_dead_tuples+=new_dead_tuples;
2038+
vacrel->num_tuples+=num_tuples;
2039+
vacrel->live_tuples+=live_tuples;
20242040
}
20252041

20262042
/*
@@ -2095,6 +2111,14 @@ lazy_vacuum(LVRelState *vacrel, bool onecall)
20952111
* not exceed 32MB. This limits the risk that we will bypass index
20962112
* vacuuming again and again until eventually there is a VACUUM whose
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* dead_tuples space is not CPU cache resident.
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*
2115+
* We don't take any special steps to remember the LP_DEAD items (such
2116+
* as counting them in new_dead_tuples report to the stats collector)
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* when the optimization is applied. Though the accounting used in
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* analyze.c's acquire_sample_rows() will recognize the same LP_DEAD
2119+
* items as dead rows in its own stats collector report, that's okay.
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* The discrepancy should be negligible. If this optimization is ever
2121+
* expanded to cover more cases then this may need to be reconsidered.
20982122
*/
20992123
threshold= (double)vacrel->rel_pages*BYPASS_THRESHOLD_PAGES;
21002124
do_bypass_optimization=
@@ -2146,7 +2170,8 @@ lazy_vacuum(LVRelState *vacrel, bool onecall)
21462170
}
21472171

21482172
/*
2149-
* Forget the now-vacuumed tuples -- just press on
2173+
* Forget the LP_DEAD items that we just vacuumed (or just decided to not
2174+
* vacuum)
21502175
*/
21512176
vacrel->dead_tuples->num_tuples=0;
21522177
}
@@ -3101,6 +3126,10 @@ lazy_cleanup_one_index(Relation indrel, IndexBulkDeleteResult *istat,
31013126
*
31023127
* Also don't attempt it if wraparound failsafe is in effect. It's hard to
31033128
* predict how long lazy_truncate_heap will take. Don't take any chances.
3129+
* There is very little chance of truncation working out when the failsafe is
3130+
* in effect in any case. lazy_scan_prune makes the optimistic assumption
3131+
* that any LP_DEAD items it encounters will always be LP_UNUSED by the time
3132+
* we're called.
31043133
*
31053134
* Also don't attempt it if we are doing early pruning/vacuuming, because a
31063135
* scan which cannot find a truncated heap page cannot determine that the

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