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Commitb0f7425

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Explain pruning pgstats accounting subtleties.
Add a comment explaining why the pgstats accounting used duringopportunistic heap pruning operations (to maintain the current number ofdead tuples in the relation) needs to compensate by subtracting away thenumber of new LP_DEAD items. This is needed so it can avoid completelyforgetting about tuples that become LP_DEAD items during pruning -- theyshould still count.It seems more natural to discuss this issue at the only relevant callsite (opportunistic pruning), since the same issue does not apply to theonly other caller (the VACUUM call site). Move everything there too.Author: Peter Geoghegan <pg@bowt.ie>Discussion:https://postgr.es/m/CAH2-Wzm7f+A6ej650gi_ifTgbhsadVW5cujAL3punpupHff5Yg@mail.gmail.com
1 parent05d8785 commitb0f7425

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3 files changed

+31
-18
lines changed

3 files changed

+31
-18
lines changed

‎src/backend/access/heap/pruneheap.c

Lines changed: 28 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -182,10 +182,29 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
182182
*/
183183
if (PageIsFull(page)||PageGetHeapFreeSpace(page)<minfree)
184184
{
185-
/* OK to prune */
186-
(void)heap_page_prune(relation,buffer,vistest,
187-
limited_xmin,limited_ts,
188-
true,NULL);
185+
intndeleted,
186+
nnewlpdead;
187+
188+
ndeleted=heap_page_prune(relation,buffer,vistest,limited_xmin,
189+
limited_ts,&nnewlpdead,NULL);
190+
191+
/*
192+
* Report the number of tuples reclaimed to pgstats. This is
193+
* ndeleted minus the number of newly-LP_DEAD-set items.
194+
*
195+
* We derive the number of dead tuples like this to avoid totally
196+
* forgetting about items that were set to LP_DEAD, since they
197+
* still need to be cleaned up by VACUUM. We only want to count
198+
* heap-only tuples that just became LP_UNUSED in our report,
199+
* which don't.
200+
*
201+
* VACUUM doesn't have to compensate in the same way when it
202+
* tracks ndeleted, since it will set the same LP_DEAD items to
203+
* LP_UNUSED separately.
204+
*/
205+
if (ndeleted>nnewlpdead)
206+
pgstat_update_heap_dead_tuples(relation,
207+
ndeleted-nnewlpdead);
189208
}
190209

191210
/* And release buffer lock */
@@ -212,10 +231,8 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
212231
* either have been set by TransactionIdLimitedForOldSnapshots, or
213232
* InvalidTransactionId/0 respectively.
214233
*
215-
* If report_stats is true then we send the number of reclaimed heap-only
216-
* tuples to pgstats. (This must be false during vacuum, since vacuum will
217-
* send its own new total to pgstats, and we don't want this delta applied
218-
* on top of that.)
234+
* Sets *nnewlpdead for caller, indicating the number of items that were
235+
* newly set LP_DEAD during prune operation.
219236
*
220237
* off_loc is the offset location required by the caller to use in error
221238
* callback.
@@ -227,7 +244,7 @@ heap_page_prune(Relation relation, Buffer buffer,
227244
GlobalVisState*vistest,
228245
TransactionIdold_snap_xmin,
229246
TimestampTzold_snap_ts,
230-
boolreport_stats,
247+
int*nnewlpdead,
231248
OffsetNumber*off_loc)
232249
{
233250
intndeleted=0;
@@ -381,13 +398,8 @@ heap_page_prune(Relation relation, Buffer buffer,
381398

382399
END_CRIT_SECTION();
383400

384-
/*
385-
* If requested, report the number of tuples reclaimed to pgstats. This is
386-
* ndeleted minus ndead, because we don't want to count a now-DEAD root
387-
* item as a deletion for this purpose.
388-
*/
389-
if (report_stats&&ndeleted>prstate.ndead)
390-
pgstat_update_heap_dead_tuples(relation,ndeleted-prstate.ndead);
401+
/* Record number of newly-set-LP_DEAD items for caller */
402+
*nnewlpdead=prstate.ndead;
391403

392404
returnndeleted;
393405
}

‎src/backend/access/heap/vacuumlazy.c

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1712,6 +1712,7 @@ lazy_scan_prune(LVRelState *vacrel,
17121712
new_dead_tuples,
17131713
num_tuples,
17141714
live_tuples;
1715+
intnnewlpdead;
17151716
intnfrozen;
17161717
OffsetNumberdeadoffsets[MaxHeapTuplesPerPage];
17171718
xl_heap_freeze_tuplefrozen[MaxHeapTuplesPerPage];
@@ -1737,7 +1738,7 @@ lazy_scan_prune(LVRelState *vacrel,
17371738
* that were deleted from indexes.
17381739
*/
17391740
tuples_deleted=heap_page_prune(rel,buf,vistest,
1740-
InvalidTransactionId,0,false,
1741+
InvalidTransactionId,0,&nnewlpdead,
17411742
&vacrel->offnum);
17421743

17431744
/*

‎src/include/access/heapam.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -186,7 +186,7 @@ extern intheap_page_prune(Relation relation, Buffer buffer,
186186
structGlobalVisState*vistest,
187187
TransactionIdold_snap_xmin,
188188
TimestampTzold_snap_ts_ts,
189-
boolreport_stats,
189+
int*nnewlpdead,
190190
OffsetNumber*off_loc);
191191
externvoidheap_page_prune_execute(Bufferbuffer,
192192
OffsetNumber*redirected,intnredirected,

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