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Commit848ef42

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committed
Add the "snapshot too old" feature
This feature is controlled by a new old_snapshot_threshold GUC. Avalue of -1 disables the feature, and that is the default. Thevalue of 0 is just intended for testing. Above that it is thenumber of minutes a snapshot can reach before pruning and vacuumare allowed to remove dead tuples which the snapshot wouldotherwise protect. The xmin associated with a transaction ID doesstill protect dead tuples. A connection which is using an "old"snapshot does not get an error unless it accesses a page modifiedrecently enough that it might not be able to produce accurateresults.This is similar to the Oracle feature, and we use the same SQLSTATEand error message for compatibility.
1 parent8b65cf4 commit848ef42

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41 files changed

+942
-85
lines changed

‎contrib/bloom/blscan.c‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -138,7 +138,8 @@ blgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
138138
blkno,RBM_NORMAL,bas);
139139

140140
LockBuffer(buffer,BUFFER_LOCK_SHARE);
141-
page=BufferGetPage(buffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
141+
page=BufferGetPage(buffer,scan->xs_snapshot,scan->indexRelation,
142+
BGP_TEST_FOR_OLD_SNAPSHOT);
142143

143144
if (!BloomPageIsDeleted(page))
144145
{

‎doc/src/sgml/config.sgml‎

Lines changed: 50 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2041,6 +2041,42 @@ include_dir 'conf.d'
20412041
</para>
20422042
</listitem>
20432043
</varlistentry>
2044+
2045+
<varlistentry id="guc-old-snapshot-threshold" xreflabel="old_snapshot_threshold">
2046+
<term><varname>old_snapshot_threshold</varname> (<type>integer</type>)
2047+
<indexterm>
2048+
<primary><varname>old_snapshot_threshold</> configuration parameter</primary>
2049+
</indexterm>
2050+
</term>
2051+
<listitem>
2052+
<para>
2053+
Sets the minimum time that a snapshot can be used without risk of a
2054+
<literal>snapshot too old</> error occurring when using the snapshot.
2055+
This parameter can only be set at server start.
2056+
</para>
2057+
2058+
<para>
2059+
Beyond the threshold, old data may be vacuumed away. This can help
2060+
prevent bloat in the face of snapshots which remain in use for a
2061+
long time. To prevent incorrect results due to cleanup of data which
2062+
would otherwise be visible to the snapshot, an error is generated
2063+
when the snapshot is older than this threshold and the snapshot is
2064+
used to read a page which has been modified since the snapshot was
2065+
built.
2066+
</para>
2067+
2068+
<para>
2069+
A value of <literal>-1</> disables this feature, and is the default.
2070+
Useful values for production work probably range from a small number
2071+
of hours to a few days. The setting will be coerced to a granularity
2072+
of minutes, and small numbers (such as <literal>0</> or
2073+
<literal>1min</>) are only allowed because they may sometimes be
2074+
useful for testing. While a setting as high as <literal>60d</> is
2075+
allowed, please note that in many workloads extreme bloat or
2076+
transaction ID wraparound may occur in much shorter time frames.
2077+
</para>
2078+
</listitem>
2079+
</varlistentry>
20442080
</variablelist>
20452081
</sect2>
20462082
</sect1>
@@ -3051,6 +3087,10 @@ include_dir 'conf.d'
30513087
You should also consider setting <varname>hot_standby_feedback</>
30523088
on standby server(s) as an alternative to using this parameter.
30533089
</para>
3090+
<para>
3091+
This does not prevent cleanup of dead rows which have reached the age
3092+
specified by <varname>old_snapshot_threshold</>.
3093+
</para>
30543094
</listitem>
30553095
</varlistentry>
30563096

@@ -3198,6 +3238,16 @@ include_dir 'conf.d'
31983238
until it eventually reaches the primary. Standbys make no other use
31993239
of feedback they receive other than to pass upstream.
32003240
</para>
3241+
<para>
3242+
This setting does not override the behavior of
3243+
<varname>old_snapshot_threshold</> on the primary; a snapshot on the
3244+
standby which exceeds the primary's age threshold can become invalid,
3245+
resulting in cancellation of transactions on the standby. This is
3246+
because <varname>old_snapshot_threshold</> is intended to provide an
3247+
absolute limit on the time which dead rows can contribute to bloat,
3248+
which would otherwise be violated because of the configuration of a
3249+
standby.
3250+
</para>
32013251
</listitem>
32023252
</varlistentry>
32033253

‎src/backend/access/brin/brin.c‎

Lines changed: 11 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -135,7 +135,7 @@ brininsert(Relation idxRel, Datum *values, bool *nulls,
135135
MemoryContexttupcxt=NULL;
136136
MemoryContextoldcxt=NULL;
137137

138-
revmap=brinRevmapInitialize(idxRel,&pagesPerRange);
138+
revmap=brinRevmapInitialize(idxRel,&pagesPerRange,NULL);
139139

140140
for (;;)
141141
{
@@ -152,7 +152,7 @@ brininsert(Relation idxRel, Datum *values, bool *nulls,
152152
/* normalize the block number to be the first block in the range */
153153
heapBlk= (heapBlk /pagesPerRange)*pagesPerRange;
154154
brtup=brinGetTupleForHeapBlock(revmap,heapBlk,&buf,&off,NULL,
155-
BUFFER_LOCK_SHARE);
155+
BUFFER_LOCK_SHARE,NULL);
156156

157157
/* if range is unsummarized, there's nothing to do */
158158
if (!brtup)
@@ -285,7 +285,8 @@ brinbeginscan(Relation r, int nkeys, int norderbys)
285285
scan=RelationGetIndexScan(r,nkeys,norderbys);
286286

287287
opaque= (BrinOpaque*)palloc(sizeof(BrinOpaque));
288-
opaque->bo_rmAccess=brinRevmapInitialize(r,&opaque->bo_pagesPerRange);
288+
opaque->bo_rmAccess=brinRevmapInitialize(r,&opaque->bo_pagesPerRange,
289+
scan->xs_snapshot);
289290
opaque->bo_bdesc=brin_build_desc(r);
290291
scan->opaque=opaque;
291292

@@ -368,7 +369,8 @@ bringetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
368369
MemoryContextResetAndDeleteChildren(perRangeCxt);
369370

370371
tup=brinGetTupleForHeapBlock(opaque->bo_rmAccess,heapBlk,&buf,
371-
&off,&size,BUFFER_LOCK_SHARE);
372+
&off,&size,BUFFER_LOCK_SHARE,
373+
scan->xs_snapshot);
372374
if (tup)
373375
{
374376
tup=brin_copy_tuple(tup,size);
@@ -647,7 +649,7 @@ brinbuild(Relation heap, Relation index, IndexInfo *indexInfo)
647649
/*
648650
* Initialize our state, including the deformed tuple state.
649651
*/
650-
revmap=brinRevmapInitialize(index,&pagesPerRange);
652+
revmap=brinRevmapInitialize(index,&pagesPerRange,NULL);
651653
state=initialize_brin_buildstate(index,revmap,pagesPerRange);
652654

653655
/*
@@ -1045,7 +1047,8 @@ summarize_range(IndexInfo *indexInfo, BrinBuildState *state, Relation heapRel,
10451047
* the same.)
10461048
*/
10471049
phtup=brinGetTupleForHeapBlock(state->bs_rmAccess,heapBlk,&phbuf,
1048-
&offset,&phsz,BUFFER_LOCK_SHARE);
1050+
&offset,&phsz,BUFFER_LOCK_SHARE,
1051+
NULL);
10491052
/* the placeholder tuple must exist */
10501053
if (phtup==NULL)
10511054
elog(ERROR,"missing placeholder tuple");
@@ -1080,7 +1083,7 @@ brinsummarize(Relation index, Relation heapRel, double *numSummarized,
10801083
BlockNumberpagesPerRange;
10811084
Bufferbuf;
10821085

1083-
revmap=brinRevmapInitialize(index,&pagesPerRange);
1086+
revmap=brinRevmapInitialize(index,&pagesPerRange,NULL);
10841087

10851088
/*
10861089
* Scan the revmap to find unsummarized items.
@@ -1095,7 +1098,7 @@ brinsummarize(Relation index, Relation heapRel, double *numSummarized,
10951098
CHECK_FOR_INTERRUPTS();
10961099

10971100
tup=brinGetTupleForHeapBlock(revmap,heapBlk,&buf,&off,NULL,
1098-
BUFFER_LOCK_SHARE);
1101+
BUFFER_LOCK_SHARE,NULL);
10991102
if (tup==NULL)
11001103
{
11011104
/* no revmap entry for this heap range. Summarize it. */

‎src/backend/access/brin/brin_revmap.c‎

Lines changed: 7 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -68,7 +68,8 @@ static void revmap_physical_extend(BrinRevmap *revmap);
6868
* brinRevmapTerminate when caller is done with it.
6969
*/
7070
BrinRevmap*
71-
brinRevmapInitialize(Relationidxrel,BlockNumber*pagesPerRange)
71+
brinRevmapInitialize(Relationidxrel,BlockNumber*pagesPerRange,
72+
Snapshotsnapshot)
7273
{
7374
BrinRevmap*revmap;
7475
Buffermeta;
@@ -77,7 +78,7 @@ brinRevmapInitialize(Relation idxrel, BlockNumber *pagesPerRange)
7778

7879
meta=ReadBuffer(idxrel,BRIN_METAPAGE_BLKNO);
7980
LockBuffer(meta,BUFFER_LOCK_SHARE);
80-
page=BufferGetPage(meta,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
81+
page=BufferGetPage(meta,snapshot,idxrel,BGP_TEST_FOR_OLD_SNAPSHOT);
8182
metadata= (BrinMetaPageData*)PageGetContents(page);
8283

8384
revmap=palloc(sizeof(BrinRevmap));
@@ -187,7 +188,8 @@ brinSetHeapBlockItemptr(Buffer buf, BlockNumber pagesPerRange,
187188
*/
188189
BrinTuple*
189190
brinGetTupleForHeapBlock(BrinRevmap*revmap,BlockNumberheapBlk,
190-
Buffer*buf,OffsetNumber*off,Size*size,intmode)
191+
Buffer*buf,OffsetNumber*off,Size*size,intmode,
192+
Snapshotsnapshot)
191193
{
192194
RelationidxRel=revmap->rm_irel;
193195
BlockNumbermapBlk;
@@ -264,7 +266,8 @@ brinGetTupleForHeapBlock(BrinRevmap *revmap, BlockNumber heapBlk,
264266
*buf=ReadBuffer(idxRel,blk);
265267
}
266268
LockBuffer(*buf,mode);
267-
page=BufferGetPage(*buf,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
269+
page=BufferGetPage(*buf,snapshot,idxRel,
270+
BGP_TEST_FOR_OLD_SNAPSHOT);
268271

269272
/* If we land on a revmap page, start over */
270273
if (BRIN_IS_REGULAR_PAGE(page))

‎src/backend/access/gin/ginbtree.c‎

Lines changed: 5 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -71,7 +71,7 @@ ginTraverseLock(Buffer buffer, bool searchMode)
7171
* is share-locked, and stack->parent is NULL.
7272
*/
7373
GinBtreeStack*
74-
ginFindLeafPage(GinBtreebtree,boolsearchMode)
74+
ginFindLeafPage(GinBtreebtree,boolsearchMode,Snapshotsnapshot)
7575
{
7676
GinBtreeStack*stack;
7777

@@ -89,7 +89,8 @@ ginFindLeafPage(GinBtree btree, bool searchMode)
8989

9090
stack->off=InvalidOffsetNumber;
9191

92-
page=BufferGetPage(stack->buffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
92+
page=BufferGetPage(stack->buffer,snapshot,btree->index,
93+
BGP_TEST_FOR_OLD_SNAPSHOT);
9394

9495
access=ginTraverseLock(stack->buffer,searchMode);
9596

@@ -115,8 +116,8 @@ ginFindLeafPage(GinBtree btree, bool searchMode)
115116

116117
stack->buffer=ginStepRight(stack->buffer,btree->index,access);
117118
stack->blkno=rightlink;
118-
page=BufferGetPage(stack->buffer,NULL,NULL,
119-
BGP_NO_SNAPSHOT_TEST);
119+
page=BufferGetPage(stack->buffer,snapshot,btree->index,
120+
BGP_TEST_FOR_OLD_SNAPSHOT);
120121

121122
if (!searchMode&&GinPageIsIncompleteSplit(page))
122123
ginFinishSplit(btree,stack, false,NULL);

‎src/backend/access/gin/gindatapage.c‎

Lines changed: 4 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1820,7 +1820,7 @@ ginInsertItemPointers(Relation index, BlockNumber rootBlkno,
18201820
{
18211821
/* search for the leaf page where the first item should go to */
18221822
btree.itemptr=insertdata.items[insertdata.curitem];
1823-
stack=ginFindLeafPage(&btree, false);
1823+
stack=ginFindLeafPage(&btree, false,NULL);
18241824

18251825
ginInsertValue(&btree,stack,&insertdata,buildStats);
18261826
}
@@ -1830,15 +1830,16 @@ ginInsertItemPointers(Relation index, BlockNumber rootBlkno,
18301830
* Starts a new scan on a posting tree.
18311831
*/
18321832
GinBtreeStack*
1833-
ginScanBeginPostingTree(GinBtreebtree,Relationindex,BlockNumberrootBlkno)
1833+
ginScanBeginPostingTree(GinBtreebtree,Relationindex,BlockNumberrootBlkno,
1834+
Snapshotsnapshot)
18341835
{
18351836
GinBtreeStack*stack;
18361837

18371838
ginPrepareDataScan(btree,index,rootBlkno);
18381839

18391840
btree->fullScan= TRUE;
18401841

1841-
stack=ginFindLeafPage(btree, TRUE);
1842+
stack=ginFindLeafPage(btree, TRUE,snapshot);
18421843

18431844
returnstack;
18441845
}

‎src/backend/access/gin/ginget.c‎

Lines changed: 12 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -73,7 +73,7 @@ scanPostingTree(Relation index, GinScanEntry scanEntry,
7373
Pagepage;
7474

7575
/* Descend to the leftmost leaf page */
76-
stack=ginScanBeginPostingTree(&btree,index,rootPostingTree);
76+
stack=ginScanBeginPostingTree(&btree,index,rootPostingTree,snapshot);
7777
buffer=stack->buffer;
7878
IncrBufferRefCount(buffer);/* prevent unpin in freeGinBtreeStack */
7979

@@ -146,7 +146,8 @@ collectMatchBitmap(GinBtreeData *btree, GinBtreeStack *stack,
146146
if (moveRightIfItNeeded(btree,stack)== false)
147147
return true;
148148

149-
page=BufferGetPage(stack->buffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
149+
page=BufferGetPage(stack->buffer,snapshot,btree->index,
150+
BGP_TEST_FOR_OLD_SNAPSHOT);
150151
itup= (IndexTuple)PageGetItem(page,PageGetItemId(page,stack->off));
151152

152153
/*
@@ -320,7 +321,7 @@ startScanEntry(GinState *ginstate, GinScanEntry entry, Snapshot snapshot)
320321
ginPrepareEntryScan(&btreeEntry,entry->attnum,
321322
entry->queryKey,entry->queryCategory,
322323
ginstate);
323-
stackEntry=ginFindLeafPage(&btreeEntry, true);
324+
stackEntry=ginFindLeafPage(&btreeEntry, true,snapshot);
324325
page=BufferGetPage(stackEntry->buffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
325326
needUnlock= TRUE;
326327

@@ -385,7 +386,7 @@ startScanEntry(GinState *ginstate, GinScanEntry entry, Snapshot snapshot)
385386
needUnlock= FALSE;
386387

387388
stack=ginScanBeginPostingTree(&entry->btree,ginstate->index,
388-
rootPostingTree);
389+
rootPostingTree,snapshot);
389390
entry->buffer=stack->buffer;
390391

391392
/*
@@ -627,7 +628,7 @@ entryLoadMoreItems(GinState *ginstate, GinScanEntry entry,
627628
entry->btree.itemptr.ip_posid++;
628629
}
629630
entry->btree.fullScan= false;
630-
stack=ginFindLeafPage(&entry->btree, true);
631+
stack=ginFindLeafPage(&entry->btree, true,snapshot);
631632

632633
/* we don't need the stack, just the buffer. */
633634
entry->buffer=stack->buffer;
@@ -1335,8 +1336,8 @@ scanGetCandidate(IndexScanDesc scan, pendingPosition *pos)
13351336
ItemPointerSetInvalid(&pos->item);
13361337
for (;;)
13371338
{
1338-
page=BufferGetPage(pos->pendingBuffer,NULL,
1339-
NULL,BGP_NO_SNAPSHOT_TEST);
1339+
page=BufferGetPage(pos->pendingBuffer,scan->xs_snapshot,
1340+
scan->indexRelation,BGP_TEST_FOR_OLD_SNAPSHOT);
13401341

13411342
maxoff=PageGetMaxOffsetNumber(page);
13421343
if (pos->firstOffset>maxoff)
@@ -1516,8 +1517,8 @@ collectMatchesForHeapRow(IndexScanDesc scan, pendingPosition *pos)
15161517
memset(datumExtracted+pos->firstOffset-1,0,
15171518
sizeof(bool)* (pos->lastOffset-pos->firstOffset));
15181519

1519-
page=BufferGetPage(pos->pendingBuffer,NULL,
1520-
NULL,BGP_NO_SNAPSHOT_TEST);
1520+
page=BufferGetPage(pos->pendingBuffer,scan->xs_snapshot,
1521+
scan->indexRelation,BGP_TEST_FOR_OLD_SNAPSHOT);
15211522

15221523
for (i=0;i<so->nkeys;i++)
15231524
{
@@ -1710,7 +1711,8 @@ scanPendingInsert(IndexScanDesc scan, TIDBitmap *tbm, int64 *ntids)
17101711
*ntids=0;
17111712

17121713
LockBuffer(metabuffer,GIN_SHARE);
1713-
page=BufferGetPage(metabuffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
1714+
page=BufferGetPage(metabuffer,scan->xs_snapshot,scan->indexRelation,
1715+
BGP_TEST_FOR_OLD_SNAPSHOT);
17141716
blkno=GinPageGetMeta(page)->head;
17151717

17161718
/*

‎src/backend/access/gin/gininsert.c‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -192,7 +192,7 @@ ginEntryInsert(GinState *ginstate,
192192

193193
ginPrepareEntryScan(&btree,attnum,key,category,ginstate);
194194

195-
stack=ginFindLeafPage(&btree, false);
195+
stack=ginFindLeafPage(&btree, false,NULL);
196196
page=BufferGetPage(stack->buffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
197197

198198
if (btree.findItem(&btree,stack))

‎src/backend/access/gist/gistget.c‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -336,7 +336,7 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
336336
buffer=ReadBuffer(scan->indexRelation,pageItem->blkno);
337337
LockBuffer(buffer,GIST_SHARE);
338338
gistcheckpage(scan->indexRelation,buffer);
339-
page=BufferGetPage(buffer,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
339+
page=BufferGetPage(buffer,scan->xs_snapshot,r,BGP_TEST_FOR_OLD_SNAPSHOT);
340340
opaque=GistPageGetOpaque(page);
341341

342342
/*

‎src/backend/access/hash/hash.c‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -278,7 +278,8 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
278278

279279
buf=so->hashso_curbuf;
280280
Assert(BufferIsValid(buf));
281-
page=BufferGetPage(buf,NULL,NULL,BGP_NO_SNAPSHOT_TEST);
281+
page=BufferGetPage(buf,scan->xs_snapshot,rel,
282+
BGP_TEST_FOR_OLD_SNAPSHOT);
282283
maxoffnum=PageGetMaxOffsetNumber(page);
283284
for (offnum=ItemPointerGetOffsetNumber(current);
284285
offnum <=maxoffnum;

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