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Commit2cc41ac

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Fix hash index vs "snapshot too old" problemms
Hash indexes are not WAL-logged, and so do not maintain the LSN ofindex pages. Since the "snapshot too old" feature counts ondetecting error conditions using the LSN of a table and all indexeson it, this makes it impossible to safely do early vacuuming on anytable with a hash index, so add this to the tests for whether thexid used to vacuum a table can be adjusted based onold_snapshot_threshold.While at it, add a paragraph to the docs for old_snapshot_thresholdwhich specifically mentions this and other aspects of the featurewhich may otherwise surprise users.Problem reported and patch reviewed by Amit Kapila
1 parent9b66aa0 commit2cc41ac

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6 files changed

+62
-6
lines changed

6 files changed

+62
-6
lines changed

‎doc/src/sgml/config.sgml

Lines changed: 13 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2077,6 +2077,19 @@ include_dir 'conf.d'
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allowed, please note that in many workloads extreme bloat or
20782078
transaction ID wraparound may occur in much shorter time frames.
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</para>
2080+
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<para>
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This setting does not attempt to guarantee that an error will be
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generated under any particular circumstances. In fact, if the
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correct results can be generated from (for example) a cursor which
2085+
has materialized a result set, no error will be generated even if the
2086+
underlying rows in the referenced table have been vacuumed away.
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Some tables cannot safely be vacuumed early, and so will not be
2088+
affected by this setting. Examples include system catalogs and any
2089+
table which has a hash index. For such tables this setting will
2090+
neither reduce bloat nor create a possibility of a <literal>snapshot
2091+
too old</> error on scanning.
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</para>
20802093
</listitem>
20812094
</varlistentry>
20822095
</variablelist>

‎src/backend/access/hash/hash.c

Lines changed: 0 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -279,7 +279,6 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
279279
buf=so->hashso_curbuf;
280280
Assert(BufferIsValid(buf));
281281
page=BufferGetPage(buf);
282-
TestForOldSnapshot(scan->xs_snapshot,rel,page);
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maxoffnum=PageGetMaxOffsetNumber(page);
284283
for (offnum=ItemPointerGetOffsetNumber(current);
285284
offnum <=maxoffnum;

‎src/backend/access/hash/hashsearch.c

Lines changed: 0 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -189,7 +189,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
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/* Read the metapage */
190190
metabuf=_hash_getbuf(rel,HASH_METAPAGE,HASH_READ,LH_META_PAGE);
191191
page=BufferGetPage(metabuf);
192-
TestForOldSnapshot(scan->xs_snapshot,rel,page);
193192
metap=HashPageGetMeta(page);
194193

195194
/*
@@ -243,7 +242,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
243242
/* Fetch the primary bucket page for the bucket */
244243
buf=_hash_getbuf(rel,blkno,HASH_READ,LH_BUCKET_PAGE);
245244
page=BufferGetPage(buf);
246-
TestForOldSnapshot(scan->xs_snapshot,rel,page);
247245
opaque= (HashPageOpaque)PageGetSpecialPointer(page);
248246
Assert(opaque->hasho_bucket==bucket);
249247

@@ -350,7 +348,6 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
350348
_hash_readnext(rel,&buf,&page,&opaque);
351349
if (BufferIsValid(buf))
352350
{
353-
TestForOldSnapshot(scan->xs_snapshot,rel,page);
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maxoff=PageGetMaxOffsetNumber(page);
355352
offnum=_hash_binsearch(page,so->hashso_sk_hash);
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}
@@ -392,7 +389,6 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
392389
_hash_readprev(rel,&buf,&page,&opaque);
393390
if (BufferIsValid(buf))
394391
{
395-
TestForOldSnapshot(scan->xs_snapshot,rel,page);
396392
maxoff=PageGetMaxOffsetNumber(page);
397393
offnum=_hash_binsearch_last(page,so->hashso_sk_hash);
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}

‎src/backend/utils/cache/relcache.c

Lines changed: 46 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -5312,6 +5312,52 @@ RelationIdIsInInitFile(Oid relationId)
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returnRelationSupportsSysCache(relationId);
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}
53145314

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/*
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* Tells whether any index for the relation is unlogged.
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*
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* Any index using the hash AM is implicitly unlogged.
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*
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* Note: There doesn't seem to be any way to have an unlogged index attached
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* to a permanent table except to create a hash index, but it seems best to
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* keep this general so that it returns sensible results even when they seem
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* obvious (like for an unlogged table) and to handle possible future unlogged
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* indexes on permanent tables.
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*/
5326+
bool
5327+
RelationHasUnloggedIndex(Relationrel)
5328+
{
5329+
List*indexoidlist;
5330+
ListCell*indexoidscan;
5331+
boolresult= false;
5332+
5333+
indexoidlist=RelationGetIndexList(rel);
5334+
5335+
foreach(indexoidscan,indexoidlist)
5336+
{
5337+
Oidindexoid=lfirst_oid(indexoidscan);
5338+
HeapTupletp;
5339+
Form_pg_classreltup;
5340+
5341+
tp=SearchSysCache1(RELOID,ObjectIdGetDatum(indexoid));
5342+
if (!HeapTupleIsValid(tp))
5343+
elog(ERROR,"cache lookup failed for relation %u",indexoid);
5344+
reltup= (Form_pg_class)GETSTRUCT(tp);
5345+
5346+
if (reltup->relpersistence==RELPERSISTENCE_UNLOGGED
5347+
||reltup->relam==HASH_AM_OID)
5348+
result= true;
5349+
5350+
ReleaseSysCache(tp);
5351+
5352+
if (result== true)
5353+
break;
5354+
}
5355+
5356+
list_free(indexoidlist);
5357+
5358+
returnresult;
5359+
}
5360+
53155361
/*
53165362
* Invalidate (remove) the init file during commit of a transaction that
53175363
* changed one or more of the relation cache entries that are kept in the

‎src/backend/utils/time/snapmgr.c

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1590,7 +1590,8 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
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&&old_snapshot_threshold >=0
15911591
&&RelationNeedsWAL(relation)
15921592
&& !IsCatalogRelation(relation)
1593-
&& !RelationIsAccessibleInLogicalDecoding(relation))
1593+
&& !RelationIsAccessibleInLogicalDecoding(relation)
1594+
&& !RelationHasUnloggedIndex(relation))
15941595
{
15951596
int64ts=GetSnapshotCurrentTimestamp();
15961597
TransactionIdxlimit=recentXmin;

‎src/include/utils/rel.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -505,5 +505,6 @@ typedef struct ViewOptions
505505
/* routines in utils/cache/relcache.c */
506506
externvoidRelationIncrementReferenceCount(Relationrel);
507507
externvoidRelationDecrementReferenceCount(Relationrel);
508+
externboolRelationHasUnloggedIndex(Relationrel);
508509

509510
#endif/* REL_H */

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