Movatterモバイル変換

[0]ホーム
URL:

Skip to content

Navigation Menu

Sign in
Appearance settings

Search code, repositories, users, issues, pull requests...

Provide feedback

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly

 Sign up
Appearance settings

Commitd441cff

Browse files
committed
Fix traversal of half-frozen update chains
When some tuple versions in an update chain are frozen due to them beingolder than freeze_min_age, the xmax/xmin trail can become broken. Thisbreaks HOT (and probably other things). A subsequent VACUUM can breakthings in more serious ways, such as leaving orphan heap-only tupleswhose root HOT redirect items were removed. This can be seen becauseindex creation (or REINDEX) complain like ERROR: XX000: failed to find parent tuple for heap-only tuple at (0,7) in table "t"Because of relfrozenxid contraints, we cannot avoid the freezing of theearly tuples, so we must cope with the results: whenever we see an Xminof FrozenTransactionId, consider it a match for whatever the previousXmax value was.This problem seems to have appeared in 9.3 with multixact changes,though strictly speaking it seems unrelated.Since 9.4 we have commit37484ad "Change the way we mark tuples asfrozen", so the fix is simple: just compare the raw Xmin (still storedin the tuple header, since freezing merely set an infomask bit) to theXmax. But in 9.3 we rewrite the Xmin value to FrozenTransactionId, sothe original value is lost and we have nothing to compare the Xmax with.To cope with that case we need to compare the Xmin with FrozenXid,assume it's a match, and hope for the best. Sadly, since you canpg_upgrade a 9.3 instance containing half-frozen pages to newerreleases, we need to keep the old check in newer versions too, whichseems a bit brittle; I hope we can somehow get rid of that.I didn't optimize the new function for performance. The new coding isprobably a bit slower than before, since there is a function call ratherthan a straight comparison, but I'd rather have it work correctly thanbe fast but wrong.This is a followup after20b6552 fixed a few related problems.Apparently, in 9.6 and up there are more ways to get into trouble, butin 9.3 - 9.5 I cannot reproduce a problem anymore with this patch, sothere must be a separate bug.Reported-by: Peter GeogheganDiagnosed-by: Peter Geoghegan, Michael Paquier, Daniel Wood,Yi Wen Wong, ÁlvaroDiscussion:https://postgr.es/m/CAH2-Wznm4rCrhFAiwKPWTpEw2bXDtgROZK7jWWGucXeH3D1fmA@mail.gmail.com
1 parent9d742e1 commitd441cff

File tree

4 files changed

+54
-11
lines changed

4 files changed

+54
-11
lines changed

‎src/backend/access/heap/heapam.c

Lines changed: 47 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -2046,8 +2046,7 @@ heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
20462046
* broken.
20472047
*/
20482048
if (TransactionIdIsValid(prev_xmax)&&
2049-
!TransactionIdEquals(prev_xmax,
2050-
HeapTupleHeaderGetXmin(heapTuple->t_data)))
2049+
!HeapTupleUpdateXmaxMatchesXmin(prev_xmax,heapTuple->t_data))
20512050
break;
20522051

20532052
/*
@@ -2230,7 +2229,7 @@ heap_get_latest_tid(Relation relation,
22302229
* tuple. Check for XMIN match.
22312230
*/
22322231
if (TransactionIdIsValid(priorXmax)&&
2233-
!TransactionIdEquals(priorXmax,HeapTupleHeaderGetXmin(tp.t_data)))
2232+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax,tp.t_data))
22342233
{
22352234
UnlockReleaseBuffer(buffer);
22362235
break;
@@ -2262,6 +2261,50 @@ heap_get_latest_tid(Relation relation,
22622261
}/* end of loop */
22632262
}
22642263

2264+
/*
2265+
* HeapTupleUpdateXmaxMatchesXmin - verify update chain xmax/xmin lineage
2266+
*
2267+
* Given the new version of a tuple after some update, verify whether the
2268+
* given Xmax (corresponding to the previous version) matches the tuple's
2269+
* Xmin, taking into account that the Xmin might have been frozen after the
2270+
* update.
2271+
*/
2272+
bool
2273+
HeapTupleUpdateXmaxMatchesXmin(TransactionIdxmax,HeapTupleHeaderhtup)
2274+
{
2275+
TransactionIdxmin=HeapTupleHeaderGetXmin(htup);
2276+
2277+
/*
2278+
* If the xmax of the old tuple is identical to the xmin of the new one,
2279+
* it's a match.
2280+
*/
2281+
if (TransactionIdEquals(xmax,xmin))
2282+
return true;
2283+
2284+
/*
2285+
* If the Xmin that was in effect prior to a freeze matches the Xmax,
2286+
* it's good too.
2287+
*/
2288+
if (HeapTupleHeaderXminFrozen(htup)&&
2289+
TransactionIdEquals(HeapTupleHeaderGetRawXmin(htup),xmax))
2290+
return true;
2291+
2292+
/*
2293+
* When a tuple is frozen, the original Xmin is lost, but we know it's a
2294+
* committed transaction. So unless the Xmax is InvalidXid, we don't know
2295+
* for certain that there is a match, but there may be one; and we must
2296+
* return true so that a HOT chain that is half-frozen can be walked
2297+
* correctly.
2298+
*
2299+
* We no longer freeze tuples this way, but we must keep this in order to
2300+
* interpret pre-pg_upgrade pages correctly.
2301+
*/
2302+
if (TransactionIdEquals(xmin,FrozenTransactionId)&&
2303+
TransactionIdIsValid(xmax))
2304+
return true;
2305+
2306+
return false;
2307+
}
22652308

22662309
/*
22672310
* UpdateXmaxHintBits - update tuple hint bits after xmax transaction ends
@@ -5719,8 +5762,7 @@ heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
57195762
* end of the chain, we're done, so return success.
57205763
*/
57215764
if (TransactionIdIsValid(priorXmax)&&
5722-
!TransactionIdEquals(HeapTupleHeaderGetXmin(mytup.t_data),
5723-
priorXmax))
5765+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax,mytup.t_data))
57245766
{
57255767
result=HeapTupleMayBeUpdated;
57265768
gotoout_locked;

‎src/backend/access/heap/pruneheap.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -474,7 +474,7 @@ heap_prune_chain(Relation relation, Buffer buffer, OffsetNumber rootoffnum,
474474
* Check the tuple XMIN against prior XMAX, if any
475475
*/
476476
if (TransactionIdIsValid(priorXmax)&&
477-
!TransactionIdEquals(HeapTupleHeaderGetXmin(htup),priorXmax))
477+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax,htup))
478478
break;
479479

480480
/*
@@ -814,7 +814,7 @@ heap_get_root_tuples(Page page, OffsetNumber *root_offsets)
814814
htup= (HeapTupleHeader)PageGetItem(page,lp);
815815

816816
if (TransactionIdIsValid(priorXmax)&&
817-
!TransactionIdEquals(priorXmax,HeapTupleHeaderGetXmin(htup)))
817+
!HeapTupleUpdateXmaxMatchesXmin(priorXmax,htup))
818818
break;
819819

820820
/* Remember the root line pointer for this item */

‎src/backend/executor/execMain.c

Lines changed: 2 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -2278,8 +2278,7 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
22782278
* atomic, and Xmin never changes in an existing tuple, except to
22792279
* invalid or frozen, and neither of those can match priorXmax.)
22802280
*/
2281-
if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
2282-
priorXmax))
2281+
if (!HeapTupleUpdateXmaxMatchesXmin(priorXmax,tuple.t_data))
22832282
{
22842283
ReleaseBuffer(buffer);
22852284
returnNULL;
@@ -2426,8 +2425,7 @@ EvalPlanQualFetch(EState *estate, Relation relation, int lockmode,
24262425
/*
24272426
* As above, if xmin isn't what we're expecting, do nothing.
24282427
*/
2429-
if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
2430-
priorXmax))
2428+
if (!HeapTupleUpdateXmaxMatchesXmin(priorXmax,tuple.t_data))
24312429
{
24322430
ReleaseBuffer(buffer);
24332431
returnNULL;

‎src/include/access/heapam.h

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -145,6 +145,9 @@ extern void heap_get_latest_tid(Relation relation, Snapshot snapshot,
145145
ItemPointertid);
146146
externvoidsetLastTid(constItemPointertid);
147147

148+
externboolHeapTupleUpdateXmaxMatchesXmin(TransactionIdxmax,
149+
HeapTupleHeaderhtup);
150+
148151
externBulkInsertStateGetBulkInsertState(void);
149152
externvoidFreeBulkInsertState(BulkInsertState);
150153

0 commit comments

Comments
 (0)
[8]ページ先頭
©2009-2025 Movatter.jp