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Fix aboriginal mistake in lazy VACUUM's code for truncating away

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no-longer-needed pages at the end of a table.  We thought we could throw away
pages containing HEAPTUPLE_DEAD tuples; but this is not so, because such
tuples very likely have index entries pointing at them, and we wouldn't have
removed the index entries.  The problem only emerges in a somewhat unlikely
race condition: the dead tuples have to have been inserted by a transaction
that later aborted, and this has to have happened between VACUUM's initial
scan of the page and then rechecking it for empty in count_nondeletable_pages.
But that timespan will include an index-cleaning pass, so it's not all that
hard to hit.  This seems to explain a couple of previously unsolved bug
reports.
This commit is contained in:
Tom Lane 2007-09-16 02:37:46 +00:00
parent 9a36a09f2e
commit 43b0c9182f
1 changed files with 14 additions and 45 deletions
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55
src/backend/commands/vacuumlazy.c
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@ -36,7 +36,7 @@
* *
* *
* IDENTIFICATION * IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.95 2007/09/12 22:10:26 tgl Exp $ * $PostgreSQL: pgsql/src/backend/commands/vacuumlazy.c,v 1.96 2007/09/16 02:37:46 tgl Exp $
* *
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
@ -784,9 +784,9 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
/* /*
* Scan backwards from the end to verify that the end pages actually * Scan backwards from the end to verify that the end pages actually
* contain nothing we need to keep. This is *necessary*, not optional, * contain no tuples. This is *necessary*, not optional, because other
* because other backends could have added tuples to these pages whilst we * backends could have added tuples to these pages whilst we were
* were vacuuming. * vacuuming.
*/ */
new_rel_pages = count_nondeletable_pages(onerel, vacrelstats); new_rel_pages = count_nondeletable_pages(onerel, vacrelstats);
@ -846,7 +846,7 @@ lazy_truncate_heap(Relation onerel, LVRelStats *vacrelstats)
} }
/* /*
* Rescan end pages to verify that they are (still) empty of needed tuples. * Rescan end pages to verify that they are (still) empty of tuples.
* *
* Returns number of nondeletable pages (last nonempty page + 1). * Returns number of nondeletable pages (last nonempty page + 1).
*/ */
@ -854,7 +854,6 @@ static BlockNumber
count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats) count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
{ {
BlockNumber blkno; BlockNumber blkno;
HeapTupleData tuple;
/* Strange coding of loop control is needed because blkno is unsigned */ /* Strange coding of loop control is needed because blkno is unsigned */
blkno = vacrelstats->rel_pages; blkno = vacrelstats->rel_pages;
@ -864,8 +863,7 @@ count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
Page page; Page page;
OffsetNumber offnum, OffsetNumber offnum,
maxoff; maxoff;
bool tupgone, bool hastup;
hastup;
/* /*
* We don't insert a vacuum delay point here, because we have an * We don't insert a vacuum delay point here, because we have an
@ -901,42 +899,13 @@ count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
itemid = PageGetItemId(page, offnum); itemid = PageGetItemId(page, offnum);
if (!ItemIdIsUsed(itemid))
continue;
tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
tuple.t_len = ItemIdGetLength(itemid);
ItemPointerSet(&(tuple.t_self), blkno, offnum);
tupgone = false;
switch (HeapTupleSatisfiesVacuum(tuple.t_data, OldestXmin, buf))
{
case HEAPTUPLE_DEAD:
tupgone = true; /* we can delete the tuple */
break;
case HEAPTUPLE_LIVE:
/* Shouldn't be necessary to re-freeze anything */
break;
case HEAPTUPLE_RECENTLY_DEAD:
/* /*
* If tuple is recently deleted then we must not remove it * Note: any non-unused item should be taken as a reason to keep
* from relation. * this page. We formerly thought that DEAD tuples could be
* thrown away, but that's not so, because we'd not have cleaned
* out their index entries.
*/ */
break; if (ItemIdIsUsed(itemid))
case HEAPTUPLE_INSERT_IN_PROGRESS:
/* This is an expected case during concurrent vacuum */
break;
case HEAPTUPLE_DELETE_IN_PROGRESS:
/* This is an expected case during concurrent vacuum */
break;
default:
elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
break;
}
if (!tupgone)
{ {
hastup = true; hastup = true;
break; /* can stop scanning */ break; /* can stop scanning */
@ -952,7 +921,7 @@ count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
/* /*
* If we fall out of the loop, all the previously-thought-to-be-empty * If we fall out of the loop, all the previously-thought-to-be-empty
* pages really are; we need not bother to look at the last known-nonempty * pages still are; we need not bother to look at the last known-nonempty
* page. * page.
*/ */
return vacrelstats->nonempty_pages; return vacrelstats->nonempty_pages;