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authorTom Lane <tgl@sss.pgh.pa.us>2007-09-16 02:37:46 +0000
committerTom Lane <tgl@sss.pgh.pa.us>2007-09-16 02:37:46 +0000
commit43b0c9182f5796f7368bc68eec08ab80c0421670 (patch)
tree9e6ece5c0566c34c767f27f0ada7d7e9ab725d7d /src
parent9a36a09f2e0e084a3983ae67bf3ebcd26245318d (diff)
downloadpostgresql-43b0c9182f5796f7368bc68eec08ab80c0421670.tar.gz
postgresql-43b0c9182f5796f7368bc68eec08ab80c0421670.zip
Fix aboriginal mistake in lazy VACUUM's code for truncating away
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.
Diffstat (limited to 'src')
-rw-r--r--src/backend/commands/vacuumlazy.c59
1 files changed, 14 insertions, 45 deletions
diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index 3dbd17de58d..3faf172acbf 100644
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -36,7 +36,7 @@
*
*
* 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
- * contain nothing we need to keep. This is *necessary*, not optional,
- * because other backends could have added tuples to these pages whilst we
- * were vacuuming.
+ * contain no tuples. This is *necessary*, not optional, because other
+ * backends could have added tuples to these pages whilst we were
+ * vacuuming.
*/
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).
*/
@@ -854,7 +854,6 @@ static BlockNumber
count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
{
BlockNumber blkno;
- HeapTupleData tuple;
/* Strange coding of loop control is needed because blkno is unsigned */
blkno = vacrelstats->rel_pages;
@@ -864,8 +863,7 @@ count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
Page page;
OffsetNumber offnum,
maxoff;
- bool tupgone,
- hastup;
+ bool hastup;
/*
* 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);
- 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
- * from relation.
- */
- break;
- 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)
+ /*
+ * Note: any non-unused item should be taken as a reason to keep
+ * 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.
+ */
+ if (ItemIdIsUsed(itemid))
{
hastup = true;
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
- * 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.
*/
return vacrelstats->nonempty_pages;