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Commit8da3183

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Fix error when trying to delete page with half-dead left sibling.
The new page deletion code didn't cope with the case the target page'sright sibling was marked half-dead. It failed a sanity check which checkedthat the downlinks in the parent page match the lower level, because ahalf-dead page has no downlink. To cope, check for that condition, andjust give up on the deletion if it happens. The vacuum will finish thedeletion of the half-dead page when it gets there, and on the next vacuumafter that the empty can be deleted.Reported by Jeff Janes.
1 parent9a65fb3 commit8da3183

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+61
-4
lines changed

1 file changed

+61
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‎src/backend/access/nbtree/nbtpage.c

Lines changed: 61 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -954,18 +954,38 @@ _bt_delitems_delete(Relation rel, Buffer buf,
954954
END_CRIT_SECTION();
955955
}
956956

957+
/*
958+
* Returns true, if the given block has the half-dead flag set.
959+
*/
960+
staticbool
961+
_bt_is_page_halfdead(Relationrel,BlockNumberblk)
962+
{
963+
Bufferbuf;
964+
Pagepage;
965+
BTPageOpaqueopaque;
966+
boolresult;
967+
968+
buf=_bt_getbuf(rel,blk,BT_READ);
969+
page=BufferGetPage(buf);
970+
opaque= (BTPageOpaque)PageGetSpecialPointer(page);
971+
972+
result=P_ISHALFDEAD(opaque);
973+
_bt_relbuf(rel,buf);
974+
975+
returnresult;
976+
}
977+
957978
/*
958979
* Subroutine to find the parent of the branch we're deleting. This climbs
959980
* up the tree until it finds a page with more than one child, i.e. a page
960981
* that will not be totally emptied by the deletion. The chain of pages below
961-
* it, with one downlink each, will be part of the branch that we need to
962-
* delete.
982+
* it, with one downlink each, will form the branch that we need to delete.
963983
*
964984
* If we cannot remove the downlink from the parent, because it's the
965985
* rightmost entry, returns false. On success, *topparent and *topoff are set
966986
* to the buffer holding the parent, and the offset of the downlink in it.
967987
* *topparent is write-locked, the caller is responsible for releasing it when
968-
* done. *target is set to the topmost page in the branch to-be-deleted,ie.
988+
* done. *target is set to the topmost page in the branch to-be-deleted,i.e.
969989
* the page whose downlink *topparent / *topoff point to, and *rightsib to its
970990
* right sibling.
971991
*
@@ -994,7 +1014,10 @@ _bt_lock_branch_parent(Relation rel, BlockNumber child, BTStack stack,
9941014
BTPageOpaqueopaque;
9951015
BlockNumberleftsib;
9961016

997-
/* Locate the parent's downlink (updating the stack entry if needed) */
1017+
/*
1018+
* Locate the downlink of "child" in the parent (updating the stack entry
1019+
* if needed)
1020+
*/
9981021
ItemPointerSet(&(stack->bts_btentry.t_tid),child,P_HIKEY);
9991022
pbuf=_bt_getstackbuf(rel,stack,BT_WRITE);
10001023
if (pbuf==InvalidBuffer)
@@ -1066,6 +1089,17 @@ _bt_lock_branch_parent(Relation rel, BlockNumber child, BTStack stack,
10661089
_bt_relbuf(rel,lbuf);
10671090
}
10681091

1092+
/*
1093+
* Perform the same check on this internal level that
1094+
* _bt_mark_page_halfdead performed on the leaf level.
1095+
*/
1096+
if (_bt_is_page_halfdead(rel,*rightsib))
1097+
{
1098+
elog(DEBUG1,"could not delete page %u because its right sibling %u is half-dead",
1099+
parent,*rightsib);
1100+
return false;
1101+
}
1102+
10691103
return_bt_lock_branch_parent(rel,parent,stack->bts_parent,
10701104
topparent,topoff,target,rightsib);
10711105
}
@@ -1234,6 +1268,13 @@ _bt_pagedel(Relation rel, Buffer buf)
12341268
lbuf=_bt_getbuf(rel,leftsib,BT_READ);
12351269
lpage=BufferGetPage(lbuf);
12361270
lopaque= (BTPageOpaque)PageGetSpecialPointer(lpage);
1271+
/*
1272+
* If the left sibling is split again by another backend,
1273+
* after we released the lock, we know that the first
1274+
* split must have finished, because we don't allow an
1275+
* incompletely-split page to be split again. So we don't
1276+
* need to walk right here.
1277+
*/
12371278
if (lopaque->btpo_next==BufferGetBlockNumber(buf)&&
12381279
P_INCOMPLETE_SPLIT(lopaque))
12391280
{
@@ -1338,6 +1379,22 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
13381379
leafblkno=BufferGetBlockNumber(leafbuf);
13391380
leafrightsib=opaque->btpo_next;
13401381

1382+
/*
1383+
* Before attempting to lock the parent page, check that the right
1384+
* sibling is not in half-dead state. A half-dead right sibling would
1385+
* have no downlink in the parent, which would be highly confusing later
1386+
* when we delete the downlink that follows the current page's downlink.
1387+
* (I believe the deletion would work correctly, but it would fail the
1388+
* cross-check we make that the following downlink points to the right
1389+
* sibling of the delete page.)
1390+
*/
1391+
if (_bt_is_page_halfdead(rel,leafrightsib))
1392+
{
1393+
elog(DEBUG1,"could not delete page %u because its right sibling %u is half-dead",
1394+
leafblkno,leafrightsib);
1395+
return false;
1396+
}
1397+
13411398
/*
13421399
* We cannot delete a page that is the rightmost child of its immediate
13431400
* parent, unless it is the only child --- in which case the parent has to

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