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Adjust "root of to-be-deleted subtree" function.

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Restructure the function that locates the root of the to-be-deleted
subtree during nbtree page deletion.  Handle the conditions that make
page deletion unsafe in a slightly more uniform way, and acknowledge the
fact that the behavior with incomplete splits on internal pages is
different (as pointed out in the nbtree README as of commit 35bc0ec7).
Also invent new terminology that avoids ambiguity around which pages are
about to be deleted.  Consistently use the term "to-be-deleted subtree",
not the ambiguous term "branch".

We were calling the subtree parent page the "top parent page", but that
was quite misleading.  The top parent page usually refers to a page
unlinked from its siblings and marked deleted (during the second stage
of page deletion).  There was one kind of top parent page that we merely
removed a downlink from, and another kind of top parent page that we
actually marked deleted.  Eliminate the ambiguity by inventing a new
term ("subtree parent page") that refers to the former kind of page
only.
This commit is contained in:
Peter Geoghegan
2020-05-11 11:01:07 -07:00
parent a8be5364ac
commit 624686abcf
5 changed files with 278 additions and 215 deletions
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14
src/backend/access/nbtree/README
View File

@ -223,9 +223,10 @@ right to make this happen --- a scan moving in the opposite direction
might miss the items if so.) Also, we *never* delete the rightmost page
on a tree level (this restriction simplifies the traversal algorithms, as
explained below). Page deletion always begins from an empty leaf page. An
internal page can only be deleted as part of a branch leading to a leaf
page, where each internal page has only one child and that child is also to
be deleted.
internal page can only be deleted as part of deleting an entire subtree.
This is always a "skinny" subtree consisting of a "chain" of internal pages
plus a single leaf page. There is one page on each level of the subtree,
and each level/page covers the same key space.
Deleting a leaf page is a two-stage process. In the first stage, the page
is unlinked from its parent, and marked as half-dead. The parent page must
@ -243,7 +244,12 @@ it, but it's still linked to its siblings.
(Note: Lanin and Shasha prefer to make the key space move left, but their
argument for doing so hinges on not having left-links, which we have
anyway. So we simplify the algorithm by moving the key space right.)
anyway. So we simplify the algorithm by moving the key space right. This
is only possible because we don't match on a separator key when ascending
the tree during a page split, unlike Lehman and Yao/Lanin and Shasha -- it
doesn't matter if the downlink is re-found in a pivot tuple whose separator
key does not match the one encountered when inserter initially descended
the tree.)
To preserve consistency on the parent level, we cannot merge the key space
of a page into its right sibling unless the right sibling is a child of

3
src/backend/access/nbtree/nbtinsert.c
View File

@ -2278,7 +2278,8 @@ _bt_finish_split(Relation rel, Buffer lbuf, BTStack stack)
* stack. For example, the checkingunique _bt_doinsert() case may
* have to step right when there are many physical duplicates, and its
* scantid forces an insertion to the right of the "first page the
* value could be on".
* value could be on". (This is also relied on by all of our callers
* when dealing with !heapkeyspace indexes.)
*
* Returns write-locked parent page buffer, or InvalidBuffer if pivot
* tuple not found (should not happen). Adjusts bts_blkno &

462
src/backend/access/nbtree/nbtpage.c
View File

@ -39,9 +39,6 @@ static void _bt_log_reuse_page(Relation rel, BlockNumber blkno,
TransactionId latestRemovedXid);
static TransactionId _bt_xid_horizon(Relation rel, Relation heapRel, Page page,
OffsetNumber *deletable, int ndeletable);
static bool _bt_lock_branch_parent(Relation rel, BlockNumber child,
BTStack stack, Buffer *topparent, OffsetNumber *topoff,
BlockNumber *target, BlockNumber *rightsib);
static bool _bt_mark_page_halfdead(Relation rel, Buffer leafbuf,
BTStack stack);
static bool _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf,
@ -49,6 +46,12 @@ static bool _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf,
bool *rightsib_empty,
TransactionId *oldestBtpoXact,
uint32 *ndeleted);
static bool _bt_lock_subtree_parent(Relation rel, BlockNumber child,
BTStack stack,
Buffer *subtreeparent,
OffsetNumber *poffset,
BlockNumber *topparent,
BlockNumber *topparentrightsib);
/*
* _bt_initmetapage() -- Fill a page buffer with a correct metapage image
@ -1316,13 +1319,16 @@ _bt_xid_horizon(Relation rel, Relation heapRel, Page page,
/*
* Check that leftsib page (the btpo_prev of target page) is not marked with
* INCOMPLETE_SPLIT flag.
* INCOMPLETE_SPLIT flag. Used during page deletion.
*
* Returning true indicates that page flag is set in leftsib (which is
* definitely still the left sibling of target). When that happens, the
* target doesn't have a downlink in parent, and the page deletion algorithm
* isn't prepared to handle that. Deletion of the target page (or the whole
* subtree that contains the target page) cannot take place.
*
* Caller should not have a lock on the target page itself, since pages on the
* same level must always be locked left to right to avoid deadlocks.
*/
static bool
_bt_leftsib_splitflag(Relation rel, BlockNumber leftsib, BlockNumber target)
@ -1356,7 +1362,7 @@ _bt_leftsib_splitflag(Relation rel, BlockNumber leftsib, BlockNumber target)
/*
* Check that leafrightsib page (the btpo_next of target leaf page) is not
* marked with ISHALFDEAD flag.
* marked with ISHALFDEAD flag. Used during page deletion.
*
* Returning true indicates that page flag is set in leafrightsib, so page
* deletion cannot go ahead. Our caller is not prepared to deal with the case
@ -1402,121 +1408,6 @@ _bt_rightsib_halfdeadflag(Relation rel, BlockNumber leafrightsib)
return result;
}
/*
* Subroutine to find the parent of the branch we're deleting. This climbs
* up the tree until it finds a page with more than one child, i.e. a page
* that will not be totally emptied by the deletion. The chain of pages below
* it, with one downlink each, will form the branch that we need to delete.
*
* If we cannot remove the downlink from the parent, because it's the
* rightmost entry, returns false. On success, *topparent and *topoff are set
* to the buffer holding the parent, and the offset of the downlink in it.
* *topparent is write-locked, the caller is responsible for releasing it when
* done. *target is set to the topmost page in the branch to-be-deleted, i.e.
* the page whose downlink *topparent / *topoff point to, and *rightsib to its
* right sibling.
*
* "child" is the leaf page we wish to delete, and "stack" is a search stack
* leading to it (it actually leads to the leftmost leaf page with a high key
* matching that of the page to be deleted in !heapkeyspace indexes). Note
* that we will update the stack entry(s) to reflect current downlink
* positions --- this is essentially the same as the corresponding step of
* splitting, and is not expected to affect caller. The caller should
* initialize *target and *rightsib to the leaf page and its right sibling.
*
* Note: it's OK to release page locks on any internal pages between the leaf
* and *topparent, because a safe deletion can't become unsafe due to
* concurrent activity. An internal page can only acquire an entry if the
* child is split, but that cannot happen as long as we hold a lock on the
* leaf.
*/
static bool
_bt_lock_branch_parent(Relation rel, BlockNumber child, BTStack stack,
Buffer *topparent, OffsetNumber *topoff,
BlockNumber *target, BlockNumber *rightsib)
{
BlockNumber parent;
OffsetNumber poffset,
maxoff;
Buffer pbuf;
Page page;
BTPageOpaque opaque;
/*
* Locate the downlink of "child" in the parent, updating the stack entry
* if needed. This is how !heapkeyspace indexes deal with having
* non-unique high keys in leaf level pages. Even heapkeyspace indexes
* can have a stale stack due to insertions into the parent.
*/
pbuf = _bt_getstackbuf(rel, stack, child);
if (pbuf == InvalidBuffer)
ereport(ERROR,
(errcode(ERRCODE_INDEX_CORRUPTED),
errmsg_internal("failed to re-find parent key in index \"%s\" for deletion target page %u",
RelationGetRelationName(rel), child)));
parent = stack->bts_blkno;
poffset = stack->bts_offset;
page = BufferGetPage(pbuf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
maxoff = PageGetMaxOffsetNumber(page);
/*
* If the target is the rightmost child of its parent, then we can't
* delete, unless it's also the only child.
*/
Assert(poffset <= maxoff);
if (poffset >= maxoff)
{
/* It's rightmost child... */
if (poffset == P_FIRSTDATAKEY(opaque))
{
BlockNumber leftsibparent;
/*
* It's only child, so safe if parent would itself be removable.
* We have to check the parent itself, and then recurse to test
* the conditions at the parent's parent.
*/
if (P_RIGHTMOST(opaque) || P_ISROOT(opaque) ||
P_INCOMPLETE_SPLIT(opaque))
{
_bt_relbuf(rel, pbuf);
return false;
}
*target = parent;
*rightsib = opaque->btpo_next;
leftsibparent = opaque->btpo_prev;
_bt_relbuf(rel, pbuf);
/*
* Check that the left sibling of parent (if any) is not marked
* with INCOMPLETE_SPLIT flag before proceeding
*/
if (_bt_leftsib_splitflag(rel, leftsibparent, parent))
return false;
return _bt_lock_branch_parent(rel, parent, stack->bts_parent,
topparent, topoff, target, rightsib);
}
else
{
/* Unsafe to delete */
_bt_relbuf(rel, pbuf);
return false;
}
}
else
{
/* Not rightmost child, so safe to delete */
*topparent = pbuf;
*topoff = poffset;
return true;
}
}
/*
* _bt_pagedel() -- Delete a leaf page from the b-tree, if legal to do so.
*
@ -1582,7 +1473,7 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
/*
* Internal pages are never deleted directly, only as part of deleting
* the whole branch all the way down to leaf level.
* the whole subtree all the way down to leaf level.
*
* Also check for deleted pages here. Caller never passes us a fully
* deleted page. Only VACUUM can delete pages, so there can't have
@ -1655,8 +1546,8 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
/*
* First, remove downlink pointing to the page (or a parent of the
* page, if we are going to delete a taller branch), and mark the page
* as half-dead.
* page, if we are going to delete a taller subtree), and mark the
* leafbuf page half-dead
*/
if (!P_ISHALFDEAD(opaque))
{
@ -1675,14 +1566,14 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
BTScanInsert itup_key;
ItemId itemid;
IndexTuple targetkey;
BlockNumber leftsib, target;
Buffer lbuf;
BlockNumber leftsib, leafblkno;
Buffer sleafbuf;
itemid = PageGetItemId(page, P_HIKEY);
targetkey = CopyIndexTuple((IndexTuple) PageGetItem(page, itemid));
leftsib = opaque->btpo_prev;
target = BufferGetBlockNumber(leafbuf);
leafblkno = BufferGetBlockNumber(leafbuf);
/*
* To avoid deadlocks, we'd better drop the leaf page lock
@ -1694,8 +1585,8 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
* Check that the left sibling of leafbuf (if any) is not
* marked with INCOMPLETE_SPLIT flag before proceeding
*/
Assert(target == scanblkno);
if (_bt_leftsib_splitflag(rel, leftsib, target))
Assert(leafblkno == scanblkno);
if (_bt_leftsib_splitflag(rel, leftsib, leafblkno))
{
ReleaseBuffer(leafbuf);
return ndeleted;
@ -1705,9 +1596,11 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
itup_key = _bt_mkscankey(rel, targetkey);
/* find the leftmost leaf page with matching pivot/high key */
itup_key->pivotsearch = true;
stack = _bt_search(rel, itup_key, &lbuf, BT_READ, NULL);
stack = _bt_search(rel, itup_key, &sleafbuf, BT_READ, NULL);
/* won't need a second lock or pin on leafbuf */
_bt_relbuf(rel, lbuf);
Assert(leafblkno == BufferGetBlockNumber(sleafbuf) ||
!itup_key->heapkeyspace);
_bt_relbuf(rel, sleafbuf);
/*
* Re-lock the leaf page, and start over to use our stack
@ -1736,7 +1629,7 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
/*
* Then unlink it from its siblings. Each call to
* _bt_unlink_halfdead_page unlinks the topmost page from the branch,
* _bt_unlink_halfdead_page unlinks the topmost page from the subtree,
* making it shallower. Iterate until the leafbuf page is deleted.
*
* _bt_unlink_halfdead_page should never fail, since we established
@ -1795,21 +1688,31 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
}
/*
* First stage of page deletion. Remove the downlink to the top of the
* branch being deleted, and mark the leaf page as half-dead.
* First stage of page deletion.
*
* Establish the height of the to-be-deleted subtree with leafbuf at its
* lowest level, remove the downlink to the subtree, and mark leafbuf
* half-dead. The final to-be-deleted subtree is usually just leafbuf itself,
* but may include additional internal pages (at most one per level of the
* tree below the root).
*
* Returns 'false' if leafbuf is unsafe to delete, usually because leafbuf is
* the rightmost child of its parent (and parent has more than one downlink).
* Returns 'true' when the first stage of page deletion completed
* successfully.
*/
static bool
_bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
{
BlockNumber leafblkno;
BlockNumber leafrightsib;
BlockNumber target;
BlockNumber rightsib;
BlockNumber topparent;
BlockNumber topparentrightsib;
ItemId itemid;
Page page;
BTPageOpaque opaque;
Buffer topparent;
OffsetNumber topoff;
Buffer subtreeparent;
OffsetNumber poffset;
OffsetNumber nextoffset;
IndexTuple itup;
IndexTupleData trunctuple;
@ -1817,8 +1720,8 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
page = BufferGetPage(leafbuf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
Assert(!P_RIGHTMOST(opaque) && !P_ISROOT(opaque) && !P_ISDELETED(opaque) &&
!P_ISHALFDEAD(opaque) && P_ISLEAF(opaque) &&
Assert(!P_RIGHTMOST(opaque) && !P_ISROOT(opaque) &&
P_ISLEAF(opaque) && !P_IGNORE(opaque) &&
P_FIRSTDATAKEY(opaque) > PageGetMaxOffsetNumber(page));
/*
@ -1847,47 +1750,57 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
* parent, unless it is the only child --- in which case the parent has to
* be deleted too, and the same condition applies recursively to it. We
* have to check this condition all the way up before trying to delete,
* and lock the final parent of the to-be-deleted subtree.
* and lock the parent of the root of the to-be-deleted subtree (the
* "subtree parent"). _bt_lock_subtree_parent() locks the subtree parent
* for us. We remove the downlink to the "top parent" page (subtree root
* page) from the subtree parent page below.
*
* Initialize topparent to be leafbuf page now. The final to-be-deleted
* subtree is often a degenerate one page subtree consisting only of the
* leafbuf page. When that happens, the leafbuf page is the final subtree
* root page/top parent page.
*/
rightsib = leafrightsib;
target = leafblkno;
if (!_bt_lock_branch_parent(rel, leafblkno, stack,
&topparent, &topoff, &target, &rightsib))
topparent = leafblkno;
topparentrightsib = leafrightsib;
if (!_bt_lock_subtree_parent(rel, leafblkno, stack,
&subtreeparent, &poffset,
&topparent, &topparentrightsib))
return false;
/*
* Check that the parent-page index items we're about to delete/overwrite
* contain what we expect. This can fail if the index has become corrupt
* for some reason. We want to throw any error before entering the
* critical section --- otherwise it'd be a PANIC.
*
* The test on the target item is just an Assert because
* _bt_lock_branch_parent should have guaranteed it has the expected
* contents. The test on the next-child downlink is known to sometimes
* fail in the field, though.
* in subtree parent page contain what we expect. This can fail if the
* index has become corrupt for some reason. We want to throw any error
* before entering the critical section --- otherwise it'd be a PANIC.
*/
page = BufferGetPage(topparent);
page = BufferGetPage(subtreeparent);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
#ifdef USE_ASSERT_CHECKING
itemid = PageGetItemId(page, topoff);
/*
* This is just an assertion because _bt_lock_subtree_parent should have
* guaranteed tuple has the expected contents
*/
itemid = PageGetItemId(page, poffset);
itup = (IndexTuple) PageGetItem(page, itemid);
Assert(BTreeTupleGetDownLink(itup) == target);
Assert(BTreeTupleGetDownLink(itup) == topparent);
#endif
nextoffset = OffsetNumberNext(topoff);
nextoffset = OffsetNumberNext(poffset);
itemid = PageGetItemId(page, nextoffset);
itup = (IndexTuple) PageGetItem(page, itemid);
if (BTreeTupleGetDownLink(itup) != rightsib)
if (BTreeTupleGetDownLink(itup) != topparentrightsib)
ereport(ERROR,
(errcode(ERRCODE_INDEX_CORRUPTED),
errmsg_internal("right sibling %u of block %u is not next child %u of block %u in index \"%s\"",
rightsib, target, BTreeTupleGetDownLink(itup),
BufferGetBlockNumber(topparent), RelationGetRelationName(rel))));
(errcode(ERRCODE_INDEX_CORRUPTED),
errmsg_internal("right sibling %u of block %u is not next child %u of block %u in index \"%s\"",
topparentrightsib, topparent,
BTreeTupleGetDownLink(itup),
BufferGetBlockNumber(subtreeparent),
RelationGetRelationName(rel))));
/*
* Any insert which would have gone on the leaf block will now go to its
* right sibling.
* right sibling. In other words, the key space moves right.
*/
PredicateLockPageCombine(rel, leafblkno, leafrightsib);
@ -1895,25 +1808,31 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
START_CRIT_SECTION();
/*
* Update parent. The normal case is a tad tricky because we want to
* delete the target's downlink and the *following* key. Easiest way is
* to copy the right sibling's downlink over the target downlink, and then
* delete the following item.
* Update parent of subtree. We want to delete the downlink to the top
* parent page/root of the subtree, and the *following* key. Easiest way
* is to copy the right sibling's downlink over the downlink that points
* to top parent page, and then delete the right sibling's original pivot
* tuple.
*
* Lanin and Shasha make the key space move left when deleting a page,
* whereas the key space moves right here. That's why we cannot simply
* delete the pivot tuple with the downlink to the top parent page. See
* nbtree/README.
*/
page = BufferGetPage(topparent);
page = BufferGetPage(subtreeparent);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
itemid = PageGetItemId(page, topoff);
itemid = PageGetItemId(page, poffset);
itup = (IndexTuple) PageGetItem(page, itemid);
BTreeTupleSetDownLink(itup, rightsib);
BTreeTupleSetDownLink(itup, topparentrightsib);
nextoffset = OffsetNumberNext(topoff);
nextoffset = OffsetNumberNext(poffset);
PageIndexTupleDelete(page, nextoffset);
/*
* Mark the leaf page as half-dead, and stamp it with a pointer to the
* highest internal page in the branch we're deleting. We use the tid of
* the high key to store it.
* Mark the leaf page as half-dead, and stamp it with a link to the top
* parent page. When the leaf page is also the top parent page, the link
* is set to InvalidBlockNumber.
*/
page = BufferGetPage(leafbuf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@ -1922,8 +1841,8 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
Assert(PageGetMaxOffsetNumber(page) == P_HIKEY);
MemSet(&trunctuple, 0, sizeof(IndexTupleData));
trunctuple.t_info = sizeof(IndexTupleData);
if (target != leafblkno)
BTreeTupleSetTopParent(&trunctuple, target);
if (topparent != leafblkno)
BTreeTupleSetTopParent(&trunctuple, topparent);
else
BTreeTupleSetTopParent(&trunctuple, InvalidBlockNumber);
@ -1932,7 +1851,7 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
elog(ERROR, "could not overwrite high key in half-dead page");
/* Must mark buffers dirty before XLogInsert */
MarkBufferDirty(topparent);
MarkBufferDirty(subtreeparent);
MarkBufferDirty(leafbuf);
/* XLOG stuff */
@ -1941,16 +1860,16 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
xl_btree_mark_page_halfdead xlrec;
XLogRecPtr recptr;
xlrec.poffset = topoff;
xlrec.poffset = poffset;
xlrec.leafblk = leafblkno;
if (target != leafblkno)
xlrec.topparent = target;
if (topparent != leafblkno)
xlrec.topparent = topparent;
else
xlrec.topparent = InvalidBlockNumber;
XLogBeginInsert();
XLogRegisterBuffer(0, leafbuf, REGBUF_WILL_INIT);
XLogRegisterBuffer(1, topparent, REGBUF_STANDARD);
XLogRegisterBuffer(1, subtreeparent, REGBUF_STANDARD);
page = BufferGetPage(leafbuf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@ -1961,7 +1880,7 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_MARK_PAGE_HALFDEAD);
page = BufferGetPage(topparent);
page = BufferGetPage(subtreeparent);
PageSetLSN(page, recptr);
page = BufferGetPage(leafbuf);
PageSetLSN(page, recptr);
@ -1969,17 +1888,21 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
END_CRIT_SECTION();
_bt_relbuf(rel, topparent);
_bt_relbuf(rel, subtreeparent);
return true;
}
/*
* Unlink a page in a branch of half-dead pages from its siblings.
* Second stage of page deletion.
*
* If the leaf page still has a downlink pointing to it, unlinks the highest
* parent in the to-be-deleted branch instead of the leaf page. To get rid
* of the whole branch, including the leaf page itself, iterate until the
* leaf page is deleted.
* Unlinks a single page (in the subtree undergoing deletion) from its
* siblings. Also marks the page deleted.
*
* To get rid of the whole subtree, including the leaf page itself, call here
* until the leaf page is deleted. The original "top parent" established in
* the first stage of deletion is deleted in the first call here, while the
* leaf page is deleted in the last call here. Note that the leaf page itself
* is often the initial top parent page.
*
* Returns 'false' if the page could not be unlinked (shouldn't happen). If
* the right sibling of the current target page is empty, *rightsib_empty is
@ -2028,7 +1951,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
page = BufferGetPage(leafbuf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
Assert(P_ISLEAF(opaque) && P_ISHALFDEAD(opaque));
Assert(P_ISLEAF(opaque) && !P_ISDELETED(opaque) && P_ISHALFDEAD(opaque));
/*
* Remember some information about the leaf page.
@ -2047,14 +1970,10 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
*/
CHECK_FOR_INTERRUPTS();
/*
* If the leaf page still has a parent pointing to it (or a chain of
* parents), we don't unlink the leaf page yet, but the topmost remaining
* parent in the branch (i.e. the "top parent")
*/
/* Unlink the current top parent of the subtree */
if (!BlockNumberIsValid(target))
{
/* No top parent, so target is leaf page */
/* Target is leaf page (or leaf page is top parent, if you prefer) */
target = leafblkno;
buf = leafbuf;
@ -2063,7 +1982,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
}
else
{
/* Target is the internal page taken from leaf's top parent */
/* Target is the internal page taken from leaf's top parent link */
Assert(target != leafblkno);
/* Fetch the block number of the target's left sibling */
@ -2155,10 +2074,9 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
* only one vacuum process running at a time.
*/
if (P_RIGHTMOST(opaque) || P_ISROOT(opaque) || P_ISDELETED(opaque))
{
elog(ERROR, "half-dead page changed status unexpectedly in block %u of index \"%s\"",
target, RelationGetRelationName(rel));
}
if (opaque->btpo_prev != leftsib)
ereport(ERROR,
(errcode(ERRCODE_INDEX_CORRUPTED),
@ -2180,7 +2098,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
elog(ERROR, "half-dead page changed status unexpectedly in block %u of index \"%s\"",
target, RelationGetRelationName(rel));
/* remember the next non-leaf child down in the branch. */
/* Remember the next non-leaf child down in the subtree */
itemid = PageGetItemId(page, P_FIRSTDATAKEY(opaque));
nextchild = BTreeTupleGetDownLink((IndexTuple) PageGetItem(page, itemid));
if (nextchild == leafblkno)
@ -2265,7 +2183,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
/*
* If we deleted a parent of the targeted leaf page, instead of the leaf
* itself, update the leaf to point to the next remaining child in the
* branch.
* subtree.
*
* Note: We rely on the fact that a buffer pin on the leaf page has been
* held since leafhikey was initialized. This is safe, though only
@ -2406,12 +2324,150 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
if (target <= scanblkno)
(*ndeleted)++;
/*
* Release the target, if it was not the leaf block. The leaf is always
* kept locked.
*/
/* If the target is not leafbuf, we're done with it now -- release it */
if (target != leafblkno)
_bt_relbuf(rel, buf);
return true;
}
/*
* Establish how tall the to-be-deleted subtree will be during the first stage
* of page deletion.
*
* Caller's child argument is the block number of the page caller wants to
* delete (this is leafbuf's block number, except when we're called
* recursively). stack is a search stack leading to it. Note that we will
* update the stack entry(s) to reflect current downlink positions --- this is
* similar to the corresponding point in page split handling.
*
* If "first stage" caller cannot go ahead with deleting _any_ pages, returns
* false. Returns true on success, in which case caller can use certain
* details established here to perform the first stage of deletion. This
* function is the last point at which page deletion may be deemed unsafe
* (barring index corruption, or unexpected concurrent page deletions).
*
* We write lock the parent of the root of the to-be-deleted subtree for
* caller on success (i.e. we leave our lock on the *subtreeparent buffer for
* caller). Caller will have to remove a downlink from *subtreeparent. We
* also set a *subtreeparent offset number in *poffset, to indicate the
* location of the pivot tuple that contains the relevant downlink.
*
* The root of the to-be-deleted subtree is called the "top parent". Note
* that the leafbuf page is often the final "top parent" page (you can think
* of the leafbuf page as a degenerate single page subtree when that happens).
* Caller should initialize *topparent to the target leafbuf page block number
* (while *topparentrightsib should be set to leafbuf's right sibling block
* number). We will update *topparent (and *topparentrightsib) for caller
* here, though only when it turns out that caller will delete at least one
* internal page (i.e. only when caller needs to store a valid link to the top
* parent block in the leafbuf page using BTreeTupleSetTopParent()).
*/
static bool
_bt_lock_subtree_parent(Relation rel, BlockNumber child, BTStack stack,
Buffer *subtreeparent, OffsetNumber *poffset,
BlockNumber *topparent, BlockNumber *topparentrightsib)
{
BlockNumber parent, leftsibparent;
OffsetNumber parentoffset,
maxoff;
Buffer pbuf;
Page page;
BTPageOpaque opaque;
/*
* Locate the pivot tuple whose downlink points to "child". Write lock
* the parent page itself.
*/
pbuf = _bt_getstackbuf(rel, stack, child);
if (pbuf == InvalidBuffer)
ereport(ERROR,
(errcode(ERRCODE_INDEX_CORRUPTED),
errmsg_internal("failed to re-find parent key in index \"%s\" for deletion target page %u",
RelationGetRelationName(rel), child)));
parent = stack->bts_blkno;
parentoffset = stack->bts_offset;
page = BufferGetPage(pbuf);
opaque = (BTPageOpaque) PageGetSpecialPointer(page);
maxoff = PageGetMaxOffsetNumber(page);
leftsibparent = opaque->btpo_prev;
/*
* _bt_getstackbuf() completes page splits on returned parent buffer when
* required.
*
* In general it's a bad idea for VACUUM to use up more disk space, which
* is why page deletion does not finish incomplete page splits most of the
* time. We allow this limited exception because the risk is much lower,
* and the potential downside of not proceeding is much higher: A single
* internal page with the INCOMPLETE_SPLIT flag set might otherwise
* prevent us from deleting hundreds of empty leaf pages from one level
* down.
*/
Assert(!P_INCOMPLETE_SPLIT(opaque));
if (parentoffset < maxoff)
{
/*
* Child is not the rightmost child in parent, so it's safe to delete
* the subtree whose root/topparent is child page
*/
*subtreeparent = pbuf;
*poffset = parentoffset;
return true;
}
/*
* Child is the rightmost child of parent.
*
* Since it's the rightmost child of parent, deleting the child (or
* deleting the subtree whose root/topparent is the child page) is only
* safe when it's also possible to delete the parent.
*/
Assert(parentoffset == maxoff);
if (parentoffset != P_FIRSTDATAKEY(opaque) || P_RIGHTMOST(opaque))
{
/*
* Child isn't parent's only child, or parent is rightmost on its
* entire level. Definitely cannot delete any pages.
*/
_bt_relbuf(rel, pbuf);
return false;
}
/*
* Now make sure that the parent deletion is itself safe by examining the
* child's grandparent page. Recurse, passing the parent page as the
* child page (child's grandparent is the parent on the next level up).
* If parent deletion is unsafe, then child deletion must also be unsafe
* (in which case caller cannot delete any pages at all).
*/
*topparent = parent;
*topparentrightsib = opaque->btpo_next;
/*
* Release lock on parent before recursing.
*
* It's OK to release page locks on parent before recursive call locks
* grandparent. An internal page can only acquire an entry if the child
* is split, but that cannot happen as long as we still hold a lock on the
* leafbuf page.
*/
_bt_relbuf(rel, pbuf);
/*
* Before recursing, check that the left sibling of parent (if any) is not
* marked with INCOMPLETE_SPLIT flag first (must do so after we drop the
* parent lock).
*
* Note: We deliberately avoid completing incomplete splits here.
*/
if (_bt_leftsib_splitflag(rel, leftsibparent, parent))
return false;
/* Recurse to examine child page's grandparent page */
return _bt_lock_subtree_parent(rel, parent, stack->bts_parent,
subtreeparent, poffset,
topparent, topparentrightsib);
}

8
src/backend/access/nbtree/nbtxlog.c
View File

@ -704,7 +704,7 @@ btree_xlog_mark_page_halfdead(uint8 info, XLogReaderState *record)
* target page or not (since it's surely empty).
*/
/* parent page */
/* to-be-deleted subtree's parent page */
if (XLogReadBufferForRedo(record, 1, &buffer) == BLK_NEEDS_REDO)
{
OffsetNumber poffset;
@ -749,8 +749,8 @@ btree_xlog_mark_page_halfdead(uint8 info, XLogReaderState *record)
pageop->btpo_cycleid = 0;
/*
* Construct a dummy hikey item that points to the next parent to be
* deleted (if any).
* Construct a dummy high key item that points to top parent page (value
* is InvalidBlockNumber when the top parent page is the leaf page itself)
*/
MemSet(&trunctuple, 0, sizeof(IndexTupleData));
trunctuple.t_info = sizeof(IndexTupleData);
@ -837,7 +837,7 @@ btree_xlog_unlink_page(uint8 info, XLogReaderState *record)
/*
* If we deleted a parent of the targeted leaf page, instead of the leaf
* itself, update the leaf to point to the next remaining child in the
* branch.
* to-be-deleted subtree
*/
if (XLogRecHasBlockRef(record, 3))
{

6
src/include/access/nbtxlog.h
View File

@ -250,7 +250,7 @@ typedef struct xl_btree_vacuum
#define SizeOfBtreeVacuum (offsetof(xl_btree_vacuum, nupdated) + sizeof(uint16))
/*
* This is what we need to know about marking an empty branch for deletion.
* This is what we need to know about marking an empty subtree for deletion.
* The target identifies the tuple removed from the parent page (note that we
* remove this tuple's downlink and the *following* tuple's key). Note that
* the leaf page is empty, so we don't need to store its content --- it is
@ -267,7 +267,7 @@ typedef struct xl_btree_mark_page_halfdead
BlockNumber leafblk; /* leaf block ultimately being deleted */
BlockNumber leftblk; /* leaf block's left sibling, if any */
BlockNumber rightblk; /* leaf block's right sibling */
BlockNumber topparent; /* topmost internal page in the branch */
BlockNumber topparent; /* topmost internal page in the subtree */
} xl_btree_mark_page_halfdead;
#define SizeOfBtreeMarkPageHalfDead (offsetof(xl_btree_mark_page_halfdead, topparent) + sizeof(BlockNumber))
@ -294,7 +294,7 @@ typedef struct xl_btree_unlink_page
*/
BlockNumber leafleftsib;
BlockNumber leafrightsib;
BlockNumber topparent; /* next child down in the branch */
BlockNumber topparent; /* next child down in the subtree */
TransactionId btpo_xact; /* value of btpo.xact for use in recovery */
/* xl_btree_metadata FOLLOWS IF XLOG_BTREE_UNLINK_PAGE_META */