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Retain original physical order of tuples in redo of b-tree splits. 

It makes no difference to the system, but minimizing the differences
between a master and standby makes debugging simpler.
This commit is contained in:
Heikki Linnakangas 2014-04-22 13:03:37 +03:00
parent 7d98054f0d
commit 7e30c186da
1 changed files with 71 additions and 47 deletions
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118
src/backend/access/nbtree/nbtxlog.c
View File

@ -27,13 +27,6 @@
* had been its upper part (pd_upper to pd_special). We assume that the
* tuples had been added to the page in item-number order, and therefore
* the one with highest item number appears first (lowest on the page).
*
* NOTE: the way this routine is coded, the rebuilt page will have the items
* in correct itemno sequence, but physically the opposite order from the
* original, because we insert them in the opposite of itemno order. This
* does not matter in any current btree code, but it's something to keep an
* eye on. Is it worth changing just on general principles? See also the
* notes in btree_xlog_split().
*/
static void
_bt_restore_page(Page page, char *from, int len)
@ -41,14 +34,35 @@ _bt_restore_page(Page page, char *from, int len)
IndexTupleData itupdata;
Size itemsz;
char *end = from + len;
Item items[MaxIndexTuplesPerPage];
uint16 itemsizes[MaxIndexTuplesPerPage];
int i;
int nitems;
for (; from < end;)
/*
* To get the items back in the original order, we add them to the page
* in reverse. To figure out where one tuple ends and another begins,
* we have to scan them in forward order first.
*/
i = 0;
while (from < end)
{
/* Need to copy tuple header due to alignment considerations */
memcpy(&itupdata, from, sizeof(IndexTupleData));
itemsz = IndexTupleDSize(itupdata);
itemsz = MAXALIGN(itemsz);
if (PageAddItem(page, (Item) from, itemsz, FirstOffsetNumber,
items[i] = (Item) from;
itemsizes[i] = itemsz;
i++;
from += itemsz;
}
nitems = i;
for (i = nitems - 1; i >= 0; i--)
{
if (PageAddItem(page, items[i], itemsizes[i], nitems - i,
false, false) == InvalidOffsetNumber)
elog(PANIC, "_bt_restore_page: cannot add item to page");
from += itemsz;
@ -332,10 +346,13 @@ btree_xlog_split(bool onleft, bool isroot,
if (BufferIsValid(lbuf))
{
/*
* Note that this code ensures that the items remaining on the
* left page are in the correct item number order, but it does not
* reproduce the physical order they would have had. Is this
* worth changing? See also _bt_restore_page().
* To retain the same physical order of the tuples that they had,
* we initialize a temporary empty page for the left page and add
* all the items to that in item number order. This mirrors how
* _bt_split() works. It's not strictly required to retain the
* same physical order, as long as the items are in the correct
* item number order, but it helps debugging. See also
* _bt_restore_page(), which does the same for the right page.
*/
Page lpage = (Page) BufferGetPage(lbuf);
BTPageOpaque lopaque = (BTPageOpaque) PageGetSpecialPointer(lpage);
@ -343,45 +360,52 @@ btree_xlog_split(bool onleft, bool isroot,
if (lsn > PageGetLSN(lpage))
{
OffsetNumber off;
OffsetNumber maxoff = PageGetMaxOffsetNumber(lpage);
OffsetNumber deletable[MaxOffsetNumber];
int ndeletable = 0;
Page newlpage;
OffsetNumber leftoff;
/*
* Remove the items from the left page that were copied to the
* right page. Also remove the old high key, if any. (We must
* remove everything before trying to insert any items, else
* we risk not having enough space.)
*/
if (!P_RIGHTMOST(lopaque))
{
deletable[ndeletable++] = P_HIKEY;
/*
* newitemoff is given to us relative to the original
* page's item numbering, so adjust it for this deletion.
*/
newitemoff--;
}
for (off = xlrec->firstright; off <= maxoff; off++)
deletable[ndeletable++] = off;
if (ndeletable > 0)
PageIndexMultiDelete(lpage, deletable, ndeletable);
/*
* Add the new item if it was inserted on left page.
*/
if (onleft)
{
if (PageAddItem(lpage, newitem, newitemsz, newitemoff,
false, false) == InvalidOffsetNumber)
elog(PANIC, "failed to add new item to left page after split");
}
newlpage = PageGetTempPageCopySpecial(lpage);
/* Set high key */
if (PageAddItem(lpage, left_hikey, left_hikeysz,
leftoff = P_HIKEY;
if (PageAddItem(newlpage, left_hikey, left_hikeysz,
P_HIKEY, false, false) == InvalidOffsetNumber)
elog(PANIC, "failed to add high key to left page after split");
leftoff = OffsetNumberNext(leftoff);
for (off = P_FIRSTDATAKEY(lopaque); off < xlrec->firstright; off++)
{
ItemId itemid;
Size itemsz;
Item item;
/* add the new item if it was inserted on left page */
if (onleft && off == newitemoff)
{
if (PageAddItem(newlpage, newitem, newitemsz, leftoff,
false, false) == InvalidOffsetNumber)
elog(ERROR, "failed to add new item to left page after split");
leftoff = OffsetNumberNext(leftoff);
}
itemid = PageGetItemId(lpage, off);
itemsz = ItemIdGetLength(itemid);
item = PageGetItem(lpage, itemid);
if (PageAddItem(newlpage, item, itemsz, leftoff,
false, false) == InvalidOffsetNumber)
elog(ERROR, "failed to add old item to left page after split");
leftoff = OffsetNumberNext(leftoff);
}
/* cope with possibility that newitem goes at the end */
if (onleft && off == newitemoff)
{
if (PageAddItem(newlpage, newitem, newitemsz, leftoff,
false, false) == InvalidOffsetNumber)
elog(ERROR, "failed to add new item to left page after split");
leftoff = OffsetNumberNext(leftoff);
}
PageRestoreTempPage(newlpage, lpage);
/* Fix opaque fields */
lopaque->btpo_flags = BTP_INCOMPLETE_SPLIT;