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Clarify why _bt_killitems sorts its items array.

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Make it clear why _bt_killitems sorts the scan's so->killedItems[]
array.  Also add an assertion to the _bt_killitems loop (that iterates
through this array) to verify it accesses tuples in leaf page order.

Follow-up to commit bfb335df58.

Author: Peter Geoghegan <pg@bowt.ie>
Suggested-by: Victor Yegorov <vyegorov@gmail.com>
Discussion: https://postgr.es/m/CAGnEboirgArezZDNeFrR8FOGvKF-Xok333s2iVwWi65gZf8MEA@mail.gmail.com
This commit is contained in:
Peter Geoghegan
2025-12-10 20:50:47 -05:00
parent 06761b6096
commit e16c6f0247
1 changed files with 9 additions and 4 deletions
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13
src/backend/access/nbtree/nbtutils.c
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@@ -222,11 +222,12 @@ _bt_killitems(IndexScanDesc scan)
so->numKilled = 0;
/*
* so->killedItems[] is in whatever order the scan returned items in.
* Items will appear in descending order during backwards scans. And
* scrollable cursor scans might have duplicate items.
* We need to iterate through so->killedItems[] in leaf page order; the
* loop below expects this (when marking posting list tuples, at least).
* so->killedItems[] is now in whatever order the scan returned items in.
* Scrollable cursor scans might have even saved the same item/TID twice.
*
* Sort and uniqueify so->killedItems[] to deal with all this.
* Sort and unique-ify so->killedItems[] to deal with all this.
*/
if (numKilled > 1)
{
@@ -271,6 +272,7 @@ _bt_killitems(IndexScanDesc scan)
minoff = P_FIRSTDATAKEY(opaque);
maxoff = PageGetMaxOffsetNumber(page);
/* Iterate through so->killedItems[] in leaf page order */
for (int i = 0; i < numKilled; i++)
{
int itemIndex = so->killedItems[i];
@@ -279,6 +281,9 @@ _bt_killitems(IndexScanDesc scan)
Assert(itemIndex >= so->currPos.firstItem &&
itemIndex <= so->currPos.lastItem);
Assert(i == 0 ||
offnum >= so->currPos.items[so->killedItems[i - 1]].indexOffset);
if (offnum < minoff)
continue; /* pure paranoia */
while (offnum <= maxoff)