Commits on Dec 10, 2025

1. Return TIDs in desc order during backwards scans. …

   Always return TIDs in descending order when returning a group of
   duplicates to the scan whose TIDs come from an nbtree posting list
   tuples during nbtree backwards scans.  This makes backwards scans tend
   to require fewer buffer hits, since the scan is less likely to
   repeatedly pin and unpin the same heap page/buffer (we'll get exactly as
   many buffer hits as we get with a similar forwards scan case).
   
   Commit 0d861bb, which added nbtree deduplication, originally did things
   this way to avoid interfering with _bt_killitems's previous approach to
   setting LP_DEAD bits on posting list tuples.  _bt_killitems made a soft
   assumption that it can iterate through posting lists in TID order,
   finding corresponding killItems[]/so->currPos.items[] entries in that
   same order (even when scanning backwards).  This worked out because of
   the prior _bt_readpage backwards scan behavior.  If we just change the
   backwards scan posting list logic in _bt_readpage, and don't alter
   _bt_killitems itself, we'll break its soft assumption.
   
   Avoid that problem by sorting the so->killedItems[] array at the start
   of _bt_killitems.  That way the order that dead items are saved in from
   btgettuple can't matter; so->killedItems[] will always be in the same
   order as so->currPos.items[] in the end.  Since so->currPos.items[] is
   now always in leaf page order, regardless of the scan direction used
   within _bt_readpage, and since so->killedItems[] is always in that same
   order, the _bt_killitems loop can continue to make a uniform assumption
   about everything being in page order.
   
   Also deduplicate the so->killedItems[] array after it is sorted.  That
   way there's no risk of the _bt_killitems loop becoming confused by
   duplicate dead items/TID.  This was possible in cases that involved a
   scrollable cursor that encountered the same dead TID more than once
   (within the same leaf page/so->currPos context).  This doesn't matter
   very much in practice, but it seems best to be as consistent as possible
   about LP_DEAD marking items within _bt_killitems.
   
   Author: Peter Geoghegan <[email protected]>
   Reviewed-By: Mircea Cadariu <[email protected]>
   Reviewed-By: Victor Yegorov <[email protected]>
   Discussion: https://postgr.es/m/CAH2-Wz=Wut2pKvbW-u3hJ_LXwsYeiXHiW8oN1GfbKPavcGo8Ow@mail.gmail.com

   

   petergeoghegan authored and Commitfest Bot committed Dec 10, 2025
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2. [CF 5824] v5 - Returning nbtree posting list TIDs in DESC order durin… …

   …g backwards scans
   
   This branch was automatically generated by a robot using patches from an
   email thread registered at:
   
   https://commitfest.postgresql.org/patch/5824
   
   The branch will be overwritten each time a new patch version is posted to
   the thread, and also periodically to check for bitrot caused by changes
   on the master branch.
   
   Patch(es): https://www.postgresql.org/message-id/CAH2-Wznq0zWu5bMEmaFZ+s3mkNpHZm-nAD=7yx_eS4t3=jZEUA@mail.gmail.com
   Author(s): Peter Geoghegan

   Commitfest Bot committed Dec 10, 2025
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