Horton

Re: Returning nbtree posting list TIDs in DESC order during backwards scans

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Mircea Cadariu <cadariu.mircea@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-07-17T19:37:31Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Clarify why _bt_killitems sorts its items array.

  2. Return TIDs in desc order during backwards scans.

  3. Optimize nbtree backwards scans.

  4. Optimize nbtree backward scan boundary cases.

Attachments

On Thu, Jul 17, 2025 at 2:26 PM Peter Geoghegan <pg@bowt.ie> wrote:
> The loop has an early check for this (for non-itemDead entries) here:
>
>         /* Quickly skip over items never ItemDead-set by btgettuple */
>         if (!kitem->itemDead)
>             continue;
>
> I really doubt that this matters

I think that this patch isn't too far off being committable. I'll need
to think about issues around adding the new kitem->itemDead bitfield.
I'm not really worried about _bt_killitems; more so about the routines
called by _bt_readpage, which must make sure that the bit is unset
every time a TID is saved in so->currPos.items[].

Attached is v3. Not much has changed. We now defensively unset each
previously set kitem->itemDead within _bt_killitems. I've also added a
pair of macros that represent 0 and 1 values for these kitem->itemDead
bits.

Thanks
-- 
Peter Geoghegan