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

On Wed, Jul 16, 2025 at 5:27 PM Mircea Cadariu <cadariu.mircea@gmail.com> wrote:
> Does the above change mean it will have to do more work in the loop?
> Whereas before it visited strictly killed, it now has to go through all
> of them?

Yes, that's true. Any item that the scan returns from the so->currPos
page needs to be considered within the loop.

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, because:

* This can only happen when we actually call _bt_killitems in the
first place, so there has to be at least one item whose index tuple
really does need to be LP_DEAD-set.

* The chances of there being a huge number of so->currPos.items[]
items but only one or two with their "itemDead" bit set seems low, in
general.

* The new loop is significantly simpler in that it iterates through
so->currPos.items[] in order, without any of the so->killedItems[]
indirection you see on HEAD. Modern CPUs are likely to skip over
non-itemDead entries very quickly.

Note that so->killedItems[] (which this patch removes) can be in
ascending leaf-page-wise order, descending leaf-page-wise order, or
(with a scrollable cursor) some random mix of the two -- even when
there's no posting list tuples involved.

--
Peter Geoghegan