I wrote:
> [ looks at that for a bit... ]  Yeah, you're right.  Once the deletion
> is completed, the F lower-bound key will disappear from the grandparent,
> which would restore consistency --- but we could have already delivered
> wrong search answers, so that won't do.

On further reflection, I think I understand why we've not realized the
existence of this bug before: in fact, it *doesn't* lead to wrong search
answers.  I think the only visible consequence is exactly the "failed to
re-find parent key" VACUUM error that Ed saw.  The reason is that the
key misordering in the grandparent level is nearly harmless.  Using your
example of

        - F D D ...

* if we happen to come across the F key first during a binary search of
the grandparent page, and we are looking for something <= F, we will
descend to its left, which is at worst a little bit inefficient:
_bt_moveright will still ensure that we find what we seek.

* if we happen to visit one of the D key(s) first, and we are looking
for something > D, we will descend to the right of that key.  Well,
that's not incorrect for the live data.  In fact, the *only* key in the
tree that we will fail to find this way is the F bounding key for the
half-dead page itself (or one of its also-deletable parents).  So that's
exactly why VACUUM can fail while trying to clean up the half-dead page,
and it's why we're not seeing reports of wrong query answers.

So that reduces the priority of the bug quite a lot in my estimation;
and makes me not want to incur a lot of additional code and locking to
fix it.  I'm wondering whether we can simply adopt a modified strategy
for searching for a half-dead page's parent during _bt_pagedel.

                        regards, tom lane

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