Re: Unsplitting btree index leaf pages

Alvaro Herrera <alvherre@commandprompt.com> writes:
> We already do something similar for page deletions.  Empty pages are not
> deleted right away, but they are marked with BTP_DEAD, and then deleted
> on a subsequent vacuum.  Or something like that, I don't remember the
> exact details.

Right, and the reason for that is exactly that there might be a
concurrent indexscan already "in flight" to the newly-dead page.
We must wait to recycle the page until we are certain no such scans
remain.

It doesn't matter whether a concurrent indexscan visits the dead
page or not, *because it's empty* and so there's nothing to miss.
So there's no race condition.  But if you try to move valid data
across pages then there is a race condition.
        regards, tom lane