Re: GiST VACUUM

Looking at the second patch, to scan the GiST index in physical order,
that seems totally unsafe, if there are any concurrent page splits. In
the logical scan, pushStackIfSplited() deals with that, by comparing the
page's NSN with the parent's LSN. But I don't see anything like that in
the physical scan code.

I think we can do something similar in the physical scan: remember the
current LSN position at the beginning of the vacuum, and compare with
that. The B-tree code uses the "cycle ID" for similar purposes.

Do we still need the separate gistvacuumcleanup() pass, if we scan the
index in physical order in the bulkdelete pass already?

- Heikki