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