digoal(at)126(dot)com writes:
> in btree, leaf page has prev and next page. but when i use order by desc,
> it's not use the prev page link? it scan from root per leaf page.
You've provided no evidence whatsoever to support that claim.
> 9 page hint = meta + root + branch + 6 leaf
> 19 page hint = meta + 6(root + branch + leaf)
I think more likely what's happening is that one end of the index is
packed full and the other end is about 50% full.
regards, tom lane