Re: Locking B-tree leafs immediately in exclusive mode

On Mon, Jul 9, 2018 at 3:23 PM, Alexander Korotkov
<a.korotkov@postgrespro.ru> wrote:
> I'm sorry, but I didn't understand this guess.  I agree that moving
> right within _bt_findinsertloc() might be worse than moving right
> within _bt_moveright().  But why should it happen more often, if both
> with and without patch that happens only after _bt_moveright() in
> exclusive mode (with patch _bt_search() calls _bt_moveright() in
> exclusive mode)?

_bt_moveright() doesn't serialize access, because it doesn't couple
buffer locks. I think that it's possible that it's slower for that
reason -- not because it moves right more often.

"The Convoy Phenomenon" research report may be worth a read sometime,
though it's very old. It's co-authored by Jim Gray. As they put it,
sometimes "a non-FIFO strategy" can be faster. In simple terms,
sometimes it can be faster to randomly grant a lock, since in practice
nobody gets starved out. I'm guessing (and it is very much a guess)
that it could be something like that, since the behavior of
_bt_findinsertloc() can be FIFO-like, whereas the behavior of
_bt_moveright() may not be.

Again, the actual queries would help a lot. It's just a guess.

-- 
Peter Geoghegan