Re: Index Skip Scan - Mailing list pgsql-hackers

> On Mon, Jan 20, 2020 at 01:19:30PM -0800, Peter Geoghegan wrote:

Thanks for the commentaries. I'm trying to clarify your conclusions for
myself, so couple of questions.

> > > - nbtsearch.c in general
> > > Most of the code seems to rely quite heavily on the fact that xs_want_itup forces _bt_drop_lock_and_maybe_pin to
neverrelease the buffer pin. Have you considered that compacting of a page may still happen even if you hold the pin?
[1]I've been trying to come up with cases in which this may break the patch, but I haven't able to produce such a
scenario- so it may be fine.
 
>
> Try making _bt_findinsertloc() call _bt_vacuum_one_page() whenever the
> page is P_HAS_GARBAGE(), regardless of whether or not the page is
> about to split. That will still be correct, while having a much better
> chance of breaking the patch during stress-testing.
>
> Relying on a buffer pin to prevent the B-Tree structure itself from
> changing in any important way seems likely to be broken already. Even
> if it isn't, it sounds fragile.

Except for checking low/high key (which should be done with a lock), I
believe the current implementation follows the same pattern I see quite
often, namely

* get a lock on a page of interest and test it's values (if we can find
  next distinct value right on the next one without goind down the tree).

* if not, unlock the current page, search within the tree with
  _bt_search (which locks a resuling new page) and examine values on a
  new page, when necessary do _bt_steppage

Is there an obvious problem with this approach, when it comes to the
page structure modification?

> A leaf page doesn't really have anything called a low key. It usually
> has a current first "data item"/non-pivot tuple, which is an
> inherently unstable thing.

Would this inherent instability be resolved for this particular case by
having a lock on a page while checking a first data item, or there is
something else I need to take into account?

> > There is a BT_READ lock in place when finding the correct leaf page, or
> > searching within the leaf page itself. _bt_vacuum_one_page deletes only
> > LP_DEAD tuples, but those are already ignored in _bt_readpage. Peter, do
> > you have some feedback for this ?
>
> It sounds like the design of the patch relies on doing something other
> than stopping a scan "between" pages, in the sense that is outlined in
> the commit message of commit 09cb5c0e. If so, then that's a serious
> flaw in its design.

Could you please elaborate why does it sound like that? If I understand
correctly, to stop a scan only "between" pages one need to use only
_bt_readpage/_bt_steppage? Other than that there is no magic with scan
position in the patch, so I'm not sure if I'm missing something here.