Re: GSoC 2017 : Patch for predicate locking in Gist index

On Mon, Oct 2, 2017 at 9:11 PM, Andrew Borodin <amborodin86(at)gmail(dot)com>
wrote:

> On Mon, Oct 2, 2017 at 8:00 PM, Alexander Korotkov
> <a(dot)korotkov(at)postgrespro(dot)ru> wrote:
> > What happen if exactly this "continue" fires?
> >
> >> if (GistFollowRight(stack->page))
> >> {
> >> if (!xlocked)
> >> {
> >> LockBuffer(stack->buffer, GIST_UNLOCK);
> >> LockBuffer(stack->buffer, GIST_EXCLUSIVE);
> >> xlocked = true;
> >> /* someone might've completed the split when we unlocked */
> >> if (!GistFollowRight(stack->page))
> >> continue;
> >
> >
> > In this case we might get xlocked == true without calling
> > CheckForSerializableConflictIn().
> Indeed! I've overlooked it. I'm remembering this issue, we were
> considering not fixing splits if in Serializable isolation, but
> dropped the idea.
>

Yeah, current insert algorithm assumes that split must be fixed before we
can correctly traverse the tree downwards.

> CheckForSerializableConflictIn() must be after every exclusive lock.
>

I'm not sure, that fixing split is the case to necessary call
CheckForSerializableConflictIn(). This lock on leaf page is not taken to
do modification of the page. It's just taken to ensure that nobody else is
fixing this split the same this. After fixing the split, it might appear
that insert would go to another page.

> I think it would be rather safe and easy for understanding to more
> > CheckForSerializableConflictIn() directly before gistinserttuple().
> The difference is that after lock we have conditions to change page,
> and before gistinserttuple() we have actual intent to change page.
>
> From the point of future development first version is better (if some
> new calls occasionally spawn in), but it has 3 calls while your
> proposal have 2 calls.
> It seems to me that CheckForSerializableConflictIn() before
> gistinserttuple() is better as for now.
>

Agree.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company