Re: Extended Prefetching using Asynchronous IO - proposal and patch

On Thu, May 29, 2014 at 6:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Claudio Freire <klaussfreire(at)gmail(dot)com> writes:
>> On Thu, May 29, 2014 at 6:43 PM, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
>>> On Thu, May 29, 2014 at 6:19 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>>> "ampeeknexttuple"? That's a bit scary. It would certainly be unsafe
>>>> for non-MVCC snapshots (read about vacuum vs indexscan interlocks in
>>>> nbtree/README).
>
>>> It's not really the tuple, just the tid
>
>> And, furthermore, it's used only to do prefetching, so even if the tid
>> was invalid when the tuple needs to be accessed, it wouldn't matter,
>> because the indexam wouldn't use the result of ampeeknexttuple to do
>> anything at that time.
>
> Nonetheless, getting the next tid out of the index may involve stepping
> to the next index page, at which point you've lost your interlock
> guaranteeing that the *previous* tid will still mean something by the time

No, no... that's exactly why a new regproc is needed, because for
prefetching, we need to get the next tid that satisfies some
conditions *without* walking the index.

This, in nbtree, only looks through the tid array to find the suitable
tid, or just return false if the array is exhausted.

> Having said that, it's probably OK as long as this mode is only invoked
> for user queries (with MVCC snapshots) and not for system indexscans.

I think system index scans will also invoke this. There's no rule
excluding that possibility.