Re: Parallel Index Scans

On Wed, Jan 18, 2017 at 6:55 PM, Rahila Syed <rahilasyed90@gmail.com> wrote:

> >+ /* Check if the scan for current scan keys is finished */
> >+ if (so->arrayKeyCount < btscan->btps_arrayKeyCount)
> >+ *status = false;
>
> >I didn't clearly understand, in which scenario the arrayKeyCount is less
> >than btps_arrayKeyCount?
> Consider following array scan keys
>
> select * from test2 where j=ANY(ARRAY[1000,2000,3000]);
>
> By the time the current worker has finished reading heap tuples
> corresponding
> to array key 1000(arrayKeyCount = 0), some other worker might have
> advanced the scan to the
> array key 2000(btps_arrayKeyCount =1). In this case when the current
> worker fetches next page to scan,
> it must advance its scan keys before scanning the next page of parallel
> scan.
> I hope this helps.
>

Thanks for the details.
One worker incremented arrayKeyCount and btps_arrayKeyCount both. As
btps_arrayKeyCount present in shared memory, so the other worker see the
update
and hits above the condition.


> >+BlockNumber
> >+_bt_parallel_seize(IndexScanDesc scan, bool *status)
>
> >The return value of the above function is validated only in _bt_first
> >function, but in other cases it is not.
> In other cases it is validated in _bt_readnextpage() which is called after
> _bt_parallel_seize().
>
> >From the function description,
> >it is possible to return P_NONE for the workers also with status as
> >true. I feel it is better to handle the P_NONE case internally only
> >so that callers just check for the status. Am i missing anything?
>
> In case of the next block being P_NONE and status true, the code
> calls _bt_parallel_done() to notify other workers followed by
> BTScanPosInvalidate(). Similar check for block = P_NONE also
> happens in existing code. See following in _bt_readnextpage(),
>
>
>             if (blkno == P_NONE || !so->currPos.moreRight)
>             {
>                _bt_parallel_done(scan);
>                 BTScanPosInvalidate(so->currPos);
>                 return false;
>             }
> So to keep it consistent with the existing code, the check
> is kept outside _bt_parallel_seize().
>

Thanks. Got it.

Regards,
Hari Babu
Fujitsu Australia