Re: Parallel Index Scans

On Sat, Feb 11, 2017 at 9:41 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Feb 10, 2017 at 11:22 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> Why can't we rely on _bt_walk_left?
>>
>> The reason is mentioned in comments, but let me try to explain with
>> some example. When you reach that point of code, it means that either
>> the current page (assume page number is 10) doesn't contain any
>> matching items or it is a half-dead page, both of which indicates that
>> we have to move to the previous page. Now, before checking if the
>> current page contains matching items, we signal parallel machinery
>> (via _bt_parallel_release) to allow workers to read the previous page
>> (assume previous page number is 9). So it is quite possible that
>> after deciding that current page (page number 10) doesn't contain any
>> matching tuples if we directly move to the previous page (in this case
>> it will be 9) by using _bt_walk_left, some other worker would have
>> read page 9. In short, if we directly use _bt_walk_left(), then we
>> are prone to returning some of the values twice as multiple workers
>> can read the same page.
>
> But ... the entire point of the seize-and-release stuff is to avoid
> this problem. You're suppose to seize the scan, read the current
> page, walk left, store the page you find in the scan, and then release
> the scan.
>

Exactly and that is what is done in the patch. Basically, if we found
that the current page is half-dead or it doesn't contain any matching
items, then release the current buffer, seize the scan, read the
current page, walk left and so on. I am slightly confused here
because it seems both of us agree on what is the right thing to do and
according to me that is how it is implemented. Are you just ensuring
about whether I have implemented as discussed or do you see a problem
with the way it is implemented?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com