On Tue, Mar 7, 2017 at 10:22 AM, Ashutosh Sharma <ashu.coek88@gmail.com> wrote:
>> I also think that commit didn't intend to change the behavior,
>> however, the point is how sensible is it to keep such behavior after
>> Parallel Append. I am not sure if this is the right time to consider
>> it or shall we wait till Parallel Append is committed.
>>
>>> I think the problem here is that compute_parallel_worker() thinks it
>>> can use 0 as a sentinel value that means "ignore this", but it can't
>>> really, because a heap can actually contain 0 pages. Here's a patch
>>> which does the following:
>>>
>>
>> - if (heap_pages < (BlockNumber) min_parallel_table_scan_size &&
>> - index_pages < (BlockNumber) min_parallel_index_scan_size &&
>> - rel->reloptkind == RELOPT_BASEREL)
>> + if (rel->reloptkind == RELOPT_BASEREL &&
>> + ((heap_pages >= 0 && heap_pages < min_parallel_table_scan_size) ||
>> + (index_pages >= 0 && index_pages < min_parallel_index_scan_size)))
>> return 0;
>>
>> The purpose of considering both heap and index pages () for
>> min_parallel_* is that even if one of them is bigger than threshold
>> then we should try parallelism.
>
> Yes, But, this is only true for normal tables not for partitioned or
> inheritance tables. I think for partition table, even if one heap page
> exist, we go for parallelism.
>
> This could be helpful for parallel
>> index scans where we consider parallel workers based on both heap and
>> index pages. Is there a reason for you to change that condition as
>> that is not even related to the problem being discussed?
>>
>
> I think he has changed to allow parallelism for inheritance or partition
> tables. For normal tables, it won't be touched until the below if-condition
> is satisfied.
>
> if (heap_pages < (BlockNumber) min_parallel_table_scan_size &&
> index_pages < (BlockNumber) min_parallel_index_scan_size &&
> rel->reloptkind == RELOPT_BASEREL)
> return 0;
>
AFAICS, the patch has changed the if-condition you are quoting.
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com
