Re: Parallel Seq Scan

On Thu, Jan 22, 2015 at 5:57 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> 1. Scanning block-by-block has negative impact on performance and
> I thin it will degrade more if we increase parallel count as that can lead
> to more randomness.
>
> 2. Scanning in fixed chunks improves the performance. Increasing
> parallel count to a very large number might impact the performance,
> but I think we can have a lower bound below which we will not allow
> multiple processes to scan the relation.

I'm confused.  Your actual test numbers seem to show that the
performance with the block-by-block approach was slightly higher with
parallelism than without, where as the performance with the
chunk-by-chunk approach was lower with parallelism than without, but
the text quoted above, summarizing those numbers, says the opposite.

Also, I think testing with 2 workers is probably not enough.  I think
we should test with 8 or even 16.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company