Re: Parallel Seq Scan

On Tue, Jul 7, 2015 at 6:19 AM, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
wrote:
>
> On Fri, Jul 3, 2015 at 10:05 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > Attached, find the rebased version of patch.
> >
> > Note - You need to first apply the assess-parallel-safety patch which
you
> > can find at:
> >
http://www.postgresql.org/message-id/CAA4eK1JjsfE_dOsHTr_z1P_cBKi_X4C4X3d7Nv=VWX9fs7qdJA@mail.gmail.com
>
> I ran some performance tests on a 16 core machine with large shared
> buffers, so there is no IO involved.
> With the default value of cpu_tuple_comm_cost, parallel plan is not
> getting generated even if we are selecting 100K records from 40
> million records. So I changed the value to '0' and collected the
> performance readings.
>

For reasonable default values for these parameters, still more testing
is required. I think instead of 0, tests with 0.001 or 0.0025 for default
of cpu_tuple_comm_cost and 100 or 1000 for default of parallel_setup_cost
would have been more interesting.

> Here are the performance numbers:
>
>
> The average table row size is around 500 bytes and query selection
> column width is around 36 bytes.
> when the query selectivity goes more than 10% of total table records,
> the parallel scan performance is dropping.
>

These are quite similar to what I have seen in my initial tests, now I
think if you add some complex condition in the filter, you will see gains
for even 25% or more selectivity (I have added factorial 10 calculation in
filter to mimic the complex filter condition).

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