Re: Parallel Seq Scan

On 11 November 2015 at 14:53, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Mon, Nov 9, 2015 at 11:15 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> Okay, I have updated the patch to make seq scan node parallel aware.
>> To make that happen we need to have parallel_aware flag both in Plan
>> as well as Path, so that we can pass that information from Path to Plan.
>> I think the right place to copy parallel_aware info from path to
>> plan is copy_path_costsize and ideally we should change the name
>> of function to something like copy_generic_path_info(), but for
>> now I have retained it's original name as it is used at many places,
>> let me know if you think we should goahead and change the name
>> of function as well.
>>
>> I have changed Explain as well so that it adds Parallel for Seq Scan if
>> SeqScan node is parallel_aware.
>>
>> I have not integrated it with consider-parallel patch, so that this and
>> Partial Seq Scan version of the patch can be compared without much
>> difficulity.
>>
>> Thoughts?
>
> I've committed most of this, except for some planner bits that I
> didn't like, and after a bunch of cleanup. Instead, I committed the
> consider-parallel-v2.patch with some additional planner bits to make
> up for the ones I removed from your patch. So, now we have parallel
> sequential scan!
>
> For those following along at home, here's a demo:
>
> rhaas=# \timing
> Timing is on.
> rhaas=# select * from pgbench_accounts where filler like '%a%';
> aid | bid | abalance | filler
> -----+-----+----------+--------
> (0 rows)
>
> Time: 743.061 ms
> rhaas=# set max_parallel_degree = 4;
> SET
> Time: 0.270 ms
> rhaas=# select * from pgbench_accounts where filler like '%a%';
> aid | bid | abalance | filler
> -----+-----+----------+--------
> (0 rows)
>
> Time: 213.412 ms
>
> This is all pretty primitive at this point - there are still lots of
> things that need to be fixed and improved, and it applies to only the
> very simplest cases at present, but, hey, parallel query. Check it
> out.

Congratulations to both you and Amit. This is a significant landmark
in PostgreSQL feature development.

Thom