Re: Parallel Seq Scan

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.

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