Re: using custom scan nodes to prototype parallel sequential scan - Mailing list pgsql-hackers

On Tue, Nov 11, 2014 at 3:29 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
> * only functions marked as "CONTAINS NO SQL"
> We don't really know what proisparallel is, but we do know what
> CONTAINS NO SQL means and can easily check for it.
> Plus I already have a patch for this, slightly bitrotted.

Interestingly, I have a fairly solid idea of what proisparallel is,
but I have no clear idea what CONTAINS NO SQL is or why it's relevant.
I would imagine that srandom() contains no SQL under any reasonable
definition of what that means, but it ain't parallel-safe.

> * parallel_workers = 2 (or at least not make it user settable)
> By fixing the number of workers at 2 we avoid any problems caused by
> having N variable, such as how to vary N fairly amongst users and
> other such considerations. We get the main benefit of parallelism,
> without causing other issues across the server.

I think this is a fairly pointless restriction.  The code
simplification we'll get out of it appears to me to be quite minor,
and we'll just end up putting the stuff back in anyway.

> * Fixed Plan: aggregate-scan
> To make everything simpler, allow only plans of a single type.
>  SELECT something, list of aggregates
>  FROM foo
>  WHERE filters
>  GROUP BY something
> because we know that passing large amounts of data from worker to
> master process will be slow, so focusing only on seq scan is not
> sensible; we should focus on plans that significantly reduce the
> number of rows passed upwards. We could just do this for very
> selective WHERE clauses, but that is not an important class of query.
> As soon as include aggregates, we reduce data passing significantly
> AND we hit a very important subset of queries:

This is moving the goalposts in a way that I'm not at all comfortable
with.  Parallel sequential-scan is pretty simple and may well be a win
if there's a restrictive filter condition involved.  Parallel
aggregation requires introducing new infrastructure into the aggregate
machinery to allow intermediate state values to be combined, and that
would be a great project for someone to do at some time, but it seems
like a distraction for me to do that right now.

> This plan type is widely used in reporting queries, so will hit the
> mainline of BI applications and many Mat View creations.
> This will allow SELECT count(*) FROM foo to go faster also.
>
> The execution plan for that query type looks like this...
> Hash Aggregate
>    Gather From Workers
>       {Worker Nodes workers = 2
>         HashAggregate
>         PartialScan}

I'm going to aim for the simpler:

Hash Aggregate
-> Parallel Seq Scan   Workers: 4

Yeah, I know that won't perform as well as what you're proposing, but
I'm fairly sure it's simpler.

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