Re: Explain says 8 workers planned, only 1 executed

[Alastair McKinley]

Hi Adrian,

Thanks for getting back to me.

Postgres version is:

PostgreSQL 11.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 
(Red Hat 4.8.5-39), 64-bit

I simplified it to the following structure:

create function f() returns setof my_type as
$$
declare
q text;
output text;
begin
    select generate_query1() into q; -- the query q executes in parallel with 8 
workers if executed standalone
    for output in execute('explain ' || q) loop
        raise notice '%',output;  -- this plan says 8 workers will be launched
    end loop;
    return query execute q; -- this launches one worker

   select generate_query2() into q;
    for output in execute('explain ' || q) loop
        raise notice '%',output;  -- this plan says 8 workers will be launched
    end loop;
    return query execute q; -- this also launches one worker
end;
language plpgsql;

Should this work in principle or am I missing something subtle about parallel 
dynamic queries in plpgsql functions?  Does the outer function need to be 
parallel safe?
Might a stored proc work better?

Best regards,

Alastair


________________________________
From: Adrian Klaver <adrian.kla...@aklaver.com>
Sent: 21 March 2020 17:38
To: Alastair McKinley <a.mckin...@analyticsengines.com>; 
pgsql-general@lists.postgresql.org <pgsql-general@lists.postgresql.org>
Subject: Re: Explain says 8 workers planned, only 1 executed

On 3/21/20 10:25 AM, Alastair McKinley wrote:
> Hi all,
>
> I have a long running query that I have tweaked along with config (e.g.
> min_parallel_table_scan_size) to execute nicely and very fast in
> parallel which works as expected executed directly from psql client.
> The query is then embedded in a psql function like "return query select
> * from function_that_executes_the_query()".

Postgres version?

What is happening in function_that_executes_the_query()?

You might want to take a look at below to see any of the conditions apply:

https://www.postgresql.org/docs/12/when-can-parallel-query-be-used.html

>
> I am checking the explain output (using execute explain $query) just
> before executing inside my function and it the plan is identical to what
> I would expect, planning 8 workers.  However, this query actually only
> uses 1 worker and takes many times longer than when ran directly on the
> psql command line with the same server configuration parameters.
>
> Why would the explain output be different from the executed plan? Is
> this a limitation of plpgsql functions? Is there any way to debug this
> further?
>
> If it is meaningful during parallel execution I notice lots of
> "postgres: parallel worker" proceses in top and when executing from my
> function just a single "postgres: $user $db $host($pid) SELECT" processes.
>
> Best regards,
>
> Alastair
>
>


--
Adrian Klaver
adrian.kla...@aklaver.com