Re: [HACKERS] proposal: schema variables

Hi

2018-09-04 9:21 GMT+02:00 Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>:

> AFAICS this patch does nothing to consider parallel safety -- that is,
> as things stand, a variable is allowed in a query that may be
> parallelised, but its value is not copied to workers, leading to
> incorrect results. For example:
>
> create table foo(a int);
> insert into foo select * from generate_series(1,1000000);
> create variable zero int;
> let zero = 0;
>
> explain (costs off) select count(*) from foo where a%10 = zero;
>
> QUERY PLAN
> -----------------------------------------------
> Finalize Aggregate
> -> Gather
> Workers Planned: 2
> -> Partial Aggregate
> -> Parallel Seq Scan on foo
> Filter: ((a % 10) = zero)
> (6 rows)
>
> select count(*) from foo where a%10 = zero;
>
> count
> -------
> 38037 -- Different random result each time, should be 100,000
> (1 row)
>
> Thoughts?
>

The query use copy of values of variables now - but unfortunately, these
values are not passed to workers. Should be fixed.

Thank you for test case.

Pavel

> Regards,
> Dean
>