Re: Re: fix cost subqueryscan wrong parallel cost

On Wed, Apr 20, 2022 at 10:01 AM bucoo(at)sohu(dot)com <bucoo(at)sohu(dot)com> wrote:
> for now fuction cost_subqueryscan always using *total* rows even parallel
> path. like this:
>
> Gather (rows=30000)
> Workers Planned: 2
> -> Subquery Scan (rows=30000) -- *total* rows, should be equal subpath
> -> Parallel Seq Scan (rows=10000)

OK, that's bad.

> Maybe the codes:
>
> /* Mark the path with the correct row estimate */
> if (param_info)
> path->path.rows = param_info->ppi_rows;
> else
> path->path.rows = baserel->rows;
>
> should change to:
>
> /* Mark the path with the correct row estimate */
> if (path->path.parallel_workers > 0)
> path->path.rows = path->subpath->rows;
> else if (param_info)
> path->path.rows = param_info->ppi_rows;
> else
> path->path.rows = baserel->rows;

Suppose parallelism is not in use and that param_info is NULL. Then,
is path->subpath->rows guaranteed to be equal to baserel->rows? If
yes, then we don't need to a three-part if statement as you propose
here and can just change the "else" clause to say path->path.rows =
path->subpath->rows. If no, then your change gives the wrong answer.

--
Robert Haas
EDB: http://www.enterprisedb.com