Re: Negative cost is seen for plan node

At Mon, 23 Mar 2020 21:13:48 +0800, Richard Guo <guofenglinux@gmail.com> wrote in
> Hi all,
>
> With the following statements on latest master (c81bd3b9), I find
> negative cost for plan nodes.
>
> create table a (i int, j int);
> insert into a select i%100000, i from generate_series(1,1000000)i;
> analyze a;
>
> # explain select i from a group by i;
>                            QUERY PLAN
> -----------------------------------------------------------------
>  HashAggregate  (cost=1300.00..-1585.82 rows=102043 width=4)

Good catch!

>    Group Key: i
>    Planned Partitions: 4
>    ->  Seq Scan on a  (cost=0.00..14425.00 rows=1000000 width=4)
> (4 rows)
>
> In function cost_agg, when we add the disk costs of hash aggregation
> that spills to disk, nbatches is calculated as 1.18 in this case. It is
> greater than 1, so there will be spill. And the depth is calculated as
> -1 in this case, with num_partitions being 4. I think this is where
> thing goes wrong.

The depth is the expected number of iterations of reading the relation.

>       depth = ceil( log(nbatches - 1) / log(num_partitions) );

I'm not sure what the expression based on, but apparently it is wrong
for nbatches <= 2.0. It looks like a thinko of something like this.

        depth = ceil( log(nbatches) / log(num_partitions + 1) );