At Mon, 23 Mar 2020 21:13:48 +0800, Richard Guo <guofengli...@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) ); regards. -- Kyotaro Horiguchi NTT Open Source Software Center