On Sun, Oct 11, 2020 at 1:39 PM Thomas Munro <thomas.mu...@gmail.com> wrote: > > Yeah, I think this is trying to fix the problem too late. Instead, we > should fix the incorrect row estimates so we don't have to fudge it > later like that. For example, this should be estimating rows=0: > > postgres=# explain analyze insert into s select * from t t1 join t t2 using > (i); > ... > Insert on s (cost=30839.08..70744.45 rows=1000226 width=4) (actual > time=2940.560..2940.562 rows=0 loops=1) > > I think that should be done with something like this: > > --- a/src/backend/optimizer/util/pathnode.c > +++ b/src/backend/optimizer/util/pathnode.c > @@ -3583,16 +3583,11 @@ create_modifytable_path(PlannerInfo *root, > RelOptInfo *rel, > if (lc == list_head(subpaths)) /* first node? */ > pathnode->path.startup_cost = subpath->startup_cost; > pathnode->path.total_cost += subpath->total_cost; > - pathnode->path.rows += subpath->rows; > + if (returningLists != NIL) > + pathnode->path.rows += subpath->rows; > total_size += subpath->pathtarget->width * subpath->rows; > } > > - /* > - * Set width to the average width of the subpath outputs. XXX this is > - * totally wrong: we should report zero if no RETURNING, else an > average > - * of the RETURNING tlist widths. But it's what happened > historically, > - * and improving it is a task for another day. > - */ > if (pathnode->path.rows > 0) > total_size /= pathnode->path.rows; > pathnode->path.pathtarget->width = rint(total_size);
Agree, thanks (bug in existing Postgres code, right?) Regards, Greg Nancarrow Fujitsu Australia
