Re: postgres_fdw: another oddity in costing aggregate pushdown paths

Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:

> As mentioned in the near thread, I think there is another oversight in
> the cost estimation for aggregate pushdown paths in postgres_fdw, IIUC.
> When costing an aggregate pushdown path using local statistics, we
> re-use the estimated costs of implementing the underlying scan/join
> relation, cached in the relation's PgFdwRelationInfo (ie,
> rel_startup_cost and rel_total_cost). Since these costs wouldn't yet
> contain the costs of evaluating the final scan/join target, as tlist
> replacement by apply_scanjoin_target_to_paths() is performed afterwards.
> So I think we need to adjust these costs so that the tlist eval costs
> are included, but ISTM that estimate_path_cost_size() forgot to do so.
> Attached is a patch for fixing this issue.

I think the following comment in apply_scanjoin_target_to_paths() should
mention that FDWs rely on the new value of reltarget.

/*
* Update the reltarget. This may not be strictly necessary in all cases,
* but it is at least necessary when create_append_path() gets called
* below directly or indirectly, since that function uses the reltarget as
* the pathtarget for the resulting path. It seems like a good idea to do
* it unconditionally.
*/
rel->reltarget = llast_node(PathTarget, scanjoin_targets);

--
Antonin Houska
https://www.cybertec-postgresql.com