Re: Problems with plan estimates in postgres_fdw

Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> wrote:

> Maybe, my explanation in that thread was not enough, but the changes proposed
> by the patch posted there wouldn't obsolete the above.  Let me explain using a
> foreign-table variant of the example shown in the comments for
> make_group_input_target():
>
>     SELECT a+b, sum(c+d) FROM foreign_table GROUP BY a+b;
>
> When called from postgresGetForeignPaths(), the reltarget for the base
> relation foreign_table would be {a, b, c, d}, for the same reason as the LIMIT
> example in [1], and the foreign_table's rel_startup_cost and rel_total_cost
> would be calculated based on this reltarget in that callback routine.  (The
> tlist eval costs would be zeroes, though).  BUT: before called from
> postgresGetForeignUpperPaths() with the UPPERREL_GROUP_AGG stage, the
> reltarget would be replaced with {a+b, c, d}, which is the final scan target
> as explained in the comments for make_group_input_target(), and the eval costs
> of the new reltarget wouldn't be zeros, so the costs of scanning the
> foreign_table would need to be adjusted to include the eval costs.  That's why
> I propose the assignments in estimate_path_cost_size() shown above.

Yes, apply_scanjoin_target_to_paths() can add some more costs, including those
introduced by make_group_input_target(), which postgres_fdw needs to account
for. This is what you fix in

https://www.postgresql.org/message-id/5C66A056.60007%40lab.ntt.co.jp

> On the other hand, the code bit added by
> 0001-postgres_fdw-Perform-UPPERREL_ORDERED-step-remotely-v3.patch handles the
> case where a post-scan/join processing step other than grouping/aggregation
> (ie, the final sort or LIMIT/OFFSET) is performed directly on a base or join
> relation, as in the LIMIT example.  So, the two changes are handling different
> cases, hence both changes would be required.

Do you mean this part?

+       /*
+        * If this includes an UPPERREL_ORDERED step, the given target, which
+        * would be the final target to be applied to the resulting path, might
+        * have different expressions from the underlying relation's reltarget
+        * (see make_sort_input_target()); adjust tlist eval costs.
+        */
+       if (fpextra && fpextra->target != foreignrel->reltarget)
+       {
+               QualCost        oldcost = foreignrel->reltarget->cost;
+               QualCost        newcost = fpextra->target->cost;
+
+               startup_cost += newcost.startup - oldcost.startup;
+               total_cost += newcost.startup - oldcost.startup;
+               total_cost += (newcost.per_tuple - oldcost.per_tuple) * rows;
+       }

You should not need this. Consider what grouping_planner() does if
(!have_grouping && !activeWindows && parse->sortClause != NIL):

    sort_input_target = make_sort_input_target(...);
    ...
    grouping_target = sort_input_target;
    ...
    scanjoin_target = grouping_target;
    ...
    apply_scanjoin_target_to_paths(...);

So apply_scanjoin_target_to_paths() effectively accounts for the additional
costs introduced by make_sort_input_target().

Note about the !activeWindows test: It occurs me now that no paths should be
added to UPPERREL_ORDERED relation if the query needs WindowAgg node, because
postgres_fdw currently cannot handle UPPERREL_WINDOW relation. Or rather in
general, the FDW should not skip any stage just because it doesn't know how to
build the paths.

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