(2019/03/01 20:00), Antonin Houska wrote:
Etsuro Fujita<fujita.ets...@lab.ntt.co.jp> wrote:
(2019/02/22 22:54), Antonin Houska wrote:
Etsuro Fujita<fujita.ets...@lab.ntt.co.jp> wrote:
So, the two changes are handling different
cases, hence both changes would be required.
+ /*
+ * If this is an UPPERREL_ORDERED step performed on the final
+ * scan/join relation, the costs obtained from the cache wouldn't yet
+ * contain the eval costs for the final scan/join target, which would
+ * have been updated by apply_scanjoin_target_to_paths(); add the eval
+ * costs now.
+ */
+ if (fpextra&& !IS_UPPER_REL(foreignrel))
+ {
+ /* The costs should have been obtained from the cache. */
+ Assert(fpinfo->rel_startup_cost>= 0&&
+ fpinfo->rel_total_cost>= 0);
+
+ startup_cost += foreignrel->reltarget->cost.startup;
+ run_cost += foreignrel->reltarget->cost.per_tuple * rows;
+ }
Yeah, but I think the code bit cited above is needed. Let me explain using
yet another example with grouping and ordering:
SELECT a+b, random() FROM foreign_table GROUP BY a+b ORDER BY a+b;
For this query, the sort_input_target would be {a+b}, (to which the
foreignrel->reltarget would have been set when called from
estimate_path_cost_size() called from postgresGetForeignUpperPaths() with the
UPPERREL_ORDERED stage), but the final target would be {a+b, random()}, so the
sort_input_target isn't the same as the final target in that case; the costs
needs to be adjusted so that the costs include the ones of generating the
final target. That's the reason why I added the code bit you cited.
I used gdb to help me understand, however the condition
if (fpextra&& !IS_UPPER_REL(foreignrel))
never evaluated to true with the query above.
Sorry, my explanation was not enough again, but I showed that query
("SELECT a+b, random() FROM foreign_table GROUP BY a+b ORDER BY a+b;")
to explain why the following code bit is needed:
+ /*
+ * 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;
+ }
I think that the condition
if (fpextra && fpextra->target != foreignrel->reltarget)
would be evaluated to true for that query.
It's still unclear to me why add_foreign_ordered_paths() passes the input
relation (input_rel) to estimate_path_cost_size(). If it passed the output rel
(i.e. ordered_rel in this case) like add_foreign_grouping_paths() does, then
foreignrel->reltarget should contain the random() function. (What I see now is
that create_ordered_paths() leaves ordered_rel->reltarget empty, but that's
another problem to be fixed.)
Do you still see a reason to call estimate_path_cost_size() this way?
Yeah, the reason for that is because we can estimate the costs of
sorting for the final scan/join relation using the existing code as-is
that estimates the costs of sorting for base or join relations, except
for tlist eval cost adjustment. As you mentioned, we could pass
ordered_rel to estimate_path_cost_size(), but if so, I think we would
need to get input_rel (ie, the final scan/join relation) from
ordered_rel within estimate_path_cost_size(), to use that code, which
would not be great.
Best regards,
Etsuro Fujita
