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postgres_fdw: Fix cost estimation for aggregate pushdown. 

In commit 7012b132d0, which added support for aggregate pushdown in
postgres_fdw, the expense of evaluating the final scan/join target
computed by make_group_input_target() was not accounted for at all in
costing aggregate pushdown paths with local statistics.  The right fix
for this would be to have a separate upper stage to adjust the final
scan/join relation (see comments for apply_scanjoin_target_to_paths());
but for now, fix by adding the tlist eval cost when costing aggregate
pushdown paths with local statistics.

Apply this to HEAD only to avoid destabilizing existing plan choices.

Author: Etsuro Fujita
Reviewed-By: Antonin Houska
Discussion: https://postgr.es/m/5C66A056.60007%40lab.ntt.co.jp
This commit is contained in:
Etsuro Fujita 2019-05-09 18:39:23 +09:00
parent 47a338cfcd
commit edbcbe277d
2 changed files with 17 additions and 3 deletions
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16
contrib/postgres_fdw/postgres_fdw.c
View File

@ -2842,12 +2842,18 @@ estimate_path_cost_size(PlannerInfo *root,
}
else if (IS_UPPER_REL(foreignrel))
{
RelOptInfo *outerrel = fpinfo->outerrel;
PgFdwRelationInfo *ofpinfo;
AggClauseCosts aggcosts;
double input_rows;
int numGroupCols;
double numGroups = 1;
/* The upper relation should have its outer relation set */
Assert(outerrel);
/* and that outer relation should have its reltarget set */
Assert(outerrel->reltarget);
/*
* This cost model is mixture of costing done for sorted and
* hashed aggregates in cost_agg(). We are not sure which
@ -2856,7 +2862,7 @@ estimate_path_cost_size(PlannerInfo *root,
* and all finalization and run cost are added in total_cost.
*/
ofpinfo = (PgFdwRelationInfo *) fpinfo->outerrel->fdw_private;
ofpinfo = (PgFdwRelationInfo *) outerrel->fdw_private;
/* Get rows and width from input rel */
input_rows = ofpinfo->rows;
@ -2909,11 +2915,13 @@ estimate_path_cost_size(PlannerInfo *root,
/*-----
* Startup cost includes:
* 1. Startup cost for underneath input relation
* 1. Startup cost for underneath input relation, adjusted for
* tlist replacement by apply_scanjoin_target_to_paths()
* 2. Cost of performing aggregation, per cost_agg()
*-----
*/
startup_cost = ofpinfo->rel_startup_cost;
startup_cost += outerrel->reltarget->cost.startup;
startup_cost += aggcosts.transCost.startup;
startup_cost += aggcosts.transCost.per_tuple * input_rows;
startup_cost += aggcosts.finalCost.startup;
@ -2921,11 +2929,13 @@ estimate_path_cost_size(PlannerInfo *root,
/*-----
* Run time cost includes:
* 1. Run time cost of underneath input relation
* 1. Run time cost of underneath input relation, adjusted for
* tlist replacement by apply_scanjoin_target_to_paths()
* 2. Run time cost of performing aggregation, per cost_agg()
*-----
*/
run_cost = ofpinfo->rel_total_cost - ofpinfo->rel_startup_cost;
run_cost += outerrel->reltarget->cost.per_tuple * input_rows;
run_cost += aggcosts.finalCost.per_tuple * numGroups;
run_cost += cpu_tuple_cost * numGroups;

4
src/backend/optimizer/plan/planner.c
View File

@ -7113,6 +7113,10 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
* confused in createplan.c if they don't agree. We must do this now so
* that any append paths made in the next part will use the correct
* pathtarget (cf. create_append_path).
*
* Note that this is also necessary if GetForeignUpperPaths() gets called
* on the final scan/join relation or on any of its children, since the
* FDW might look at the rel's target to create ForeignPaths.
*/
rel->reltarget = llast_node(PathTarget, scanjoin_targets);